ROOFS. ROOF GENERAL FUNCTION: To give a protective covering to the building, so rain, wind or snow may not damage the building Functional requirements.

ROOFS

ROOF

GENERAL FUNCTION:• To give a protective covering to the building, so

rain, wind or snow may not damage the building

Functional requirements

1. Weather resistance2. Structural stability

3. Provide good appearance4. Provide thermal insulation

Functional Requirements (cont’d)

Weather Resistance1. Required to protect a building from the damaging effects of rain & wind

2. The ingress of rain: causes damage to decorations & structural damage to timber due to attacks by fungus & insects

3. The roof is covered by a layer of impermeable material which prevents the passage of rain into a building

4. The deflection of building may be cause of wind: accurance of suction on the windward of slope of roof

Functional Requirements (cont’d)

Structural Stability• Provide a means of support for the roof covering

• In designing a roof structure, the designer must take into consideration the span of structural members

• If span at joist too great – bend

• To overcome : introducing a structural element which reduces the span

Functional Requirements (cont’d)

Provide Good Appearance1. Might be a major visual element in the design of a building

2. The detailing of roof can significantly affect the appearance of a building

Functional Requirements (cont’d)

Provide Thermal Insulation

1. The roof constitutes a substantial proportion of the external surface area of a building & consequently has considerable potential for heat loss

2. Pitched roofs are usually insulated between the ceiling joists

3. Alternative insulation might be applied between rafters

4. Flat roof insulation is usually laid above the decking, either below or above the roof covering

•General types of roofs:

SlopingFlatsShell

Domes

SLOPING ROOF / PITCHED ROOF• Sloping roof may be categorized into: Medium to high slope – 4:12 to 12:12 Low slope - up to 3:12

• Sloping roofs shed water easily to eave gutters• Roof height & area increase with its horizontal dimension

SLOPING ROOF (cont’d)• Sloping roofs may have a structure of:1. Rafters & sheathing2. Beams, purlins & decking3. Trusses

SLOPING ROOF /PITCHED ROOF

• Medium & high slope roofs may be covered with shingles, tiles or sheet materials

• Low slope roofs require roll or continuous membrane roofing; some shingle & sheet materials may be used on 3:12 pitches

• The roof slope also affects the design loads & the requirements for underlayment & eave flashing

• Slope roof planes may be combined to form a variety of roof forms

FLAT ROOFS

• Slope may be achieved by sloping the structural members or by tapering the deck or rigid insulation

• Slope usually leads to interior drains; perimeter scuppers can be used as overflow drains

•Pitch from 0 to 10 degree

FLAT ROOFS (cont’d)• Roof structure may consist of:-1. Joist & sheathing2. Beams & decking3. Flat trusses4. Concrete slabs• Flat roofs require continuous membrane roofing

Deck

Shell Roof• A thin shell is defined as a shell with a thickness which is small compared to

its other dimensions and in which deformations are not large compared to thickness

• These elements are typically curved and are assembled to large structures. Example: factories and roof structures in some buildings.

• Advantage in construction : a lot of material is saved as the section needed is very thin.

Domes • A roof of semi-spherical or semi elliptical shape

• Constructed of stone or brick or concrete, & supported on circular or regular polygon shaped walls

• The structure is such that within certain height & diameter ratios, very small thickness is needed

Common Term UsedTerm Description

Ridge piece A member which runs horizontally at the highest level of the roof

Hip External angle of a sloping roof at which the roof slopes are turned down

Common rafter Members supporting the battens or boardings under the roof covering

Hip rafter Members which form the hip of the roof & to which the common rafters are attached

Ridge

Common Term Used (cont’d)

Ridge Apex line of a sloping roof

Eaves The lower edges of the sloping surface of a roof

Eave-board Wooden board fixed along the eaves connecting the common rafters

Gable Wall which follows the slope of the roof from eave to ridge & covers the end of a roof

Barge board

Wooden planks used to fix the ends of common rafters projecting beyond the sloping top of a gable wall

Common Term Used (cont’d)Valley A depression formed at the intersection of two sloping roofs at their

junction

Jack rafter Short lengths of rafters fixed at the hip rafters & eaves board

Valley rafter Member at the intersection of 2 inverted slopes to which rafters are fixed

Dragon beam Short member of wood which carries the foot of a hip rafter which is attached to the tie fixed across the angle formed by the intersection of 2 wall plates at the corner of a building

Purlins Members that laid horizontally to support the common rafters. They transmit the loads to the trusses or walls

Common Term Used (cont’d)

Purlin cleat Short piece of timber bolted to the rafters of roof truss for fixing the purlins

Truss A framework of triangles

ROOF COVERINGS• Is a materials which gives a protective surface to the roofing

structure• The function is only to prevent ingress or egress of heat & moisture

into the building• Various types of coverings depending on The character of the building The type of the roofing structure Local conditions, cost, etc.

Types of Roof Coverings1. Thatch

2. Wood

3. Shingles

4. Tiles

5. Slates

6. Asphalt

7. Asbestos cement sheets

8. Corrugated iron sheets

9. Bituminous felt

10. Glass

11. Sheet metal roof coverings (copper, zink, etc.)

Tiles

Thatch

Asbestos

Wood

ROOF DESIGN & CONSTRUCTION FACTORS

• Roof structure must be designed to carry: Dead loads – roof structure, deck, insulation, roofing & any

equipment located on top of or suspended from roof Live loads – accumulated rain, snow & ice & traffic if any. Wind loads – pressure or suction from wind

• Type of roofing required against: Surface water from the outside Water vapor diffusion from the inside Moist air flow Heat flow & solar radiation