Roof Trusses & Purlins Notes

Last reviewed 16 Sept 2026 · 6 min read

Components of a steel roof

Roof covering — galvanised iron (GI) or pre-coated steel sheets, asbestos-cement (older), aluminium, polycarbonate for daylight.
Purlins — beams spanning between trusses that carry the sheeting.
Trusses — principal rafters (top chord), main tie (bottom chord), struts and ties (web members), gusset plates.
Bracing — rafter bracing in the roof plane, bottom-chord bracing, vertical bracing between columns — resists wind and provides stability.
Eaves girder / gable girder, ridge, bearing plates on columns or walls.

Types of trusses

Truss	Span range (typical)	Feature
King post	up to about 8 m	Simple, timber or steel
Queen post	about 8–12 m	Two vertical posts
Pratt	about 6–30 m	Diagonals in tension under gravity
Howe	about 6–30 m	Diagonals in compression under gravity
Fink (French)	about 10–40 m	Short compression members; economical for pitched roofs
Fan	about 10–15 m	Top chord divided into more panels
North-light (saw-tooth)	factories (bays)	Steep glazed face towards north for diffuse light
Quadrangular / Warren	large spans	Parallel chords
Bowstring, arched	30 m and above	Curved top chord

Beyond about 40 m, space frames, portal frames or lattice girders are usually preferred.

Geometry

Pitch $= \frac{rise}{span}$ . Common pitch for GI sheeting: about 1/4 to 1/6 of span (slope roughly 18°–30°); flatter roofs with profiled sheets or built-up roofing.
Spacing of trusses: commonly span/4 to span/5, about 3–4.5 m for small sheds, up to 6–9 m for larger ones with heavier purlins.
Purlin spacing: decided by the sheeting's safe span, typically 1.2–1.8 m for GI sheets; purlins placed at panel points so the top chord carries no bending (if not, the rafter is designed for bending too).
Economical truss depth at the centre commonly about span/4–span/5 for pitched trusses.

Loads on roof trusses

Dead load

Sheeting, fixtures, purlins, bracing and the truss self-weight. An empirical estimate of the self-weight of a steel truss per m² of plan area is often taken as

w \approx \frac{L}{3} + 5 (kg/m^{2}) (L = span in m)

(or about 0.10–0.15 kN/m² for ordinary spans).

Imposed (live) load on roofs — IS 875 Part 2

Code ProvisionIS 875 Part 2 — imposed load on sloping roofs (key values)

Flat, sloping or curved roofs with slopes up to 10°, with access: 1.5 kN/m²; without access (except maintenance): 0.75 kN/m².
Sloping roofs with slope greater than 10°: $0.75 - 0.02 (α - 10°)$ kN/m², but not less than 0.40 kN/m² ( $α$ = slope in degrees).
Members of roof sheeting and purlins are also checked for a concentrated load (for maintenance) as specified.

Wind load — IS 875 Part 3

V_{z} = V_{b} k_{1} k_{2} k_{3} k_{4}, p_{z} = 0.6 V_{z}^{2} (N/m^{2}), p_{d} = K_{d} K_{a} K_{c} p_{z}

$V_{b}$ = basic wind speed for the location (from the wind zone map, 33–55 m/s).
$k_{1}$ = probability (risk) factor; $k_{2}$ = terrain roughness and height factor; $k_{3}$ = topography factor; $k_{4}$ = importance factor for cyclonic regions.
$K_{d}$ , $K_{a}$ , $K_{c}$ = directionality, area averaging and combination factors.
Force on a roof surface $F = (C_{p e} - C_{p i}) A p_{d}$ — external pressure coefficients depend on roof slope and wind direction; internal pressure coefficients (commonly ±0.2 for buildings with low permeability, ±0.5 for medium) depend on openings.

For light roofs, wind suction (uplift) often governs, reversing member forces — ties may go into compression, so slenderness limits for reversal apply.

Load combinations

Typical: (DL + LL), (DL + WL), (DL + LL + WL) with IS 800 partial safety factors. For uplift, dead load is taken with factor 0.9.

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Roof Trusses & Purlins