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

Functional Requirements (cont’d)

Structural Stability• Provide 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 will cause bending

• 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. Roof represents a large proportion of the external surface area of a building has 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 (drain/sewer).• Roof height & area increase with its horizontal dimension

This type of roof depends on:-

•Area covered•Materials available•Type of lighting•Ventilation needed inside•Available appliance

Type of roof Description

Shed roof Simplest type and slopes only in 1 direction.Use for smaller span

Gable roof Slopes in 2 directions, commonly used

Hip roof Slopes in 4 directions

Gambrel roof Slopes in 2 directions, but there is a break in slope

Mansard roof Slopes in 4 directions, but there is a break in slope

Saw-tooth / north light roof Used in factories where light is admitted from the glazing fixed on the steep sloping sides of the roof.

Shed

Mansard

Gambrel Hip

Saw tooth

Gable

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

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

SLOPING ROOF (cont’d)

FLAT ROOFS• Pitch from 0 to 10 degree• 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

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- is the craft of building a roof with dry vegetation such as straw, water reed, sedge, rushes, or heather. Now become choice for rustic

look and more ecologically friendly roof.

2. Wood- thin, tapered pieces of wood primarily used to cover roofs and walls of buildings to protect them from the weather

3. Shingles- roof covering consisting of individual overlapping elements. Can be made of various materials such as wood, composite or ceramic. Thatch

Wood

Tiles

4. Tiles-made of a ceramic material and is hard and brittle, poorly suited for places where tree limbs can fall on a house's roof.

5. Slates - is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash

6. Asphalt – known as bitumen, viscous liquid or semi solid form of petroleum.

7. Asbestos cement sheets-known as fibre cement sheet. It is a building material in which asbestos (silicate mineral) fibres are used to reinforce thin rigid cement sheets.

8. Corrugated iron sheets-The corrugations increase the bending strength of the sheet. Normally each sheet is manufactured longer in its strong direction.

9. Bituminous felt-bitumen mixed with a filler component such as limestone or sand or polymers to gives rigidity and tear resistance.

10. Sheet metal roof coverings (copper, zink, etc.)-roofing system made from metal piece, or tiles.

Asbestos

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