About this calculator
How many roof trusses a gable roof needs, at what centres, and the member sizes SANS 10400-L allows for a simple Howe truss.
How it’s calculated
Trusses = building length ÷ truss centres, rounded up, plus one. SANS 10400-L table 4 sets the maximum centres by roof covering: 760 mm for tiles and slates (650 mm where they weigh over 55 kg/m²), 1,200 mm for metal or fibre-cement sheets, and 1,000 mm for metal tiles. It also sets the pitch range (17.5° to 35° for tiles, 15° to 30° for sheets and metal tiles) and the members: 38 × 114 mm grade 5 top chords and webs, with a 38 × 152 mm bottom chord over 6.5 m clear span. Trusses are 2-bay up to 3 m, 4-bay up to 6 m and 6-bay up to 8 m. Top chord length = half the span ÷ cos(pitch) plus the overhang ÷ cos(pitch); a 114 mm top chord may overhang 600 mm, a 152 mm one up to 900 mm.
Worked example
With these inputs:
- Clear span (wall plate to wall plate)
- 6 m
- Roof covering
- Concrete or clay tiles, or slates
- Roof pitch (degrees)
- 26
- Overhang (horizontal)
- 300 mm
- Building length (for count)
- 10 m
the result is:
- Roof trusses
- 15 trusses
- Truss centres (maximum)
- 760 mm
- Bays (Howe truss)
- 4
- Top chord
- 38 × 114 mm, grade 5
- Bottom chord (tie beam)
- 38 × 114 mm, grade 5
Tips from the trade
- Measure the clear span between the wall plates, and check the walls are parallel before ordering.
- Trusses must bear only at the heel joints, not on internal walls.
- Most truss manufacturers design and supply trusses for your plans. Give them the span, pitch, covering and overhang.
- A geyser of up to 150 L may sit on a platform across trusses with a clear span of up to 8 m, one geyser per three trusses (SANS 10400-L figure 24).
Common questions
How far apart should roof trusses be?
Under SANS 10400-L table 4, no more than 760 mm apart under tiles or slates (650 mm for heavy tiles), 1,000 mm under metal tiles and 1,200 mm under metal or fibre-cement sheets, unless the covering manufacturer says otherwise.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and SANS 10400 (the National Building Regulations) and your local municipality before ordering.