Most South African houses have a timber roof of trusses: triangulated frames made from SA pine that carry the roof covering and the ceiling. Truss manufacturers design and make most of them, but SANS 10400-L also gives deemed-to-satisfy rules for simple Howe trusses, which builders can follow without an engineer. This guide explains those rules and the timber grades behind them.
When the simple rules apply
SANS 10400-L 4.4.1.1.1 limits the deemed-to-satisfy trusses to softwood roofs that:
- use Howe-type trusses, fully triangulated;
- span no more than 4 m for a monopitch truss or 8 m for a double-pitched one;
- are built gable to gable, with no hips or valleys;
- sit within the pitch limits for their covering;
- bear only at the heel joints, not on internal walls.
Hipped roofs, longer spans, other pitches and trusses that sit on internal walls need trusses designed by a truss manufacturer or an engineer.
Spacing and pitch by roof covering
| Covering | Pitch | Truss centres (max) |
|---|---|---|
| Concrete or clay tiles and slates on 38 × 38 mm battens | 17.5° to 35° (26° recommended) | 760 mm (650 mm where tiles or slates weigh more than 55 kg/m²) |
| Metal or fibre-cement sheets on 50 × 76 mm purlins | 15° to 30° (17.5° recommended) | 1,200 mm |
| Metal tiles on 38 × 38 mm battens | 15° to 30° (20° recommended) | 1,000 mm |
The covering manufacturer’s instructions may set other spacings, and SANS 10400-L 4.4.1.1.4 allows those instead of the table.
Member sizes and grades
Table 4 of SANS 10400-L sets the members. All timber is 38 mm thick (36 mm if planed):
- top chords and webs: 38 × 114 mm, grade 5;
- bottom chord (tie beam): 38 × 114 mm grade 5 up to 6.5 m clear span, and 38 × 152 mm grade 5 from 7 m to 8 m, where 38 × 114 mm grade 7 may be used instead;
- bays: 2-bay trusses up to 3 m, 4-bay up to 6 m and 6-bay up to 8 m;
- heel and splice joints: two M12 bolts with 40 mm washers at each end;
- overhangs: a 114 mm top chord may overhang up to 600 mm, and a 152 mm one up to 900 mm.
“Grade 5” and “grade 7” are structural stress grades. SA pine sold for roofing is graded to SANS 1783, and structural grade 5 is marked S5, for example on 38 × 114 mm kiln-dried rafters. SANS 10400-L asks for all roof timber to be stress-graded and stamped with its grade mark.
Treatment and support
- In the coastal municipal areas shown in SANS 10400-L, roof timber must be treated against rot, fungus and insects, and cut ends brushed with two coats of preservative.
- Trusses sit on wall plates at least 38 × 76 mm, and are tied down to the walls.
- A geyser of up to 150 L may stand on a platform across trusses with a clear span of up to 8 m, one geyser per three trusses.
Tying down and bracing
SANS 10400-L 4.4.1.2 adds the details that hold a truss roof together:
- trusses are tied down to the supporting walls with galvanised steel straps or wires built into the walls (4.4.1.2.5);
- top chords are braced as the standard’s figures show, with different details for tiled and sheeted roofs, and diagonal bracing at the wall plate for stub-ended trusses (4.4.1.2.6);
- bottom chords are braced as well (4.4.1.2.7), and where there is no boarded ceiling, permanent 38 × 76 mm runners are fixed to the bottom chords at the centre of each bay (4.4.1.2.8);
- water tanks and geysers should, wherever possible, sit on timber bearers over internal walls (4.4.1.2.10).
Counting trusses
Trusses = building length ÷ truss centres, rounded up, plus one. Worked example: a 10 m long tiled roof at 760 mm centres takes 10 ÷ 0.76 = 13.2, so 14 spaces and 15 trusses. Under IBR sheeting at 1,200 mm, the same roof takes 10 trusses. The roof truss calculator also gives the bays and member sizes for your span, and the roof truss and IBR calculator adds the sheets, purlins and screws.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check SANS 10400 (the National Building Regulations) and your local municipality’s building control officer for your project.