Guidance Roofing

Guidance Roofing

Flat Roof - Types

Introduction

A roof is defined in BS 6229 as a flat roof if it has a pitch of 10 degrees or less.

A flat roof must be strong, durable and stable throughout its lifetime. It must provide

adequate protection against the elements, keeping the buildings structure and

interior dry.

Flat roofs have a reputation of failing early but improvements in strength, flexibility,

ageing and weather resistance mean that, if built with care, using the correct

materials, today’s high performance felts can have a life span of up to 20 years.

Roof Types

Flat roof constructions are generally classed as either ‘Cold’ or ‘Warm’ roofs depending on the position of the thermal insulation.

The Cold Roof

Although not recommended today, and actually banned in Scotland, until recently

cold flat roofs were fairly common.

In a cold roof the thermal insulation is laid between the joists below the structural

deck.

As the insulation is not required to take any loads, quilts and other loose fill materials

can be used as well as rigid insulation.

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Cold Flat Roof Detail, Felt Covering

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Because the structural elements of a cold roof are not protected by from the heat of

the sun by a layer of insulation they are liable to suffer the damaging effects of

thermal movement.

Ventilation is required above the insulation in a cold roof to prevent the build up of

moisture vapour in the roof void.

Foundations

Floors

Warm Deck Roof

In a warm deck roof the insulation is positioned above the structural deck and no

ventilation is required.

Throughout the course of the year the roof deck and all below it is kept at a

temperature close to that of the inside of the building, therefore the roof structure is

protected from extremes of hot and cold, lessening the potential for damage caused

by thermal movement.

Warm Flat Roof Detail, Felt Covering

A warm deck also provides added protection from the dangers of condensation as the

structure is kept warm, at a temperature above dewpoint, by the insulation above it.

Therefore water vapour which enters the roof structure from the room below will not

have a cold surface on which to condense. NHBC recommend that this type of roof be

considered as the standard form of construction.

Types of Warm deck roof

There are two forms of warm deck roof:

Sandwich

Inverted

Warm Deck: Sandwich Roof

The sandwich warm deck roof is the most common type of flat roof. The insulation is

placed below the waterproof covering and is either mechanically fixed or bitumen

bonded on to the top of the deck.

Inverted warm deck roof

The insulation boards in an inverted warm deck are laid over the structural deck and

the waterproof covering. The insulation is secured by a layer of ballast or paving

slabs to prevent wind uplift.

The waterproofing membrane has the added protection of the insulation from foot

traffic and degradation caused by exposure to solar radiation. However, it may be a

more difficult to locate defects in the membrane.

Proprietary systems are available now which combine the insulation and ballast

layers.

Flat Roof - Components

Components of a Flat Roof

A flat roof is a multi-layer construction typically comprising of the following elements:

Waterproof membrane with a protective covering - this should prevent water

reaching the roof structure and the room below.

Thermal insulation – ensures thermal comfort is maintained inside the building and

often provides support for the waterproofing membrane (warm roof).

Roof deck - provides a base for the waterproof membrane or the insulation.

Vapour control layer - helps reduce the risk of condensation.

Load bearing supportive structure - transmits the weight of the roof and any loads

acting on it onto the loadbearing walls. In domestic buildings the structure is usually

in the form of timber joists although it may be concrete or even steel structures can

be used.

Ceiling - usually plasterboard.

Outlets and gutters.

The choice of materials for the different components will depend upon type of roof, whether cold deck, warm deck or inverted (see below). Components must be compatible with each other as some components, for example, may react badly when in contact with other materials.

Roof Covering Waterproofing System

The covering for a flat roof will normally comprise of one or more layers of roofing sheet material.

This material should be of adequate durability and remain weather tight, resisting the action of rain, snow and ice, and preventing any water entering the building.

The designer will need to ensure the roof covering is robust enough to handle all the dead, imposed and wind loads, and UV solar radiation.

There are various types of roof coverings including:

Built–up reinforced bitumen membrane (often named ‘felt’).

Mastic asphalt roofing.

Single-ply roofing systems.

Green roofs (intensive and extensive).

Metal sheet roofing, including zinc, copper and lead.

Priming decks and substrates

In order to achieve a good key, ensure a longer lasting watertight finish or to stabilise and seal porous substrates, primers can be applied before applying the waterproofing covering.

Built up bituminous membranes (felt)

Built up bituminous membranes are probably the most economic and common form of roof covering material for domestic flat roofs. They can be used on timber, metal and concrete decks.

Today’s high performance felts are reinforced with polyester or glass or a mixture of the two, and are coated and impregnated with bitumen.

High performance felts are supplied in rolls and can be laid in two layers bonded together with hot bitumen. The multiple layering reduces the risk of failure as any damage in one layer will usually be covered by the next layer.

High performance felts are often referred to by the additive used in their manufacture:

AAP (atactic –polypropoylene) – normally used for torch-on technique.

SBS (stryrene – butadiene- styrene) - normally used for pour-and-roll application.

Roofing felt should conform to the requirements of BS EN 13707:

Flexible sheets for waterproofing - reinforced bitumen sheets for roof waterproofing.

The most common tyoes of felts are described below:

Type 5

Class 5B or 5U felt can be used for the base or intermediate layer.

Class 5B can only be used for the top layer when provided with surface protection.

Class 5E can be used for the top layer and is already provided with a surface

protection in the form of mineral granules.

Type 3

Type 3 glass-fibre-based felts are less durable and are only suitable for intermediate

layers, but can be used as a base layer when partially bonded.

Type 3G is perforated and can be used as a venting base layer to give a partial bond.

Type 1 traditional organic woven rag based felts have been removed from the British standards and should not be used.

Laying the felt

All felts should be laid in the following manner:

with staggered joints.

with side laps of 50mm minimum and end laps of 75mm minimum.

the sheets should be laid, starting from the bottom, laying progressively up the slope

of the roof so that water will not run into the joints of the laps.

a vapour control layer is required beneath the insulation.

Torch-on membranes

Torch-on bitumen felts are pre-coated with bitumen. The top layer is usually polyester based and the underlay may be polyester reinforced or glass fibre based.

Application

Melt the bitumen by heating on the underside of the membrane from a gas torch.

Roll membrane out on to the substrate where it can form a bond.

The torch-on technique is very reliant on good workmanship and is unsuitable for laying on to timber decks or flammable materials.

Traditional ‘pour and roll’ method of bituminous flat roofing

The pour and roll method is the more traditional way of laying the roofing membrane.

Application

Heat the bitumen to over 200ºC.

Pour the heated bitumen onto the substrate in front of the felt.

Roll the felt onto the hot bitumen.

Full or partial bonding

The felt membrane can either be partially or fully bonded to the substrate. The pour and roll and torch on technique will provide a full bond, this has the advantage of providing a high level of resistance to wind uplift and ensuring no flow path for water is provided under the membrane which could cause moisture to become trapped and blister the membrane.

However, a full bond will not accommodate thermal movement between the membrane and the substrate which could possibly cause the membrane to split or crack.

Therefore, the first waterproofing layer is usually partially bonded. This can be achieved by using one of the following methods:

Providing a perforated underfelt, for example Type 3G, laid loose over the decking or

over rigid insulation boards before applying the bitumen.

Pouring the hot bitumen in a series of strips, before rolling the felt into it.

Mechanically fixing the felt by nailing it to the substrate using 20mm galvanized clout

nails at approximately 150mm centres in both directions. This is the usual method of

fixing when laying the felt on to timber board substrates.

Subsequent layers of felt membrane should always be fully bonded.

Partial bonding will allow the waterproofing layer to be isolated from the substrate so that it accommodates differential movement that might otherwise cause it to split.

However, the reduced contact between membrane and substrate in a partial bond will mean there is a greater potential for wind uplift.



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Cold applied bitumen felt roofing

Cold applied membranes are a safer alternative to using hot bitumen and gas torches, but are only really suitable for small jobs such as small repairs or shed roofs.

Cold applied bitumen membranes usually comprise of fibre or glass fibre and bitumen and are bonded with a suitable cold adhesive of brushable consistency.

Membranes are applied in one, two or three layers on concrete, asphalt, metal and timber decks, although the manufacturer's details should be checked to ensure compatibility. Absorbent or dusty surfaces should first be primed with a suitable proprietary roof primer.

Application

The deck must be free from any dust, dirt, moisture etc and may need to be primed

with special primer.

The adhesive is applied evenly onto the roof surface.

The roofing felt is unrolled onto the adhesive.

All laps are sealed with the cold adhesive.

Pressure contact may be required to ensure full adhesion.

Laps should be a minimum of 75mm and sealed or heat welded.

Joints should be staggered.

Self Adhesive Membrane

Self-adhesive membranes are bituminous compounds with a polyester and/or fiberglass reinforcement with the adhesive already on the underside of the membrane.

Normally laid in two layers, they can be applied without using hot gas torches and can be laid with only basic skills.

Application

The deck must be free from any dust, dirt, moisture etc and may need to be primed

with special felt primer.

The backing paper should be pealed off and the membrane stuck to the prepared

deck.

Pressure contact is required to ensure full adhesion.

Laps should be a minimum of 75mm and sealed or heat welded.

Joints should be staggered.

Self adhesive membranes are ideal for the diy market for application on small roofs, garages, sheds. Self adhesive membranes should not be laid in cold weather.

Single Ply Roofing

A single-ply roofing membrane is suitable for use on timber, metal and concrete decks; although compatibly with other materials should always be checked with the with manufacturer.

Single-ply membranes are mostly used on large commercial buildings. They can be laid in a single sheet and allow more choice in roof design.

Today’s single-ply membranes are strong, flexible and durable and have a typical life span of about 20 years but are often known to last longer.

However, there are no national standards for single-ply membranes and installation should be in strict accordance the manufacturer's details, the British Board of Agreement (BBA) certificate or WIMLAS certificate.

Single-ply membranes are made from synthetic polymers or rubber often reinforced with glass fibre or polyester. They can be categorised as either thermoplastic or elastomeric.


Warm Flat Roof Detail, Single-Ply Membrane

Thermoplastic

Thermoplastic membranes include PVC (polyvinyl chloride). These can soften when heated, allowing them to be hot welded.

However, thermoplastic membranes can be susceptible to damage when in contact with other chemicals including bitumen, therefore compatibitly with other materials should always be checked.

Elastomeric

Elastomeric membranes include Butyl rubber and EPDM. Common in the UK, these membranes are less sensitive to temperature than thermoplastic and provide good weather resistance.

Installation

Single-ply membranes can be laid as one entire piece with the eaves and verge already formed in the factory or in rolls which are laid in strips. The laps can then be joined by heat welding or proprietary solvent to melt and fuse laps together or adhesive tapes, depending on the type of membrane.

Single-ply sheets should always be laid starting from the lowest point of the roof so that rainwater is shed over all lapped joints rather seeping between the sheets.

Insulation manufacturers often recommend an additional 12mm plywood to be provided between the insulation and the membrane.

Fixing to the substrate

Single-ply membranes can be attached to the substrate using any of the following methods;

Mechanical fixing

This is the most common type of fixing.

The membrane is loosely laid and mechanically fixed through the insulation to the

substrate using proprietary metal or plastic screw fastenings.

Single-plys can also be mechanically fixed by heat or chemically welding special

membrane coated discs or strips to the deck.

Adhesive bonding

Adhesive bonding provides a full bond to the insulation or deck. The membrane is

rolled into a proprietary adhesive applied directly to the substrate. A few single-plys

come with the adhesive already bonded to the membrane.

Bitumen bonding

Some single-ply membranes are fully compatible with bitumen and can be rolled

directly into hot bitumen poured onto the substrate.

Loose laid and ballasted

This method of fixing is suitable for warm and inverted roofs.

In a warm roof design, the single-ply membrane is laid loose over the insulation. The

ballast, in the form of round washed stones, paving slabs or soil and planting (green

roof systems), is then laid over the membrane. The weight of the ballast holds the

membrane in place.

A polymeric layer should always be provided between the membrane and the ballast

to protect the membrane from abrasion.

Mastic Asphalt

A mastic asphalt roof gives a seamless covering and can be used on timber, metal and concrete decks. It is not as common as built up felt for small domestic roofs, even though it is known to be more durable and generally performs better as a waterproofing material.

Mastic asphalt is a mixture of asphalt, bitumen and aggregates. Asphalt occurs naturally in asphalt lakes and rock but is more commonly obtained from the process of refining crude oil. Modern mastic asphalt products incorporate polymer which enables an extremely durable and flexible material to be produced.

Mastic asphalt should be laid to the recommendations of BS 8218 Code of Practice for mastic asphalt roofing.

Mastic asphalt is delivered on to the site as solidified blocks and is re-melted before applying to the deck. The number of coats should be appropriate to the waterproofing requirements and anticipated traffic, but will usually be two coats a minimum 20mm thick over an isolating membrane of type 4A black sheathing felt to BS747.

The sheathing felt should be laid loose with 50mm overlaps and will protect the mastic asphalt covering from damage caused by thermal movement of the deck. Insulation manufacturers often specify an additional 13mm fibreboard over the insulation.

The effects of solar radiation on the mastic asphalt can be reduced by rubbing sand into the surface of the topcoat breaking up any build-up of bitumen.

Liquid Applied Coatings

Liquid applied systems are ideal for diy projects and carrying out repairs. They provide a seamless finish with a full bond to substrate and are often a practical option for curved roofs.

Normally, bitumen-based liquid applied systems are emulsified flexible polyurethane systems or synthetic rubber systems. Generic types are defined as ETAs.

Most liquid applied systems are cold-applied, although hot-applied systems are available.

Application

Apply a primer on a clean dry surface.

Brush, roll or spray the liquid applied membrane on to the substrate in three or four

layers.

Some form of reinforcement fabric(scrim) to deal with the tensile stresses, either

glass fibre or polyester fleece, should be laid loose before the second coat is applied

(some types are already fibre-filled and do not require additional reinforcement).

A high standard of workmanship is essential.

No standards exist for this type of system and the manufacturer's guidance should always be followed. Compatibility with insulation, deck etc must be checked. Most types of liquid-applied membranes should not be applied below 5°C.

However, liquid applied membranes do not offer the same durability as some other coverings, with a life expectancy of only around 10 years.

Sheet Metal Roofing

Sheet metal materials commonly used for flat roof covering include aluminium, copper, lead, stainless steel, terne coated stainless steel (TCSS) and zinc.

They are usually in the form of preformed panels with the sheet metal already bonded to a plywood background. The sheets are laid over the supporting structure on a slip layer of polythene to accommodate movement.

Copper is a particularly durable choice of metal but can be expensive. Lead is a more common choice as it is less expensive and can still last 100 years or more.

Sheet membrane should not be laid in temperatures of 5°C or below.

Lead

Lead is specified in codes from Code 3 to 8. The higher the code number, the thicker and more durable the lead will be. Flat roofs will require a minimum of Code 6 lead. Expansion joints known as ‘rolls’ and ‘drips’ must be formed in lead sheeting.

Service Perforations

Air leakage and water ingress should be prevented by sealing around pipes or ducts passing through the roof, for example around soil and vent pipes or roof ventilators. The roofing membrane should be taken up around the pipe or duct and provided with a cover flashing.

Any services through the vapour control layer must also be sealed.

Flat Roof - Surface Protection

Surface Protection

Most flat roofs will need some form of surface protection for the following reasons.

UV rays - The covering of a flat roof will spend many years exposed to ultra violet

light from the sun, this can lead to oxidisation of the roof surface causing the

plasticisers leak out and the surface to become brittle. This can significantly affect

the performance and durability of the roof.

Thermal movement - Heat from the sun can also cause solar radiation, where heat is

transmitted to the roof system causing continual expansion and contraction which

can be very damaging. This is a particular problem for built-up felt and asphalt roofs

as dark membranes absorb more heat.

Differential movement - The temperature difference between the roof covering and

the structure below, in a warm roof, may be high, leading to thermal stresses and

possible splitting of the membrane.

Fire protection - The Building Regulations Approved Document B4 sets out certain

specified performance levels of surface finishes on flat roof to control the spread of

flame.

Surface protection should be light in colour because it will provide good solar reflectiveness and emit absorbed heat more efficiently.

Surface Protection Methods

The various methods of surface protection are detailed below.

Factory Applied Protection

Self finished mineral- surfaced felts are available which limit UV radiation, these

should be of Class 3E or 5E to BS747. Some of these felts also give an AA external

fire rating although AB is more common.

The felts are ready surfaced with a granular material of fine sand, green mineral

aggregate, small crushed slate flakes or granite. Membranes faced with metal

(aluminium, copper, stainless steel) are also available.

However some self finished felts have little reflective quality and do not provide

protection from the damaging effect of temperature rises in the membrane surface or

solar gain.

Single-Ply Membranes

These membranes are generally self finished with good reflectivity and resistance to UV ageing and so do not usually require additional solar protection. However the fire rating of a single ply membrane must be checked with the manufacture.

Solar Reflective Painted Finishes

Proprietary solar reflective paints can be painted on the surface of flat roof to provide solar reflection. This helps protect the roof from the potentially damaging effect of the ultraviolet solar radiation (compatibility with bitumen based materials should be checked).

However these paints are not very durable and reflectiveness deteriorates rapidly, therefore repainting is required every 3 to 5 years. They also do not attain the AA fire rating required for most flat roofs.

Solar Reflective Chippings

Solar protection is often achieved by the application of white reflective chippings,

usually limestone or light coloured spar, not less than 12.5mm. These are very

effective at providing protection from ultra-violet light and reducing the surface

temperature in hot weather. The density of the stones will also slow the rate of heat

gain and heat loss from the building.

The chippings can be laid loose or bonded in hot or cold bitumen compound to the

roof covering.

Chippings can be used provide a ballast layer to help hold down the roof surface in

exposed locations.

However, the chippings can move around or conceal defects in the roof and should

not be walked on as they may puncture the membrane.

Chippings should only be used on roofs at less than 10° as complete adhesion at

steeper pitched is not possible.

Foot Traffic

Where flat roofs are designed for regular access such as a terrace or balcony the surface layer will need additional protection.

The form of protection will depend upon the anticipated usage and the appearance required, but will usually be in the form of slabs or paving tiles, placed on raised supports to allow rainwater to drain away.

The roof structure, the covering and insulation should be designed to support the dead weight of the paving and live loads from foot traffic.

Cold Flat Roof Detail, Felt Covering

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Flat Roof - Deck

Deck

A ‘deck’ is the structural substrate of a flat roof and should be capable of supporting the static and dynamic design loads as well as any loads occurring during construction from machinery and plant.

The deck should be of adequate strength and stiffness to ensure structural integrity and provide suitable support to the roof covering system, the VCL and insulation board (warm roof) without deflection.

The deck must be dry, clean and primed as necessary before laying the waterproofing and insulating system to ensure good adhesion.

The material used for the roof deck must be moisture resistant and will usually be plywood, OSB or timber boards although concrete and woodwool slab or profiled metal can also be used.

Ordinary chipboard is not recommended as it may absorb moisture when damp and lose its structural integrity.

Timber Boarding

Softwood timber boarding is the traditional material for the deck.

The timber used for the deck should be:

of a naturally durable species or preservative-treated in accordance with BS EN 599-

1:2009

a minimum of 19mm thick.

double nailed at each joist.

laid with staggered joints.

laid with the edges of the board corresponding with joists or noggins.

Tongued and grooved boards are preferred over plain-edged to help minimise warping after natural shrinkage, which may cause damage to the waterproof covering.

Plywood

Plywood for a roof deck should be of marine grade and suitable for external use to BS EN 636 or a bond Class 2 or better, complying with BS EN 314 (check).

Plywood boards used for the deck should be:

a minimum thickness of 18mm (these will be able to span up to 600mm).




provided with expansion gaps of 3mm should be provided between the boards.

Oriented Strand Board (OSB)

OSB is a type of wood panel comprising of timber strands and should be manufactured in accordance with BS EN 300: Part 3: 1992.

OSB Boards used for the deck should be:

a minimum thickness of 18mm (these will be able to span up to 600mm).




provided with an expansion gap of 3mm between boards.

Concrete

A concrete deck is more common on blocks of flats and commercial buildings.

Concrete is a strong and steady deck material providing both the loadbearing

supportive structure and roof deck.

Concrete decks are either reinforced cast in situ (poured on site), or supplied as pre-

cast concrete units.

The surface of a concrete deck should be free from ridges and hollows. A screed of

sand and cement is usually provided over the surface to get a smooth base for the

insulating or waterproofing layer above and to create the desired fall. The fall can

also be produced by proprietary decking or insulation products which are produced

ready-cut to a suitable fall.

Concrete decks should only be covered by non-bonded or partially bonded roof

coverings.

Wood wool Slab

Woodwool slabs are a rigid timber and cement based panels, which can be reinforced

if long spans are required.

Woodwool slabs should be no less than 50mm thick and conform to BS EN 13168.

Profiled Metal Decking

This type of deck is usually formed in galvanised steel or aluminium.

The lightweight construction has the advantage of being able to cope with very long

spans, enabling large room spaces to be created and reducing the load on the

building's structure.

Profiled metal decking does not provide a continuous surface and therefore can only

be used when supporting a timber deck or rigid insulation which can then be covered

with a bitumous or polymeric membrane.

This type of decking is not suitable for cold roof design unless it is subsequently

overlaid with plywood or OSB.

Flat Roof - Insulation

Insulation

Insulation is required to minimise heat losses in the winter and control excessive solar gains in the summer. This will help to conserve energy and ensure thermal comfort is maintained within the building.

Insulation can also help to limit condensation and contribute to acoustic insulation (depending on the type of insulation used).

U value of a flat roof

To comply with Part L of the building regulations 2010, all roofs to rooms (except

those in unheated buildings) should be thermally insulated to a maximum U-value of

of 0.18 W/m²K.

A lower U-value indicates better insulation properties, hence U-value requirements

usually specify a maximum value.

U-values calculations take into account the thermal resistivity and thickness of the

insulation.

Thermal resistance is the rate of heat transmission through a unit area for each

degree of temperature difference from inside to outside the building.

When calculating the U-value of a flat roof, screeds, timber, air spaces, plasterboard

etc contribute to the thermal performance and should all be considered.

Insulation Materials

The choice of a insulation will be usually dictated by the roof design. Some of these are fibrous materials which derive their performance from air trapped between fibres. Several different insulation materials are available including:

Synthetic foam plastics

Natural materials (e.g. cork)

Inorganic materials (e.g. glass fibre. cellular)

Mineral wool

Loose fill

Quilt

Cellular glass CG. Cellular thermal insulation materials are composed of materials of

polymeric and mineral origin.

Polymeric materials

Polyurethane PUR

Polyisocyanurate PIR

Rigid urethane foam (PUR/PIR) RUF

Phenolic foam PF

Polystyrene - expanded EPS

Polystyrene - expanded - extruded XPS

About Materials

Some manufacturers of PVC waterproofing membranes recommend that a

separation/protection fleece is installed on top of the membrane before the insulation

board is laid in place.

Where a roof is to be finished in gravel, a water permeable filler fabric can be laid on

top to protect the membrane from any fines which may be washed down below the

insulation.

If the waterproofing is asphalt or BUR felt, depending on the size of the gravel being

used, it may not be necessary to use a filler fabric.

Rigid board cellular material which derives its performance from the thermal

resistance of gases trapped in the cell structure and from the thermal resistance of

the cell walls.

Insulation in a warm roof

The insulation for a warm roof will usually be boards of extruded polystyrene or rigid compressed boards of glass fibre or rock fibre. These are can be supplied with rebated edges which interlock reducing the risk of uplift.

The insulation material for a warm flat roof should:

be able to cope with extremes of temperature and UV light without deterioration in

thermal performance.

be able to resist pressure from any anticipated foot traffic.

be durable and suitable for external use.

be able to provide support for, or protection of, the weatherproofing membrane.

be able to resist moisture. This is particularly important for inverted roofs where the

insulation is directly exposed to the rain.

be capable of withstanding loads imposed on it during construction and future

maintenance.

An inverted warm roof must also:

be capable of receiving the extra load of the ballast.

An overlay between the membrane and the insulation will protect the insulation from heat when laying hot bitumen and provide a slip layer to account for any thermal movement.

Tapered Insulation

Tapered insulation boards are designed and pre-formed to provide the appropriate drainage falls.

Composite

Composite board can be found on both warm and inverted roof applications.

These often have a deck of plywood with a cellular insulation and metal foil vapour control layer bonded to the underside and a top layer with an overlay factory-bonded to it.

Bonding

The insulation may be fully or partially bonded to the vapour control layer or laid

loose.

If the insulation has a sufficiently high vapour resistance, a separate vapour control

layer may not be required and the insulation can be bonded directly to the deck.

However, the insulation must be compatible with the bonding materials used.

Where insulation is mechanically fixed, the fixings must be of sufficient length to

ensure they can be secured through to the deck into the structure.

Avoiding Thermal Bridges

Thermal bridging occurs where the continuity of the insulation has been broken.

This can lead to condensation, mould growth or staining.

Cold bridging is likely to occur:

At Junctions with a wall.

Where pipes penetrate the roof.

Flat Roof - Drainage

Drainage

A correctly build flat roof will drain the rainwater quickly and effectively into the gutters without allowing water to collect in depressions and ‘pond’ on the surface of the roof leading to the build-up of silt deposits on the roof and stresses in the membrane when the water freezes (a small amount of ponding is evitable and it should not reduce the performance of the roof covering).

Ponding will be reduced by ensuring the roof is provided with a decent fall allowing the rainwater to be drained effectively towards the outlets and gutters at the edge of the roof.

Design Falls

When specifying a fall for a flat roof, the designer should take account of:

Any potential deflection of the structural members and decking under dead and

imposed loads (a particular problem with timber and metal deck roofs).

Possible inaccuracies in construction.

Type of weatherproofing material used.

Because the desired fall may be difficult to achieve in practice, the designer may need to adopt a design fall considerably steeper (often twice) than the recommended finished fall.

The building regulations encourage a minimum fall of 1:40.

Minimum finished fall required according to BS6229 is 1:80.

Recommendations for specific materials are as follows:

Aluminium 1:60

Copper 1:60

Zinc 1:60

Lead sheet 1:80

Built-up bitumen sheet 1:80

Mastic asphalt 1:80

Single ply membranes 1:80

Liquid waterproofing systems (hot- or cold-applied) 1:80

Green roofs should have a fall of not less than 1:60 and be built in accordance with

manufacturer’s details and British Board of Agrément certification.

The desired fall may be created in one of the following ways:

Sloping decks

The fall of the roof may be created in the structure itself by laying the supporting

beams, or joists at a slope (giving a sloping soffit) or by installing tapered beams with

horizontal soffits.

Firrings

It is normal practice for the joists of a flat roof to be set level thus creating a perfectly

level ceiling. The required fall is then formed using strips of tapered shaped timbers

known as firrings fixed along the tops of the joists before the deck is laid. These

firings should be the same width and length of the joists. The firrings may also

provide the fall by each one decreasing in thickness along the slope of the roof.

The firrings can be fixed at right angles to the joists instead of along the length of the

joists, this will provide a better level of ventilation. However, when fixing in this

manner it is essential that the firrings are of suitable structural strength to span

between joists and should not be less than the following sizes.

Size of FirringsDistance between Joists Width of Firrings Depth of Firrings

400 / 450 mm 38 mm 38 mm600 mm 38 mm 50 mm

Pre-formed insulation boards

Proprietary preformed tapered insulation boards can provide drainage falls to a warm

decked roof (manufacturers recommend falls not less than 1:60). The boards should

be laid onto a vapour control layer and be covered with the waterproofing system.

Concrete and Screeds

Falls on a flat roof with a concrete deck are either provided within the structural

concrete itself or created within the screed which is laid over the concrete deck (as

for in-situ cast concrete slab).

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Foundations

Guttering

Gutters and downpipes should be adequately sized to deal with storm conditions in

accordance with BS EN 12056-3. Flat roofs should be designed to drain the roof to

one or two edges towards gutters and outlets.

There should be a smooth transition into the gutters which can often be lined using

the roofing membrane to achieve a completely uniform finish (check the

manufacturer's details).

Internal gutters can be used but conventional eaves gutters are preferable.

If unavoidable, internal outlets should be fitted with leaf and gravel guards.

Welted drips should extend to the middle of the rainwater gutter, and there should

be a minimum of 50mm turndown.

To ensure the rainwater is directed towards the gutters, the sides of the roof surface

(apart from the gutter edge) require a minimum of 50mm upstand verge provided

using triangular pieces of timber called ‘tilt fillets’ nailed to the edges of the decking,

not to the wall.

All gutters and downpipes should be accessible for maintenance.

Weathering Details

Upstands

At the point where the roof deck meets the external wall or at parapets, careful

detailing will be required to prevent water ingress.

To avoid a sharp bend where the membrane is folded up the wall, which could cause

a split, a small strip of timber ‘angle fillet’ a minimum 75mm x 75mm is fitted under

the felt and fixed firmly to the deck.

The waterproofing membrane must be taken up the wall at least 150mm (providing

an upstand) above the roofing surface.

Flashing

The membrane upstand should be protected by a cover flashing to give a good

watertight joint.

Lead should be cut into the horizontal mortar joint and overlap the top edge of

roofing material not less than 50mm, finishing no closer than 75mm above the roof

surface.

Cavity Tray

If the wall is a cavity wall, a cavity tray must be fitted, stepping downwards at least

150mm and weep holes should be provided.

Flat Roof - Structure

Loading

Flat roofs shall be designed to carry all imposed and dead loads acting on the structure of the roof. These may include:

Dead loads- from the self weight of the roof construction.

Environmental loads - such as snow, water and wind loads should be calculated in

accordance with BS EN 1991.

Plant and equipment placed on it during its construction.

Traffic loads from roof maintenance equipment and personnel.

Roof terraces and gardens must be able to carry the additional loads expected from foot traffic, planting and furniture etc.

Uplift

Flat roofs should be constructed to provide protection from wind uplift this can be achieved by either

being sufficient weight to prevent lifting or

by fixing the joists to the wall plates and securing the wall plate with vertical holding

down straps, at least 1m long and 30mm x 2.5mm in cross section at 2.0m centres.

Wall plates are to be fixed to masonery using hardened nails 4mm in diameter x

75mm long or 50mm long wood screws if fixed into timber.

The roof covering membrane may also require additional fixing using mechanical fasteners to prevent uplift high wind load areas.

Timber Roof joists

All roof timbers including joists, wall plates, blocking, strutting, battens, firings and noggings must be to be preservative treated unless the timber used is naturally durable.

Sizing Joists

The joists should be sized using either the TRADA span tables or BS EN 1995.

The sizing and spacing will depend upon the loads imposed on them and the required

span.

Wherever possible joists should span the shortest distance.

Common joists sizes are 200 x 50mm, 175 x 50mm, and 150 x 50mm.

The joists will normally be placed at 400mm centres but no more than 600mm

centres.

Packing

Hard packing, for example tiles or slates bedded in mortar, can be use to ensure the

joists are level.

Loose or soft packing including timber should not be used.

Fixing Joists

The joists can be built into the wall or hung from joist hangers.

The joist hangers must be the correct size for the joists and conform to BS EN 845.

The joists should be placed on and fixed to timber a wall plate which is bedded in

mortar on top of the inner skin and strapped down using galvanised holding-down

straps.

Fixing joists to an existing wall

Where a new flat roof abuts an existing external wall, the joists can be hung on metal

joist hangers which are fixed to a horizontal timber plate wall bolted to the wall.

A cavity closer should be provided to the top of the walls to prevent fire spread and

to stop damp air entering the roof space.

Lateral Restraint

Lateral restraint should be provided to walls by:

ensuring concrete elements and timber joists where they are built in have a

minimum bearing of 90mm.

proving restraint straps at 2m centres where joists run parallel to walls.

Strutting

Strutting is required in order to prevent twisting of the joists at the following points:

one row when the span of the joists exceeds 2.5m.

two rows or more when the span exceeds 4.5m.

Strutting should be either:

herringbone type (timber 38mm x 38mm).

solid blocking (38mm thick timber x ¾ depth of joist).

or

proprietary steel strutting.

Any strutting provided should not obstruct the cross ventilation. required to cold deck roofs.

Flat Roof - Condensation

Introduction

Condensation within a flat roof mainly occurs during cold weather when moisture vapour in the air which has been generated within the heated building rises from the room below into the cold roof void above the ceiling.

When the temperature of the vapour falls to or below its dew point the water vapour condenses on cold surfaces.

The warmer than air the more water vapour it can contain and the higher the moisture content in the air (relative humidity; RH) the lower the dew point temperature will be.

Condensation is a particular problem in roofs above rooms which generate a lot of moisture such as kitchens and bathrooms.

A flat roof should be designed to minimise condensation and a condensation risk analysis should be undertaken taking into account positioning of insulating materials, vapour control layers, ventilation, thermal insulation and the choice of materials. This can be calculated using computer programmes.

Surface Condensation

Another type of condensation is surface condensation which is visible on surfaces within the building and occurs when the temperature of the surface is at or below the dew point of the moist air.

This type of condensation is often identifiable by black mould on the walls, windows, ceilings etc.

Interstitial Condensation

Condensation which occurs within the roof structure is called interstitial condensation. It is particularly dangerous because it can cause unseen decay in roof timbers and fixings.

Condensation in a cold roof

Interstitial condensation is a particular problem in cold deck roofs where the insulation is placed in-between the joists in the void above the ceiling.

The position of the insulation means that the roof deck and most of its structure has no protection from low temperatures during the winter.

These elements then become much colder than the interior of building, and moisture vapour which has made its way up from the room below is then liable to condense on the timber structure possibly leading to decay.

Cold deck roofs are not recommended for new work, and actually banned in Scotland.

Cross-Ventilation

To help disperse the moisture vapour, the building regulations require cross ventilation to be provided in the form of a 50mm air gap between the deck and the insulation and continuous gap of about 25mm at the eaves.

This can be difficult to achieve where roofs abut external walls and propriety mushroom vents to provided the equivalent 25mm continuous ventilation are available.

Vapour control layer for a cold roof

However the cross ventilation does not completely remove the moisture vapour in the ceiling void and a vapour control layer sealed at joints and penetrations is required under the insulation and over the plasterboard to provide a barrier against moisture vapour rising up from the room below.

Alternatively metalized polyester lined plasterboard can be used.

Vapour control layer for a warm roof

A vapour layer should be positioned under the insulation or, in some situations, immediately below the roof covering to minimise water vapour condensing beneath the membrane.

Vapour control layer may be formed using any of the following:

A polythene sheet membrane loose laid and restrained by mechanical fasteners or

nailed to the deck (timber decks) all laps should be sealed with an appropriate

adhesive.

A reinforced bitumen sheet. - one layer of BS 747 Type 5U felt, fully bonded or

mechanically nailed to the deck.

Two layers of Type 5U felt fully bonded in hot bitumen. check - all laps must be

sealed with bitumen.

For single-ply membranes, the VCL should be either polythene or reinforced

aluminium foil.

Designers often specify readymade ‘composite’ decking that combines together plywood, insulation, vapour control layer and felt covering into one product.

Flat Roof - Green Roofs

Green Roofs

Green roofs offer a sustainable and ecologically friendly solution, whereby plants and vegetation can live on the deck of the roof.

The plants on a green roof contribute to improving air quality by producing oxygen and absorbing air pollutants, dust and CO2.

The thermal mass of a green roof provides a good level of thermal performance providing passive heat storage, as well as significantly reduce cooling requirements in the summer.

Green roofs contribute to suds (sustainable urban drainage).

Rainwater is stored in the substrate and vegetation reducing the volume of rainwater runoff from the roof which provides a more natural drainage process for storm water than a traditional flat roof.

There are two main types of green roof:

Extensive

Supplied as a complete system this type of roof can be built using a only 50mm soil

base and lightweight construction.

However vegetation for this type of roof is limited to sedum, grasses, and mosses

contained within a sedum blanket.

This type of green roof requires minimal maintenance is required, although an

irrigation system may be required for use in dry weather feeding with a slow release

fertilizer a biodiverse roof is the soil seeds naturally and no watering is required.

Intensive (roof garden)

Intensive roofs may contain a variety of plants including shrubs and trees, as well as

garden furniture and even small ponds.

Intensive roofs can only used on roof with a maximum pitch of 20o.

They require a soil base at least 150mm deep and a substantial steel or concrete

deck is needed to support the weight of the soil and plants.

Regular intensive maintenance is required for this type of roof.

Construction

The majority of green roofs both intensive and extensive are build-from the following elements:

Vapour control layer - placed above the roof structure.

Rigid slab insulation – must be strong enough to withstand the additional loads.

Where the insulation is above the weatherproofing, only extruded polystryrene (XPS)

should be used.

Waterproof root barrier - for an intensive roof this should be reinforced bitumen

membrane (RBM) or mastic asphalt.

This will prevent roots penetrating the roof membrane. Suitable materials include,

rubber mats, bitumen, slate-surfaced layers or polyethylene.

Drainage layer- controls the rate of rainwater runoff.

Filter layer - prevents the soils etc from blocking the drainage layer or causing

damage – includes foams, mineral wool, plastic sheets and granular drainage.

Growing medium (soil) - the depth of the growing layer will depend on the type of

roof. For an extensive roof the growing medium could be just a 20mm geotexile

mats. However soil 400mm deep is typical for an intensive roof.

Vegetation – this is the planted layer which could be sedum, mosses and grasses for

an extensive roof or shrubs and trees for an intensive roof.

Green roof systems should be installed by a specialist installer to drainage falls of 1:60 min.

Pitched Roofs - Overview

Introduction

The function of a roof is to protect the building below from the weather. In order to satisfactorily fulfil this function over a period of years it must be strong, stable and durable.

In addition, roofs must provide good thermal insulation and prevent the spread of fire from adjacent or adjoining properties.


Pitched Roof Detail, Ceiling-Level Insulation

The majority of houses in this country are constructed with pitched roofs.

The angle of the pitch may be dictated by aesthetic or structural factors.

It may also be influenced by the nature of the roof covering.

Modern tiles permit shallow pitch but some of the older traditional coverings, such as

hand made clay tiles, require quite steep slopes to ensure rain does not penetrate

the roof covering.

Shallow pitches are generally cheaper to construct with savings in both timber and

tiling.

In traditional pitched roofs, the rafters are supported by a wall plate at the bottom

and a ridge board at the top.

Intermediate support is supplied by a horizontal purlin.

Most modern roofs for new houses are built from prefabricated components erected

on site.

Lean-to Roofs

The simplest of pitched roofs is the Lean-to roof, commonly found forming the rear extensions to terraced housing.

The sloping timbers are known as rafters and are supported at the top by building

them into the solid wall (not recommended nowadays) and at the bottom by securing

to a wall plate.

The wall plate is a strip of timber which is bedded in mortar on top of the wall, and

which evenly distributes the load from the roof and provides a good fixing for the

rafters.

Note: Mortar cannot actually bond the timber to the wall.

The wall plate should be 100mm x 75mm 0r 100mm x 50mm.

Ceiling joists are often built into the wall or supported on a wall plate bedded within

the wall (not recommended).

The rafters are skew nailed to the plate

The depth of the rafter depends on its span and loading, and the width is primarily to

prevent twisting and to provide a sufficiently wide surface on which to nail the

battens supporting the tiles

It is good practice to notch the bottom of the rafter where it sits on the plate as this

gives a good bearing and aids alignment of the rafters.

The rafters are usually spaced at 400mm centres.

In most houses, the guttering is supported by the facia board. This is fixed to the feet

of the rafters and it can be flush against the wall or it can form an overhang.

Lean-to roofs can achieve only modest spans.

Drainage - Foul Water

Foul Water Drainage

Foul water comprises of ‘black’ soil water from toilets and ‘grey’ waste water from baths, basins, sinks, washing machines etc.

A foul water drainage system carries foul water from the building to an underground sewer pipe, a cesspool, a septic tank or a wastewater treatment system. (Public sewers are not

considered in this guidance and consultation with the Local water authority should be sought when building near or over a public sewer – see below.)

The Building Regulations 2010 require that the foul water should be discharged ‘in order of priority’, therefore where it is reasonably practicable to do so, a connection should be made:


Drainage Pipe Through Wall Lintel

1) to a public sewer.

2) to a private sewer communicating with a public sewer.

3) to a septic tank or other wastewater treatment plant.

4) to a cesspool.

The first solution on the list should preferably be adopted, where this is not possible move down the list to the next.

Combined and Separate systems of drainage

Underground drainage systems can be divided into two types :

1) Combined system - In a combined system the foul water and the rainwater are

carried in the same underground pipes. The advantages of this system are firstly that

the rainwater helps to clean the pipework minimising blockages, and secondly less

pipe work is required, although the pipe sizes may need to be increased.

2) Separate system - In a separate drainage system separate sewer pipes are

provided for the foul water and the rainwater.

The water authorities discourage combined systems because:

1) They can cause an overload of the drainage system.

2) They produce an increased amount of water to treat. New developments are often

required to provide a separate system of drainage up to the point of connection with

a combined sewer.

Pipe materials

In the past, drainage pipes have been made from a variety materials, including pitch fibre, vitrified clay, uPVC, concrete, glassfibre reinforced plastics (GRP), iron and asbestos cement.

Not all of the materials used have performed well, for example, pitch fibre has a tendency to collapse under heavy clay soils and expensive repair work may be required.

Today, most new domestic drainage is constructed using plastic (flexible) pipes or clay (rigid) pipes, usually 100mm and 110mm in diameter, or 150mm and 225mm in diameter for public sewer pipes.

Types of pipes

Socketed clayware.

Plain-end clay c/w coupling.

Plastic 100/150mm.


Drainage Pipe Through Wall

Clay

Clay drainage pipes date back to the Victorian times, their inherent strength means they are more durable and less likely to deform under loads than plastic. They are also:

Highly frost resistant.

Rodent resistant.

Able to be laid directly into a well trimmed trench (Class D bedding).

Clay pipe manufacturers promote their environmental advantages claiming clay uses less energy in production as well as requiring less aggregates for bedding and backfilling.

Plastic

Typically golden brown in colour, 110mm plastic pipes are the most common material used for underground drainage today.

The advantage of plastic pipework is that it is:

Relatively cheap compared to clay.

Frost resistant.

Simple to cut to size with a hacksaw and lightweight, making the pipes easy to

handle and work with.

Flexible and can remain watertight and resist fracture when subject to small amounts

of movement.

However, excessive pressure from loads or ground movement may cause plastic pipes to deform. Therefore they must be surrounded by a good bedding material such pea-shingle to provide support and prevent the pipe from cracking.

http://www.buildingregs4plans.co.uk/guidance_flat_roof_types.php

http://www.buildingregs4plans.co.uk/guidance_flat_roof_types.php