Concrete Centre Basements for Housing

Basements for housing

Benefits and solutions for sustainable housing

2Basements for housing

IntroductionThis document considers the role of basements as a viable part of the solution to sustainable housing in the UK, through consideration of their specific attributes in relation to planning policy and the Code for Sustainable Homes.

Outlining the key issues to be considered at each stage of the design

and construction process for single-storey basements in domestic

housing, guidance is additionally offered on sources of more detailed

advice.

The Code of Practice for protection of below-ground structures against

water from the ground (BS 8102) was revised and re-issued in December

2009.

About this publicationDomestic basements can aid in the creation of desirable, sustainable

homes, providing greater flexibility and adaptability of space; thus

extending the design life of the building.

Including a basement maximises usage of available land, provides a

stable construction base and improves thermal efficiency.

Modern basements offer the possibility of additional, alternative

living spaces in dry, warm, day-lit rooms with good ceiling heights and

ventilation levels. Usage potential ranges from ideal quiet areas for

home working or leisure to additional space for storage and parking.

The use of full or partial basements in housing can play a significant role

in meeting current and future needs for new homes in the UK. Basement

design supports basic sustainability principles such as longevity, durability

and adaptability, as well as providing useful space for many of the additional

requirements needed to comply with the Code for Sustainable Homes.

Definition: Basement

Throughout the document, the term basement refers to a usable part

of a building that is situated partly or entirely below ground level, as

defined by the British Standards Institute [1].

Building regulations in England and Wales define a basement storey as

at least 1.2m below adjoining ground level [2].

Occasional reference is made in this document to partial- or semi-

basements to remind readers that usable domestic basements are likely

to include windows and doors for natural lighting and ventilation. All

walls enclosing a basement may not, therefore, be fully below ground

level; for example on sloping sites or with lowered courtyards.

This document is principally concerned with the issues associated

with new basement construction rather than existing cellars since the

latter is, by definition, limited to space for storage below ground with

less requirement for daylighting and ventilation. Much of the guidance

is, however, relevant to the conversion of old cellars into habitable

basements.

ContentsTypes of basements 4

Benefits of basements 6

Optimising development potential 9

Construction techniques 11

Design issues 15

Costs 19

Building legislation 20

Appendix and further reading 22

An example of a new build development with a basement.


The case for basements todayChanging requirements for housing

Houses with below-ground space used to be common in the UK but

their construction has declined over the last century. In mainland

Europe and North America, however, basements have continued to

be incorporated into new dwellings. In Germany, they are included in

almost 98% of housing stock.

The decline in the UK was due, in part, to changes in living requirements.

Improved transport, convenience food and the use of electric fridges

diminished the need for naturally cool areas to store food and, together

with changes in social attitude, the need for a space below stairs for

domestic help. Until recently, reliance on cheap gas and electricity also

reduced the need for areas in the home to store fuel.

However, housing in the UK is currently undergoing a radical re-

evaluation of its design and performance, as a result of evolving

legislation and attitudes towards sustainability and energy efficiency.

Established construction techniques and space-planning solutions

for housing are being challenged and tested against a new set of

sustainable performance criteria, and basements are once more

considered relevant and attractive additions to housing.

In addition to the drive for low energy housing, there is a movement

towards increasing the size of new housing, in response to reports by

various organisations, including the Commission for Architecture and

the Built Environment (CABE), Homes and Communities Agency and

Mayor of London Office. The provision of homes that have sufficient

space to develop and grow with the requirements of a family is a

significant sustainability issue for housing.

The sustainability benefits of basements are described in the benefits of

basements section of this document, and a summary of scoring credits

applying to basements under the Code for Sustainable Homes is given

in the Appendix.

In Germany, and much of continental Europe, the basement provides the solution to the problem.Over there, asking for a house without a basement is like asking for a car without wheels.

Chris Drury - Weber House, Germany, commenting on the

lack of storage space in UK housing.

Government targets for housing and limited land availability

A combination of Government targets for new housing and

limited availability of land for new construction have led to greater

consideration of increased densities and the development of difficult

plots of land, such as sloping sites or those with poor soil stability.

Planning Policy Statement 3: Housing (PPS3), underpinning delivery of the

Governments strategic housing policy objectives, encourages increased

density of housing developments. In addition, high land prices mean

housebuilders are under increasing pressure to maximise potential

returns through efficient use of land.

The use of full or semi-basements can be a cost effective means of

increasing the density of housing developments, without reducing

amenity levels; particularly if planning requirements restrict the building

footprint or height.

Strategies for optimising residential development are explored on page

9 and 10 of this document.

Improved construction techniques and contractor warranties

The reliability and design of construction and waterproofing techniques

has significantly improved over the years; backed by many successful

examples both in the UK and abroad.

Advice regarding the detail, design and construction of basements

is well documented and available from The Basement Information

Centre (TBIC), The Concrete Centre and National House-Building

Council (NHBC), amongst others. NHBC arrangements with builders

and developers provide cover under their standard building assurance

system for the construction of basements [3].

Accreditation and indemnity schemes exist for specialist basement

contractors. Details of accredited contractors (construction and

waterproofing) may be found on The Basement Information Centre

website. In addition, various manufacturers of specialist water-resisting

concrete offer guarantees.

An accreditation and insurance scheme, administered by the Association

of Underpinning Contractors (ASUC), is available for underpinning of

existing structures; most commonly required for refurbishment or retro-

fit basements.

The correct procedures for design, soil investigation and construction

are key to achieving robust and reliable basement construction.

The various methods of construction on offer, and design issues, are

summarised in the construction techniques and design issues sections

of this document.


Types of basementsBasements can be broadly sub-divided into five categories, depending upon their location, time of construction relative to the main property and depth. A brief summary of the differences, and the key issues related to each, is outlined below.

Table 1: Types of basements.

Type of basement

Description Details Benefits Other comments

Refurbishment Alterations to existing space

below ground.

May require lowering the

floor to increase head room;

underpinning adjacent walls;

improving ventilation and

lighting. Typically involves

improvements to, or new,

waterproofing system; and new

finishes and fixtures.

Addsvalueanddesirability of property

Additionalusablespace Addressespotentialexisting damp problems

Providesopportunitiesfor new activities in property e.g.

games room/gym/storage

Building regulations approval

will be required for any works.

Planning permission may be

required, depending on extent

of works and intended use. *

New garden basement

New basement

adjacent to existing

property, usually in

garden space.

New structure below ground,

with planted green roof or

terrace at garden level. Access

from main house via new

external covered staircase.


Additionalusablespace Providesopportunitiesfor new activities in property

Potentialfornaturaldaylight and ventilation through

roof lights

Planning and building

regulations approval required.

Likelihood of need for structural

support to existing house is

reduced as distance from house

increases.

New basement under new housing

Basement space

built as part

of a new build

development.

Arrangement of windows and

internal and external access

vary. Designed to suit current

and future use requirements,

site conditions, cost and

constructability.


Futureadaptability Usablespaceforsustainable technologies and recycling

Potentiallyreducesfootprint of house

Increasesthermalperformance

No additional building

or planning regulation

requirements provided it is

included in initial application.*

Retrofit New space created through excavation

below ground

floor of an existing

property.

Allows creation of additional

space below business or homes,

which benefit from staying in

same location.

Preservation of existing building

possible (e.g. listed building)

Underpinning works required.

Addsvaluetoproperty Additionalspaceforbusiness to develop in same location or

family to expand

Releasespotentialofempty property

Planning and building

regulations approval required.

Specialist work. Generally only

economically viable for high

land value properties.

Deep basements Spaces below one storey deep.

Frequentlyusedforcarparking,plant/services space and

storage below larger residential

development and other uses

including commercial, retail or

mixed use schemes in urban areas.

Buildingfootprintand development potential

optimised above ground

Parkingandspacefor deliveries possible

RefertoDesign and Construction of Concrete Basements [10].

*AttimeofwritingworksmaynotbeallowableunderPermittedDevelopmentRights,dependingonindividualLocalAuthority.Thismayberevisedin the near future. The situation should be confirmed with the Local Authority Planning Department.

Note: The Party Wall Act could apply to each type of basement. See page 21.


Some examples of basement arrangements

Figure 2: Basement with gym, shower and sauna. External and internalaccess with light well for daylight and additional ventilation.

Figure 1: Basement garage and storage. External access via the driveway.

Figure 5: Split level design with separate external access and light well. Potential for office or workshop.

Figure 6: Garden basement with internal access and roof light above.

Figure 3: Basement with games room, wine cellar and storage. With external courtyard for daylight and additional ventilation. Access via

courtyard and separate internal stair.

Figure 4: Basement with additional bedroom or annex to the property (granny flat or similar) with internal and external access.Extra light

provided via conservatory.

6Benefits of BasementsThere are many reasons for the provision of basements in a housing development including: added value; increased development potential; occupant or purchaser attraction and sustainability. This section explains the key benefits.

Desirable and adaptable spaces

There is an evident desire for the provision of basements as part of

our housing solution in the UK, illustrated by the significant number of

basements constructed in the self-build market.

In addition, studies by the Traditional Housing Bureau [4] indicate

significant demand from home owners for more space. In the 2005 CABE

report What home buyers want: Attitudes and decision making among

consumers, basements are cited as particularly valuable in this regard.

One of the major benefits of basements is allowing the creation of a single

large space. Due to economies and method of construction, the floor above

the basement level can be created in a single span, providing flexibility in

the location of internal walls and allowing simple future alterations.

Pre-subdivision, the resultant space is typically the single largest area

in the house and, due to its location on a different level from the rest

of the house, provides opportunity for uses not always possible to

accommodate in more basic dwellings. In North America, Canada and

continental Europe, it is common for basements to serve as multi-

purpose areas, for example games or utility rooms or storage areas.

Basements can provide comfortable day-lit rooms, with natural

ventilation and external access, as an extension to the living spaces

above. They also provide the opportunity for more unique uses, such as

gyms, music rooms and swimming pools. Alternatively, basements can

simply provide practical space for games or hobby rooms, home offices

parking or storage.

Good sound insulation

Good acoustic attenuation is provided by the concrete walls

surrounding basement rooms by the earth itself and the ground floor if

it is built from concrete. Basement spaces are therefore inherently well

insulated for sound and ideal for locating noisy activities such as music

practice, home cinemas or other loud equipment that could disturb

neighbours or the rest of the house.

Conversely, the quiet nature of the space provides a peaceful place

for reading, relaxing or working; away from ground-level noise, in and

outside of the house.

Martin Grant Homes - Riverview Court development.

Developers in the UK are now looking at basements as a solution for creating spacious homes whilst using the land available efficiently. RiverviewCourtdevelopmentwasbuiltonaformer water treatment works on a flat site, besidetheRiverCam,Cambridgeshire.35homes were built with sunken patios which provide natural daylight and ventilation to the basement kitchen and dining room.

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Added value and space

The provision of additional floor space in a home clearly increases the

value of a property and the addition of a basement adds space with little

or no effect on the garden area. The costs associated with constructing a

basement are explored in the Costs section of this document.

Sustainability benefits

Including a basement in the design of domestic properties helps

developers to address many of the fundamental principles of

sustainable design, including improved energy efficiency, longevity and

adaptability and support of local employment.

Energy efficiency

Basements benefit from the surrounding ground improving their energy

efficiency. As a consequence, the amount of insulation needed to reduce

heat loss through a basement wall is less than that required on upper floor

levels [5]. The simple construction methods and minimum wall penetrations,

associated with basement construction, also lead to minimal heat loss

throughcoldbridging.StudiesbyTBICandBuildingResearchEstablishment(BRE)highlightapotential10percentsavinginspaceheatingforatwo-storey house with a full ground basement compared with its three-

storey equivalent above ground (both having the same amount of added

insulation). The potential space heating saving rises to around 14 per cent

for a single storey property with full basement, compared to its two-storey

equivalent above ground [6].

Thermal mass

The heavyweight nature of basement construction can be utilised to

naturally regulate the internal temperature of a home and can be part

of an energy efficient strategy for controlling the temperature of the

whole house. The Met Office has projected average daily temperature

rises throughout the UK, indicating the increasing need for low energy

solutions to cool homes, which heavyweight construction - including

basements-iswellplacedtoprovide.ForfurtherinformationrefertoThe Concrete Centre publications Thermal Mass Explained and Thermal

Mass for Housing. www.concretecentre.com/publications.

The thermal mass properties of concrete are optimised by omitting

insulating internal surface finishes. If insulated and waterproofed

externally, basement concrete walls will offer greater thermal mass. This

could be achieved with a fair-faced or painted finish, or alternatively

awetplasterfinish.Fair-facedconcreteofferspotentialcostandprogramming benefits, by omitting subsequent use of finishing

materials and trades and associated waste produced on site.

Air tightness and mechanical ventilation

The construction of sustainable dwellings using low air permeability

and mechanical ventilation with heat recovery, such as the Passiv Haus

technique, is a means of improving the energy efficiency of the building

fabric of dwellings. This is simply provided by basements, since the

structure below ground is inherently more air tight.

The addition of a basement increases the living space in a property, without compromising the garden. Courtesy of The London Basement Company.


Basements and the Code for Sustainable HomesSince 2008, all new homes require rating under The Code for Sustainable

Homes (CSH) as part of Home Improvement Packs (HIPs). The inclusion

of a full or partial basement can provide additional credits under the

assessment criteria of the Code; for example, storage of waste, recycling,

alternative fuel supplies such as wood pellets, other equipment

associated with renewable energy or water recycling and even bikes.

In addition, basements can be used to reduce the overall size of the

building footprint, relative to the number of storeys, scoring points in the

Ecology Section of the Code. A full list of how basements can assist with

obtaining credits for Code for Sustainable Homes is found in the appendix

of this document.

Space for working from home

Basements offer the possibility of quiet, private spaces for working

from home, with the potential for direct access from the outside, and

therefore designated work-based visits or deliveries.

The solid nature of basement construction provides excellent conditions

for workshop spaces and activities that create noise, require support for

heavy equipment or require robust, cleanable surfaces.

Changing uses for basement spaces during the life of a family home

Extra storage (recycling/chest freezers)

DIY work shop

Utility room

Hobby room

Home Gym/Sauna

Wet room outdoor gear

All weather play room

Band practice/music room

Teenage den/bedsit

Home office/studio

Wine cellar

Home cinema

Granny annex

Living room

Basements provide flexible multipurpose spaces through the lifetime of a home. Courtesy of The London Basement Company.


Optimising development potential

Higher density development - minimal extra heightThe use of a basement provides increased floor area in a

development without significantly increasing the apparent

size of the building; thereby improving the viability of a

development, particularly in areas where building height or

size of footprint is restricted. The potential to increase the

number of proposed dwellings in one property is boosted

if both a basement and habitable roof space are proposed;

thereby potentially creating two additional storeys in a

similar building envelope.

Utilise existing slopePartial basements on sloping sites become viable compared

to alternative substructure construction options to make up

ground levels.

Maximise site layout - build up to street boundaryBy lifting the ground floor level above street level to

create upper and lower ground floors, properties can be

constructed close to site boundaries while maintaining

privacyforoccupants.ConsiderationofBuildingRegulationPart M requirements for access are required.

10


Protect amenity spaceThe use of a fully below-ground basement or partially

below-ground basement gives increased usable space

within the building footprint. Larger dwellings can therefore

be built on small sites without losing amenity space around

the buildings.

By incorporating facilities such as garages, utility rooms

or habitable space at basement level, it is possible to

reduce the footprint of a proposed property; thereby

increasing the number of houses on a given site or

along a fixed street frontage.

Better use of poor sites with poor soilWhere poor ground conditions necessitate deep foundations, the

additional cost can be mitigated by including a basement to add space

and therefore value to the proposed new properties.

Where large areas of contaminated soil are removed from site, the

viability of including a basement level is increased. If constructed before

original ground levels are reinstated, the amount of replacement ground

material is reduced and further excavation is unlikely. Back fill will need

to be compacted around the walls, but in general the programme of

construction is likely to benefit from improved access conditions.

Stable building stockBasements create a good stable structural base, capable of supporting

heavy loads above. By combining foundation design with the provision

of habitable space, the extra depth of structure provides the building

with greater ability to cope with climate change effects in the soil, such

as shrinkage or tree roots. This means buildings with basements are less

prone to movement and cracking as a result of potential future changes

in soil conditions.

Increased number of plots per hectare

Housing using a basement garage requires less street frontage,

compared to houses with garages located alongside at ground

level.

Shading indicates equivalent accommodation areas located

beside or below a dwelling, impacting on available garden

space.

9 plots possible with basement garages, compared to 8 plots on the same site.

10.0m

9.0m

7.8m

6.0m

6.0m7.8m

9.0m

7.6m

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Basement constructionTypes of waterproofing protectionThere are three main methods of providing protection against ground

water for residential basements. These are defined in BS 8102 as types

A, B and C. Their application is influenced by the ground conditions and

proposed building use. The definition of terms and guidance related to their

appropriate usage, are highlighted in the 2009 revision.

Alternative approaches

The traditional method of waterproofing domestic basements in Britain

was a single barrier method (Type A) or drained protection (Type C).

Earlier materials used in basement construction have since been

developed into more durable waterproofing membranes.

Furtheralternativeapproacheshavebeendeveloped,wherebyhydrostaticpressure on the wall structure is eliminated. These methods incorporate a

drainage blanket around the perimeter of the basement, allied to effective

drainage below the floor slab and around the building. Suitability depends

on the drainage characteristics of the ground and topography.

A combination of systems can also be an appropriate design solution.

Forexample,theapplicationofadditionalwaterproofingsystemstoaType B structure will improve water vapour control or provide further

protection against water ingress.

Each of these methods is viable for domestic basements in Britain,

depending upon the specifiers preference, site conditions, the type of

development and perceived risk. Table 2 on page 12 provides a summary

of appropriate waterproofing protection for varying risks associated

with water table levels and useful additional measures to reduce risk

dependant on project particulars [7]. BS 8102 should be consulted for

further details.

Type C: drained protection any water seeping through external walls and floor is drained to a sump via an internal

cavity, typically created by a proprietary cavity system and

pumped or drained away.

Type A: barrier protection reinforced concrete or block-work with waterproofing located either externally, internally

or sandwiched.

Type B: structural integral protection - reinforced or prestressed concrete designed through composite and

integrated details, such as water bars, to be water resistant.

Internalwaterproofing

Sandwichedwaterproofing

Externalwaterproofing

Water resistant reinforced concrete wall and slab

Slab with integral kicker

Drained cavity

Inner skin

Wall cavity

Internal block wall

Access point(s) to drainageConcrete/steel piled wall

Drainage channel

Waterstop at junction to follow wall profile

Floor slab with integral protection and/or added membrane (internal or external)

Pump

Sump formed in situ or separate drain which may be solid or perforated

May incorporate drainage channel with pipe connection to setup

Slab with kickerless construction

External or internal waterstop as required Waterstop required

at junction between wall and slab and at all construction joints. e.g Crystallisation, hydrophilic or injected waterstop

Water-resistent reinforced concrete wall and slabA non-integral kicker

should be avoided as it will require one water-stop where it adjoins the slab and another at the intersection with the wall






Drained cavity

Inner skin

Wall cavity

Internal block wall


Drainage channel



Pump













Drained cavity

Inner skin

Wall cavity

Internal block wall


Drainage channel



Pump








Double height concrete basement extension to existing property. Courtesy of pH+ architects.

Diagrams from The Design Guide, courtesy of TBIC, 2010.

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Table 2: Types of waterproofing protection

Risk associated with water table

Water table classification*

Waterproofing protection

Type A Type B Type C

Piled Wall Reinforced concrete wall to BS EN 1992

Low

High

Low Acceptable Acceptable Acceptable Acceptable

Variable Acceptable if the variable classification is due to surface water. The manufacturers advice should be sought.

Acceptable where:a) the piled wall is directly accessible for repair and maintenance from inside the structure; orb) the piled wall is combined with a fully bonded waterproofing barrier; orc) the piled wall is faced internally with a concrete wall to BS EN 1992.

Acceptable Acceptable

High Acceptable where:a) an appropriate cementitious multi-coat render or cementitious coatings are used;b) the wall is of concrete to BS EN 1992.

Acceptable Acceptable

* The water table classifications are defined as follows:

Low-wherethewatertableorperchedwatertableisassessedtobepermanently below the underside of the base slab, this only applies to free-draining strata. Variable-wherethewatertablefluctuates High-wherethewatertableorperchedwatertableisassessedtobepermanently above the underside of the base slab. Ground permeability might affect risk under a low or variable water table

Measures to reduce risk

Usecombinedprotection Incorporateappropriatelydesignedsub-surfacedrainageandensurethat this is maintained Useafullybondedwaterproofingbarrier Lowerthepermeabilityofthemainstructuralwall Useconcretewithawaterproofingadmixture,e.gtoBSEN934 Ensurethatdischargesystems,e.gpumps,aremaintainedsothatthe system remains effective

Waterproofing barriersThis section describes in broad terms some of the options and issues

associated with the choice of waterproofing system for domestic

basementsusingTypeAbarrierprotection.Formoredetailedguidancerefer to BS 8102 and the Waterproofing Design Guide, by The Basement

Information Centre.

There are six categories of waterproofing barrier materials available. The

following table shows where they can be located, subject to the form

of supporting structure. In addition to those noted below, there are

waterproofing membranes used in Type C construction.

Water stops

Water stops are an essential part of the waterproofing design solution;

for Type B protection used at the junction of structural panels, between

walls and floors or along day-work joints for cast in situ concrete, the

principle types can be classified as:

a) Passive sections e.g. PCV water bars, located outside or within

the structure to obstruct water transmission.

Table 3: Categories of barrier protection

Categories External Sandwich Internal

Bonded sheet membranes / /

Liquid applied membranes / /

Geosynthetic (bentonite) clay liners / /

Mastic asphalt membranes / /

Cementitious crystallisation slurries

and powders

/ /

Cementitious multi-coat renders,

toppings and coatings

/ /

b) Active strips or slurries (hydrophilic or crystallization) that react with water to prevent its further progression. These are set within the section of the structure, or post-injected.c) Specialist sealing resin injected into pre-positioned permeable hoses or similar.

Design issues

Particular attention should be paid to the specification of waterproofing

systems - particularly for deep basements - relating to areas of high

water table and in soils with aggressive chemicals. An appropriate

specialist should be contacted for early advice and help on

waterproofing design.

Good design and workmanship are primary factors in achieving

waterproof construction. Key considerations are compatibility of

waterproofing systems, sealing around joints and junctions of the

waterproof membrane and, for integral structural waterproofing

systems, attention to the construction joints.

Structural design may affect the choice of waterproofing and

compatibilitybetweenthetwoisessential.Forexample,thestressandpermissible crack width of a structure is controlled by reinforcement.

In plain wall structures (i.e. not reinforced) the applied waterproof

membrane needs to be appropriate to the anticipated movement

of the structure, as the allowable movement or cracking may exceed

the strain capacity of some waterproofing membranes. This is also a

key consideration when refurbishing or extending basements, since

movement between existing and new structures must also be anticipated.

Details and construction profiles should be simple, avoiding nibs and

thickening of structure wherever possible to prevent complicated

junctions. Adequate details must be provided for each junction and

considered in three dimensions (3D) for thoroughness.

Although discontinuity with respect to waterproofing might be

acceptable - subject to careful detailing and an appropriate assessment

of risk - in practice this may not be allowed due to the need to manage

radon, methane and other ground gases and contaminants.

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Concrete constructionConcrete is the most common and appropriate material used in the

construction of new basement walls and floors. This is due in part to cost

and availability but also its inherent resistance to water, durability under

ground and ability to provide a stable structural surface for the support

of waterproofing membranes.

The method of construction chosen will depend upon consideration of

various factors including: potential repetition of construction elements;

accessibility for labour and cranes; cost; and fundamentally, the type

of construction system permitted according to water table and use, as

described in Table 2. Most forms of concrete construction can provide a

variety of wall thicknesses to suit the particular structural requirements

of each basement.

Masonry construction or concrete blockwork

Masonry construction or concrete blockwork is a traditional form

of basement construction in the UK. It can be used with Type A

waterproofing protection, for cases in which it is recommended that

render or a similar smooth, continuous layer is applied to the blockwork

face to provide continuous support to the waterproof membrane. Walls

are typically reinforced and particular care is required at corner details

and the wall slab junctions to cope with ground pressure.

Masonry walls can also be effective as internal lining to create a drained

cavity basement wall (Type C).

Cast in situ concrete

Cast in situ concrete is appropriate for all types of basement

construction. It is a common form of basement construction for

residential use, due to its relatively simple application, adaptability and

cost. In-situ concrete is often the only appropriate form of construction

for retrofit basements under existing properties, due to its relative ease

of placement on site.

As with masonry, in-situ walls are most commonly installed as reinforced

structures but can be used plain (without reinforcement) following

guidance provided in Addendum 1- Plain masonry and plain in-situ concrete retaining walls by TBIC.

Typically, cast in situ walls are constructed with steel reinforcement

bars to control cracking in the structure, with particular attention given

to reinforcement of the corner junctions. Plain concrete walls are not

generally specified as Type B construction due to the more critical need

to control crack dimensions. Workmanship is a key issue for successful

implementation of Type B protection.

Water stops are included in the construction joints and particular

attention is required with regards to day-working joints and the

constituents of the concrete mix. Cast in situ concrete requires time to

dry out before water sensitive finishes can be applied.

Water-resisting concrete

Concrete is inherently water-resistant and robust, making it suitable for

subterranean construction. Its water resistance can be further enhanced

by the introduction of admixtures. These admixtures (hydrophobic and

pore blocking) act to reverse the capillary or sucking action of the tiny

capillaries on the concrete surface and to effectively block the pores

within the concrete when subjected to hydrostatic pressure. The result is a

dry concrete that protects from water ingress. Such proprietary concrete

mixes are available for this purpose from a number of specialist suppliers.

Warranties can be obtained for products and workmanship on site.

It is still possible for small levels of water vapour to pass through these

types of concrete but they are generally very low and so unlikely

to cause a problem. Additional membranes or ventilation may be

considered, depending upon site conditions, proposed use and client or

designers assessment of, and attitude to, risk.

Insulating concrete formwork (ICF)

ICFsystemsuseeitherlightweighttwin-walledexpandedpolystyrene(EPS) or extruded polystyrene (XPS) in panels or blocks to create

formwork walls, for in-situ concrete walls, typically 100 or 150mm

thick. Once in place, the formwork is filled with ready mixed concrete

and, unlike conventional formwork, is left in place to act as insulation.

Forbasementconstruction,polystyreneprovidesgoodbackgroundfor waterproofing barriers. Care should be taken to ensure that the

specification of the waterproofing membrane and its fixing methods are

appropriate for application to polystyrene.

ICFprovidesacosteffective,simpleandinexpensivemeansforplacingcastin situ walls; most appropriate for new build, rather than retrofit, basements.

AnewbuildbasementusingICF. An example of concrete twin wall construction.

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Precast concrete modular units

Precast concrete units are increasingly used in Britain and

elsewhere as a form of basement construction and provide

an excellent support for waterproof membranes, either as a

tanked membrane system or as the outer wall of a drained

cavity or even as proprietary Type B system. Precast sections

can be fabricated to specific design requirements for just-in-

time delivery, providing rapid on-site construction, integrated

water bars, low site waste and high quality finishes. They

are particularly appropriate for developments potentially

benefitting from a high number of repeated standardised

elements and the use of a crane on site. The waterproofing

detail should be designed to suit the manufacturers

established method of joining panels.

Twin wall

This construction method is a hybrid of precast and cast in

situ concrete walls and floors. Each wall unit comprises of two

plates of precast concrete with a cavity between, linked by a

lattice of steel reinforcement and placed on site; effectively

as permanent concrete formwork. Once units and water bars

are in place the cavity is filled with ready mixed concrete to

complete the structural wall.

Twin wall systems offer all the benefits of precast concrete

described above but with the added benefit of continuous

cast in situ concrete across the whole wall and, potentially,

floor above.

Concrete piles

Piles are more commonly used for deep basement

construction, rather than domestic situations and come in

various forms, but can be useful for the creation of retaining

walls to facilitate excavation in areas of restricted access

or close to site boundaries. Secant or fair-faced contiguous

piles can effectively become the outer wall of a Type C

construction, or be faced with concrete or waterproofed to

provideTypeBorTypeAprotection.Furtherinformationonthis and other forms of retaining structures can be found

in The Concrete Centre technical publication Design and

Construction of Concrete Basements.

Concrete floors

At basement level, floors are typically cast in situ concrete.

The choice of system will be driven in part by coordination

withthewallconstruction.Floorsatgroundfloorlevelinhousing can be constructed using a variety of different

concrete construction techniques, including in-situ, block

and beam, hollow core precast units or hybrid systems.

Typically, it is possible and beneficial to span the full width of

the basement space with the floor structure. Concrete easily

exceeds the minimum building regulations requirements

for fire and imposed loads and provides excellent sound

insulation between the spaces.

14


15


Design principlesThe appropriate design of basements is well established and achievable,

provided design and construction guidance is implemented.

The general principle is to assess the risk of water reaching the below

ground structure and to select an appropriate form of construction,

structure and system of waterproofing to achieve the required internal

environment.

To do this the designer needs to understand the expectations of the

client, the proposed and likely future use of the basement space and its

associated performance requirements in terms of building regulations.

It is essential that an appropriate site investigation is carried out to

establish the soil and ground water conditions. Evaluation of these

factors provides the basis for selection of an appropriate construction

method, structural solution and system of waterproofing.

It is strongly advised that a three dimensional (3D) review of structure

and waterproofing is undertaken to identify and avoid any complex

geometries, which will not be readily identified from normal two-

dimensional details.

Design issues

Basement design process (simplified)1 Establish basement use; current and future flexibility2 Site survey and exploratory works3 Design proposals to define type of construction, water tight class and thermal performance

4 Detailed structural design integrated with design of waterproofing

Roles and responsibilities

Aspects of the design process are inter-related and there are likely to be a number of options available; particularly for straightforward residential properties.

Of particular importance for new-build basements is a unified approach to establishing an appropriate design solution and defining the roles and responsibilities of the design team from the outset. It was common for the design of the waterproofing system to be the responsibility of the architect however, in BS 8102: 2009 there is emphasis on including a specialist waterproofing advisor as part of the design team so that an integrated waterproofing system is created. This can be an architect or another consultant, manufacturer or supplier, provided they have the relevant expertise. An exception to this is when the construction method is classified as structurally integral protection; when it may form part of the structural engineers brief, a specialist waterproofing advisor may still be required.

The client should be advised of any implications related to choice of construction and waterproofing with regards to the expected building use, future flexibility and associated maintenance requirements.

Minimising risk in basement design:Initial design should consider: Anticipatedcurrentandfutureuseofbasement Anticipatedcurrentandfuturegroundwaterconditions Orientationofbuildingrelativetogroundwater Currentandfuturedaylightingandventilation requirements

Simplifyingshapetofacilitatewaterproofing Locationandaccessonsitetofacilitateconstruction Avoidingpenetrationofwaterproofmembranefor services where possible

Site investigations should include: Appropriatequalitativeassessmenttoappropriatedepth Geotechnicalinvestigationtoindicatecurrentand anticipated future ground water regime

Teststoindicatesoilpropertiesandsurfaceloadingto establish lateral earth pressures

Detailed design should consider: Correctchoiceofconstructionandwaterproofingto suit ground conditions and use

Integrationofstructuralandwaterproofingdesignto best practice recommendations

Three-dimensionalstructuralloadsofbuilding,ground and water pressure with attention to corners

Accessforfuturemaintenanceandalterations Obtainingspecialistadviceparticularlyforhigh water tables

Construction should include: Supervisionandchecking(bothessential) Experiencedandskilledoperatives Instigationofconstructionwarranties

In use: Maintenanceandoperationofdrainage,pumpsand ventilation systems

16


Table 4: Grades of basements

Grade Basement Usage Performance Level

1 Car parking; plant rooms (excluding electrical

equipment); workshops

Some seepage and damp areas tolerable, depending on the intended use*

Local drainage might be necessary to deal with seepage

2 Workshops and plant rooms requiring drier

environment (than grade 1); storage areas

No water penetration acceptable

Damp areas tolerable; ventilation might be required

3 Ventilated residential and commercial areas

including offices, restaurants etc; leisure centres

No water penetration acceptable

Ventilation, dehumidification or air conditioning necessary, appropriate to the intended use

* Seepage and damp areas for some forms of construction can be quantified by reference to industry standards, such as the ICSs Specification for piling

and embedded retaining walls.

Basement use - current and future

It is essential that the current and proposed use of a basement space is established early in design development, in order to provide the relevant performance criteria for the subsequent choice of waterproofing system, construction method and structural design.

BS 8102 designates building uses against three grades of water tightness. These range from car parking areas, where some seepage and damp patches are tolerated, to ventilated residential and commercial areas where no water penetration is acceptable. Standards and forms of construction and waterproofing suitable for each grade of usage are provided.

The previous edition of the British Standard (still referenced in the Approved Document - Basements for Dwellings) referred to Grade 4 environments. This was omitted in the later version since the only difference from Grade 3 is the performance level related to ventilation, dehumidification or air conditions. BS 5454 provides specific guidance related to the storage of exhibition or archival documents.

A Grade 2 environment may be acceptable for permanent workshops or garages. However, since usage may change, it is better to construct a basement to a Grade 3 environment than to upgrade it later. In a high risk situation, the client and designer may wish to opt for additional waterproofing or vapour control.

Site investigationThe location and potential fluctuation of the water table is the key factor

effecting basement design and construction. High water tables present

the greatest risk for a basement and must therefore be identified at an

early stage in the design. A watercourse or water table that rises and

falls, and the potential for a perched water table, must also be identified.

A high water table refers to, by definition, groundwater level consistently

above the level of the basement floor. A permanently low water table

involves a water table consistently below the level of the basement floor.

A variable water table refers to levels varying between the two extremes.

The installation of drainage systems can artificially lower the water

table but is not always beneficial due to potential detrimental effects on

neighbouring properties.

The draining ability of the soil and existence of contaminants can effect

the choice of concrete construction and waterproofing method, as will

the location of nearby drains and an assessment of the likelihood of

their flooding.

Typical factors to be assessed in site investigation

Existenceofwatercourseorseasonalposition of water table

Topographyoflandanddirectionofground water movement

Locationofdrainsandlanddrains Soiltypeandconditions Movementrisks-potentialsubsidence Presenceofnaturalgasese.g.radon/methane Evidenceofgroundcontaminants Boundaryconditions

Mirrors facing and adjacent to, window openings can significantly increase the perceived light levels. Courtesy of The London Basement Company.

17


Orientation and site layout

The shape and orientation of a building should be considered because

of the potential to dam the flow of ground water and the resultant

build up of hydrostatic pressure. If unavoidable, additional subground

drainage may need to be provided to discharge the water elsewhere.

The form of construction of a basement and its cost will be influenced by

the proximity of its walls to existing boundaries and adjacent buildings.

The installation of external waterproofing and insulation, for example,

requires sufficient space around the outside of the basement walls to

provide a safe working area and may require temporary shoring.

Fornewbuildconstructionsintight-usuallyurban-plots,permanentunderpinning of adjacent boundary walls or properties can allow

valuable additional basement floor areas, but is expensive. An alternative

is to install sheet piling to contain the ground supporting the structure

while the new basement is under construction.

Flowofgroundwater

Flowofgroundwater

Plan form of building avoids possibility of damming the flow of ground water

Provide subground drain discharging to a suitable outfall to alleviate hydrostatic

pressure where necessary

Effect of building orientation on flow of ground water

Daylight The need to provide daylighting and comply with building regulation

requirements for ventilation of habitable rooms will generally be met by

incorporating openable windows in the same manner as above ground.

This may entail adjusting the external ground levels in partially below-

ground basements, and would mean forming open areas for windows in

fully below-ground basements.

A primary factor in improving the quality of a room in a basement is the

provision of natural light. Inclusion of glazed windows or doors provides

greater possibility of future adaptation and uses, as well as sustainability

benefits by reducing dependence upon artificial lighting.

There are many techniques for improving the level of natural daylight and

ventilation in basement spaces; determined by various factors including

the proposed use of the space, proximity to boundary and plot size.

Solutions for habitable spaces include simple direct lighting through

windows, glazed doors or roof lights. Other supplementary solutions

include the use of sun pipes or use of borrowed light with mirrors,

glazed floors or stairwells.

Daylighting techniques

Partiallysunkenlightwellsandwindows.

Fulldepthexternalspaceswithglazeddoorsproviding separate private amenity space and potential access to

the garden or alternative entrance from the street.

Sunpipesandpavementlightsaresuitablefor basement spaces extending beyond the footprint of

the building above. They provide permanent natural

lighting with additional security but limited views.

Glazedrooflightscanwashspaceswithnaturallight and provide sky views and natural ventilation if

openable.

Mirrorsfacingandadjacentto,windowopeningscan significantly increase the perceived light levels and

provide depth of field. Light and/or polished surfaces

will generally improve the sense of space and daylight

levels in a room.

Glazedfloors,particularlybelowupperfloorrooflights or windows, can be useful additional sources of light

but will require fire-rated glazing to maintain fire

compartmentation between floors.

Lightfromupstairsroomscanbrightenlowerground floor spaces via the stairwell. This arrangement will

depend upon the specific fire arrangements of

individual properties and may require an upgraded

fire resistance or detection system.

Light from the upstairs room can brighten lower ground floor spaces via the stair-well. Image courtesy of Loates Taylor Shannon architects, Paul Avis photography.

18


Drainage It is advisable that drainage, or any service connections, should not be

made through the basement retaining walls. Even if invert levels are

lower than the outlet point, it is best to provide an up-and-over system,

due to the potential for reverse flow.

The location of utility spaces and bathrooms in basements has

been facilitated by readily available pumped drainage systems and

macerators. Consideration should be given to easy access for future

maintenance and replacement.

Structural designCoordination of the structural design with the construction and

waterproofing system is essential. At a domestic scale, the correct masonry

construction to back up Type A barrier protection may be determined

from Approved Document Basement for Dwellings.Reinforcedconcretewalls and basement slabs, especially those used as Type B structurally

integral protection, will require detailed structural calculations. The

calculations take into account the ground, groundwater, the construction

method and the required performance to determine the amounts of

reinforcement required in the sections and specification of the concrete.

Where piling is required, for instance as part of a Type C protection

solution, then a more specialist design will be required and that must be

integrated into the overall structural design.

With respect to Type A protection, simple design i.e. with limited

protrusions and corners will facilitate the installation of waterproofing

membranes. Drainage and granular fill in front of the wall will minimise

build up of hydrostatic pressure. Avoid in-plan inverted corners that face

uphill they can trap groundwater.

Foradviceonthestructuraldesignofbasements,seeApproved Document Basement for Dwellings [9], or, for larger basements, see Design and Construction of Concrete Basements [10].

Ventilation Building regulations require the provision of ventilation to all basements

(heated or unheated) to adequately control moisture vapour, be it

generated internally or brought through from the structure. Cross-

ventilation or passive stack ventilation are the most effective forms of

natural ventilation although continuous mechanical ventilation may be

required depending upon proposed use and internal arrangement of

rooms.

Forspaceswithanticipatedhighlevelsofhumidity,suchasutilityrooms,bathrooms or gyms, mechanical ventilation is essential.

Ventilation should be directly applied to exposed external walls where

possible i.e. not through the basement retaining walls. Stack ventilation

(i.e. ventilation through a vertical vent duct) or mechanical ventilation

which can be the preferred method of providing natural cross-

ventilation, provided it does not compromise the fire compartmentation

strategy of the development. This can be effectively provided by

the staircase linking basement and ground levels, provided no fire

separating doors are required. See the Building Legislation section of this

document for more details.

Passive stack ventor mechanical vent

Ventilationduct(s)

Basement ventilation

Natural ventilation and daylight provided with open two-storey design. Double height basement courtesy of pH+ architects.

The flow of air through a basement using natural

cross-ventilation.

The flow of air through a basement using passive stack or

mechanical ventilation.

Courtesy of TBIC 2004 [8]

19


Analysis of the costs of constructing new domestic basements has been carried out by TBIC in 2005 and updated in 2010 [11]. The study provides approximate construction costs for basements based on a variety of parameters, including flat and sloping sites, full and partial basements, and in-situ concrete and masonry construction. The calculations are based on two-storey detached, semi-detached and terraced houses, with varying widths of frontage.

The schematic design of a two-storey detached dwelling of 129m2 is illustrated below, along with a similar area of house, designed over three storeys, one of which is a basement. The cost model exercise by TBIC concluded that building the three-storey version with a basement fully below ground, only cost an additional 3.8 per cent to construct and is even 0.8 per cent lower if constructed as a partial basement. Offset against the saving in land value through the reduced plot size, or the potential additional return from development of more plots on the same site, the cost exercise illustrates how basements can be a viable option for increasing profitable development, particularly in areas with high land values.

Costs The cost of a basement, and its viability for construction as part of any development, will be determined by a number of factors including, most significantly, land value. Previous examples have illustrated how the inclusion of a partial or full basement can increase the potential floor area of a single dwelling and density of a whole development, thereby yielding higher returns.

Table 5: Extract summary of costs for basement construction as a percentage of construction costs [11].

Groundworks Fitting-out

Fullyfinished 18-34% 29-47%

Empty Shell 27-44% 10-19%

The empty shell specification basement An idea resulting from the cost analysis is that a basement, or semi-basement, completed to a very basic, or empty shell space specification, can yield even greater potential profit margins for housebuilders, whilst offering a reduced cost for the purchaser.

This is an attractive proposition, since the basement offers an economic

and realistic way of creating a large single room, with the flexibility

for future adaptation to suit the lifestyles and requirements of the

occupants.

The concept of providing spaces, or a blank canvas, for residents to

adapt to their specific needs sits well with the concept of design based

on resource efficiency and minimising waste to landfill.

Elemental breakdown of constructionFactorsaffectingthecostofbasementconstructionincludegroundconditions such as excavation costs, type of waterproofing system and

access for construction.

The TBIC cost analysis [11] provides an elemental breakdown of the

range of costs associated with basement construction and summarises

the varying influencing factors including type of basement, construction

type, plan form and sloping site.

Ground works associated with basement construction can amount

to between 18 and 44 per cent of overall construction costs. There

is potential scope for cost savings to be made, such as retention of

excavated soil on site for landscaping, or adoption of an empty shell

specification leading to a reduction in fitting-out costs.

Plot area264.5sqm

129sqm house with habitable basement plus garage at the side

129sqm house without a basement requires approximately 26% more land

3.0m increase in size width

Plot area332.9sqm

11.6

14.6

The schematic design of a two-storey detached dwelling of 129m2.

22.8

20


Building regulationsBuilding regulation approval is required for the construction, adaptation and extension of all basements. Comprehensive guidance on all building regulations related to basement construction is provided by the Approved Document - Basements for Dwellings produced by TBIC.

When planning new basements for housing, particular attention is required to the provision of fire separation between the basement and ground floor, a fire escape from the basement and disabled access and entrance.

Below is a summary of the issues related to basements under two-storey houses with typical floor to ceiling height. Designers should consult the relevant approved documents to check requirements related to their specific design and for other housing types.

Fire resistance and separation

The basement, as defined above, is not counted when assessing the numbers of storeys for fire resistance and means of escape. Typically, for a two-storey house over a basement, 30 minutes fire resistance is required for the structure, increasing to 60 minutes where the number of storeys is four or more. Both requirements are easily exceeded using concrete.

Fireseparationbetweenthebasementandupperstoreysisrequirediftheheight of the top floor is more than 4.5 metres above the lowest external ground level. This situation is only likely to occur in two-storey dwellings if the basement floor level is less than 1.2m below the external ground level, or located on a very sloping site. The 30-minute separation required can be simply and cost effectively achieved using concrete.

The walls and floor between garage and house requires 30 minutes fire separation which also applies if located in a basement.

Ground floor flats or maisonettes with a basement level and direct main entrances require no fire separation over and above typical fire separation between apartments. Since concrete floor construction can easily provide the fire and acoustic separation needed for a separating floor, it can be possible to convert basements into separate dwellings, provided all the necessary fire escapes and ventilation etc. are provided, where such floors are utilised.

Means of escape

Habitable rooms in basements require a safe means of leaving the building. This could be provided by the main stair of the house, provided it is protected and is connected to a final exit. Alternatively, escape can be provided by an additional stair, leading to an alternative final exit. The stair can be internal, but more commonly external. Escape through windows is also permissible if designed to permit escape as defined by the building regulations. The last two options offer cost effective solutions, particularly in terms of optimising usable space, provided the external stair is positioned away from other windows.

It is worth noting that non-habitable rooms, such as kitchens, utility rooms and bathrooms can be classed as inner-rooms and, depending upon the layout, may not require separate means of escape.

It is permissible to exit into gardens or courtyards, provided they have an exit to a place of safety or are at least as long as the height of the house.

Health and SafetyAs with all forms of construction, consideration of health and safety

issues is required at all stages of design and construction. Particular

issues related to the construction of basements depend upon the exact

nature of the work, but may include working in confined spaces, falls

from height, temporary stability and craning of large structural elements.

Planning permission Currently, planning permission is required for the construction and

extension of basements, even when not visible above ground level. At

the time of writing, the extension of a property below ground is not

directly covered by permitted development rights but submissions have

been made to address this apparent anomaly.

A detailed analysis of the role of basements within the planning

guidelines of the UK has been produced by TBIC and is published on their

website as The Hidden Potential. Basements: a planning review document.

Size of development

While planning approval is required for the construction of a basement,

often the size of the proposed construction below ground is less

contentious than an over-ground structure. This is particularly useful

for increasing the proposed floor area of an existing or new property

in areas with strict planning policy controlling the construction

of new buildings, such as a National Park or Conservation area.

In-fill development in urban settings can also benefit from the

accommodation and value added by inclusion of a basement.

Building legislation

NewForestHouse,designedbyPerringArchitectureandDesign. Photographer:NigelRigden.

21


New structure is less than six metres away and lower than a line drawn downwards at 45o from the bottom of the neighbours foundation.

Excavation and construction of foundations and basement walls within three metres of an adjacent building or structure owned by others.

Alowenergyhouse,recentlyconstructedintheNewForestNationalPark was limited above ground to the size of the original existing

single-storey structures on site. Development of the three-bedroom

family home was possible through the construction of a large basement,

containing study area, two double bedrooms, wine storage and plant

area, and a large library and TV room.

Increased density

As described in section Optimising potential development, the inclusion

of a basement level can assist in obtaining planning permission by

raising the density of a development through increasing the number of

homes without reducing the amenity levels.

Flood risk areas

There is a resistance, through planning controls and the insurance

industry, to build houses on areas prone to flooding. The provision of

any habitable rooms in basements in flood risk areas is generally not

supported by planning legislation but can be feasible if addressed

directly.Forexample,theprovisionofanescapestairtoanareaabovethe flood risk level could be an acceptable solution, rendering the

proposed development feasible with basements.

The construction of concrete ground structures or sacrificial basements

is a recognised solution for construction in areas of high flood risk. The

habitable spaces are raised a minimum of 600mm above the level of

design flood risk, while the basement area can provide additional non-

habitable storage space. Concrete is a flood resilient material and the

design and construction of the basement and ground floor can deliver

best practice both in terms of water-entry prevention to the habitable

areas and recovery from the effects of flooding.

FloatingconcretebasementshavebeenpioneeredintheNetherlands,where 48 floating homes have been constructed in Maasbommel on the

banks of the Maas, by Dura Vermeer [2].

Party Wall ActThe Party Wall Act exists to protect the concerns of neighbouring

landowners and to facilitate an agreements between them with regards

to construction works. It will most likely be necessary to issue a Party

Wall Notice, as required by the Act, if a basement is being constructed or

extended. The diagrams below show the summary of criteria for serving

Party Wall Notice.

Less than 6m Less than 3m

Adjoining Owner

Building owners excavation

AdjoiningOwner

Building owners excavation

45o

Summary of criteria for serving Party Wall Notice under the Party Wall Act 1996 [13].

22


Section Benefit potential through basements

Associated credits

The role of the basement

Energy/CO

2

Drying space 1 Potential space to house a permanent fixture for four to six linear metres of drying space, where external

options are not practical or in addition to external options for use during inclement weather. Suitable

ventilation is required to comply with Building Regulations Approved Document F Ventilation and is equivalent

to requirements applying to a bathroom or utility room.

Bicycle storage 2 Space for secure, dry storage of bicycles. Direct access to a public right of way is required, either via stairs at

the front of the house or via the garden.

Home office 1 Ideal space to accommodate the home office requirement of a minimum 1.8m wall length to allow for a

desk, chair and filing cabinet. The office would need a window with an opening casement window of 0.5m2

in order to provide ventilation and have a daylight factor of at least 1.5%. (This provision more than satisfies

the requirement for an alternative means of escape as defined by the building regulations). The inclusion of

a home office in the basement will influence the daylighting factor for the overall dwelling under the Health

& wellbeing section.

Fuelstorage None directly.

Supports potential

up to 2 points.

A basement can also provide storage for biomass materials. While this does not attract points directly, it

supports the use of biomass heaters and combined heating and power (CHP) plants which help score points

under the Energy and Efficiency calculation.

Materials The environmental impact of the ground floor of the basement would be assessed on the elements

contained in the BRE Green Guide Domestic Ground Floor Construction. At the time of writing there is no

Green Guide rating for a ground floor designed as a basement. The CSH is likely to need to make an individual

assessment of the specific construction. The floor at ground level in the dwelling (i.e. between basement

andfirstfloors)wouldbeassessedasanUpperFloorConstructionintheBREGreenGuide.Basement

walls represent the substructure of the dwellings and are currently not considered in the CSH assessment of

environmental impacts. The external walls above the basement would be assessed against the External Wall

ConstructionelementscontainedintheBREGreenGuide.Anyinternalwallsorseparatingwallswouldbe

assessedagainsttherelevantbuildingelementsintheBREGreenGuide.

Surface water run-off

1 The inclusion of sacrificial basements in houses with a medium to high level of flooding risk could support

gaining an additional point. The basement raises the ground above the design flood level, while providing

additional non-habitable storage space below.

Waste Space for waste

storage

4 In order to obtain credits the facilities need to be adjacent to the kitchen and positioned for disabled access.

Health and well-being

Daylighting 3 To maximise the number of points available, this would require all living rooms, dining rooms or studies that

may be located in a basement to also have a daylighting factor of 1.5%. If a kitchen is located in the basement,

this must have a daylight factor of 2%. To gain additional points, these rooms would also require 80% of the

working plane in each room to receive direct light from the sky.

Sound insulation 3 or 4 A basement in a detached house would score maximum points in this area [4]. Basements provide excellent

sound insulation. Where a basement contains separating walls between dwellings, these can be built to existing

RobustDetailsspecificationswheretheappropriateconcrete/masonrywallconstructionwillallowthehighest

score(threecredits)currentlyavailableforadjacentdwellings.FulluseofRobustDetailingcreditsdepends,

however, upon the external wall construction and flanking conditions. Solid external concrete walls can provide

goodacousticinsulation,butatthetimeofwritingarenotincludedasaRobustdetail.

Private space 1 External courtyards at basement level count as private external space.

Ecology Optimise foot print 2 A basement can increase the footprint ratio of the net internal floor area over the net ground floor of most

standard design houses to achieve at least 2.5:1 and often 3:1. The latter allows maximum points to be

scored under the Code.

Use of basements and potential sustainable homes credits

ForfurtherinformationontheCodeforSustainableHomesandhowtouseconcreteandmasonryaspartofthesolution,refertoEnergy and CO2

masonry solutions and Concrete and the Code for Sustainable homes, both available at www.concretecentre.com/publications.

Appendix

Basements and credits scored under Code for Sustainable Homes

23


References1. BRITISHSTANDARDSINSTITUTION.BS6100-1(2004) Building and Civil engineering - Vocabulary General Terms. London, BSI, 2009 pp. 16

2. CLG Approved Document B (fire safety)- Volume 1: Dwelling Houses (2006 Edition). Appendix E Definitions. London, HMSO, 2006 pp. 83

3. NHBCFOUNDATIONRisks in domestic basement construction NF4.NHBCFoundation,Amersham,2007pp.14

4. TRADITIONALHOUSINGBUREAUAttitides towards house construction - MORI survey. 1994 (pp. 25) 1999 (pp. 30) and 2001 (pp. 30)

5. THEBASEMENTINFORMATIONCENTRE:Approved Document: Basements for Dwellings. Section 5 (update pending) TBIC, Blackwater, 2010 (ref TBIC/001)

6. THEBASEMENTINFORMATIONCENTREThermal Performance of houses with basements (BasedontheRegulationsandSAPin-placeatthetimeofthispublication).TBIC,Blackwater,2010.pp.24(Ref:TBIC/005)

7. BRITISHSTANDARDSINSTITUTIONBS8102(2009)Code of practice for the protection of structures against water from the ground pp. 38

8. THEBASEMENTINFORMATIONCENTREApproved Document: Basements for Dwellings,TBIC,Blackwater,2005pp.67-68(RefTBIC/001)

9. THEBASEMENTINFORMATIONCENTREApproved Document: Basements for Dwellings, TBIC,Blackwater,2005pp.67-68(RefTBIC/001)

10. NARAYANANRS&GOODCHILDCH,DesignandConstructionofConcreteBasements,MPA-TheConcreteCentre,due2010

11. THEBASEMENTINFORMATIONCENTRE,CoststudyofHouseswithBasements,TBIC,Blackwater,2010(pending)

12. Innovation and Research Focus Issue 65 May 2006, pp.3

13. CLGThePartyWalletcAct1996:explanatorybooklet02BR008622004pp.18

Further reading BS 8102: Code of practice for the protection of below ground structures against water from the ground, revised and re-issued in 2009, provides guidance on methods of dealing with, and preventing the entry of water from, surrounding ground into a building below-ground level for all below

ground structures.

Basement waterproofing: Design Guide and Basement Waterproofing: Site Guide by the former BCA, offers comprehensive basic guidance on design, use and application of different water-resisting methods and systems. The Design Guide is being revised for issue by TBIC, with support

from The Concrete Centre, 2010.

TheCIRIAGuide:Water-resisting basement construction - a guide safeguarding new and existing basements against water and dampness, (Report139)providesadditionalcomprehensiveguidance,withausefulsummaryprovidedbyReport 140.

Approved Document Basements for dwellings brings into one document all of the relevant building regulations for dwellings that are affected by the inclusion of a basement and is supplemented by Approved Document - Basements for dwellings. Addendum 1 Plain masonry and plain in-situ concrete retaining walls.

British Board of Agrment certificates are available for some water membrane products, which are not covered by the British Standards for asphalt or bituminous felt and for basement tanking systems.

Design and Construction of Concrete Basements will provide comprehensive guidance on the design issues for the design of deep basements, focusing on structural calculations. To be published by MPA - The Concrete Centre in 2010.

IHSBREPress.Good Building Guide 72 , Parts 1 and 2. September 2007 are short publications providing some practical guidance on a range of issues associated with basement design and construction, some replicating information from the Approved Document Basements for Dwelling.

All advice or information from MPA -The Concrete Centre is intended only for use in the UK by those who will evaluate the significance and limitations of its contents and take responsibility for its use and application. No liability (including that for negligence) for any loss resulting from such advice or information is accepted by Mineral Products Association oritssubcontractors,suppliersoradvisors.ReadersshouldnotethatthepublicationsfromMPA-TheConcreteCentrearesubjecttorevisionfromtimetotimeandshouldthereforeensure that they are in possession of the latest version.

Printedonto9Livessilkcomprising55%recycledfibrewith45%ECFvirginfibre.CertifiedbytheForestStewardshipCouncil.

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www.concretecentre.com

The Concrete Centre,

RiversideHouse,4 Meadows Business Park,

Station Approach, Blackwater,

Camberley, Surrey GU17 9AB

Ref.TCC/04/12ISBN 978-1-904818-94-6

Firstpublished2010 MPA - The Concrete Centre 2010

The Concrete Centre is part of the Mineral

Products Association, the trade association for the

aggregates, asphalt, cement, concrete, lime, mortar

and silica sand industries.

www.mineralproducts.org

Concrete Centre Basements for Housing

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