Last reviewed 16 Sept 2026 · 10 min read
Roofs
A roof is the uppermost part of a building that covers and protects it from weather.
Requirements
- Strength and stability to carry dead, live, wind, snow and seismic loads.
- Weather protection — watertight, efficient drainage of rainwater.
- Thermal insulation and sound insulation.
- Fire resistance and durability.
- Lightweight where possible; economical; good appearance; low maintenance.
Classification of roofs
| Type | Slope | Examples |
|---|---|---|
| Flat roofs | Nearly level — small slopes (commonly up to about 10°) for drainage | RCC slabs, jack arch, filler slabs, precast slabs |
| Pitched (sloping) roofs | Slope greater than about 10° | Timber roofs, steel trusses with sheets or tiles |
| Curved roofs | Shells, vaults, domes | Barrel vaults, domes, hyperbolic paraboloid shells, folded plates |
Pitched roof terms
| Term | Meaning |
|---|---|
| Span | Horizontal distance between the internal faces of supporting walls (or supports) |
| Rise | Vertical distance between the top of the wall plate and the ridge |
| Pitch | Rise ÷ span (e.g. ¼ pitch: rise = span/4); slope angle for a symmetric roof |
| Ridge | Apex line where two opposite slopes meet |
| Eaves | Lower edge of the sloping roof projecting beyond the wall |
| Hip | Sloping ridge formed by the intersection of two slopes at an external (salient) angle |
| Valley | Line of intersection of two slopes at an internal (re-entrant) angle — collects water |
| Gable | Triangular upper part of an end wall of a pitched roof |
| Verge | Edge of a pitched roof at the gable end |
| Wall plate | Timber member laid on the wall top to receive rafters and distribute loads |
| Common rafters | Inclined members from wall plate to ridge carrying battens/covering |
| Principal rafters | Inclined members of a truss |
| Hip rafter / valley rafter | Rafters along hips and valleys |
| Jack rafters | Short rafters meeting hip or valley rafters |
| Purlins | Horizontal members spanning between trusses or walls, supporting common rafters or sheeting |
| Battens | Thin strips on rafters to support tiles or slates |
| Ridge board / ridge piece | Board at the ridge against which rafters are fixed |
| Cleats | Short blocks fixed to principal rafters to prevent purlins slipping |
| Barge board | Inclined board fixed along the verge of a gable |
| Fascia board | Board fixed to the ends of rafters along the eaves (supports eaves gutter) |
| Eaves board / soffit | Boards closing the underside of the eaves projection |
| Lean-to roof | Single slope roof against a wall |
| Hipped roof | Slopes on all sides (hips at corners) |
Timber roofs
Single roofs (common rafters only — small spans)
| Roof | Description / typical span range |
|---|---|
| Lean-to (pent) roof | Rafters rest on a wall plate at one end and on a wall/corbels at the higher end — verandahs, sheds (small spans) |
| Couple roof | Pairs of rafters meeting at a ridge and resting on wall plates — tends to spread walls outward; very small spans |
| Couple-close roof | Couple roof with a tie connecting the feet of rafters to prevent spreading |
| Collar beam roof | Tie (collar) placed higher up between rafters — more headroom; moderate spans |
Double (purlin) roofs
Common rafters supported at intermediate points by purlins, which in turn rest on walls or trusses — reduce rafter size; used for moderate spans.
Trussed (framed) roofs
| Truss | Description / use |
|---|---|
| King post truss | Principal rafters, tie beam, a central king post and two struts — timber roofs of moderate spans (commonly about 5–8 m) |
| Queen post truss | Two queen posts, a straining beam between them, principal rafters, tie beam and struts — larger spans (commonly about 8–12 m) |
| Combination of king and queen post | Larger spans |
| Mansard truss | Double-pitched on each side (steep lower slope, flatter upper slope) — extra attic space |
| Truncated truss | Flat top section |
| Bowstring / Belfast truss | Curved top chord (bowstring) — large timber spans |
| Steel roof trusses | For larger spans (see below) |
Steel roof trusses
| Truss | Features / use |
|---|---|
| Pratt truss | Verticals in compression, diagonals in tension (under gravity) — medium spans |
| Howe truss | Diagonals in compression, verticals in tension — historically timber |
| Fink (French) truss | Subdivided diagonals — economical for long spans with steep pitches, sheds and warehouses |
| Fan truss | Struts radiating in fan shape |
| North-light (saw-tooth) truss | Asymmetric profile with steep glazed side facing north (in the northern hemisphere) — uniform, glare-free daylight in factories and workshops |
| Quadrangular, parallel chord (flat) trusses | Flat roofs, large spans |
| Pre-engineered frames (portal frames) | Tapered rigid frames — warehouses, industrial buildings |
- Spacing of trusses commonly about 3–4.5 m for timber/steel roof trusses in ordinary buildings (economic spacing often taken as a fraction of span, e.g. about one-third to one-fifth).
- Trusses are anchored to supports with holding-down bolts; one end may allow expansion; bracing is provided for wind and stability.
- Loads considered: dead load (sheeting, purlins, truss self-weight), imposed load on roofs, wind load (uplift — critical for light roofs, IS 875 Part 3), snow load in hilly areas.
Roof coverings for pitched roofs
| Covering | Features |
|---|---|
| Thatch | Straw, grass, palm leaves — cheap, insulating; fire hazard, short life |
| Country tiles (pot tiles) | Half-round burnt clay tiles — cheap, rural |
| Mangalore tiles | Interlocking clay tiles on battens — widely used in south and coastal India; good ventilation and insulation |
| Slates | Thin natural stone sheets — durable, hilly areas |
| Wooden shingles | Thin timber pieces — hill areas |
| Galvanised iron (GI) corrugated sheets | Light, economical, quick; noisy in rain and hot (poor insulation), corrosion if coating fails |
| Colour-coated / galvalume steel sheets | Pre-painted profiled sheets — durable, aesthetic, common for industrial and PEB buildings |
| Fibre cement (asbestos-free) sheets | Corrugated or profiled — cheaper, better insulation than GI; brittle |
| Asbestos cement sheets | Formerly widely used; health hazards from asbestos fibres have led to restrictions and replacement by fibre cement sheets in many applications |
| Aluminium sheets | Light, corrosion resistant, reflective |
| Polycarbonate and FRP sheets | Translucent — skylights, canopies, daylight panels |
| Sandwich (insulated) panels | Steel skins with PUF/EPS core — insulated roofs, cold storages |
| Stone slabs | Kota/Shahabad stone on steel or timber supports — traditional in some regions |
Sheets are laid with end laps and side laps, fixed to purlins with J-hooks/self-drilling screws with washers, and sealed at ridges with ridge caps and flashing.