Roof pitch decides which coverings you may use, how they must be laid and whether you need a membrane underneath. Get it wrong and the roof leaks in driving rain no matter how well the tiles are laid. This guide sets out the pitch rules in SANS 10400-L and the rule for undertile membranes, with the reasoning behind them.
Measuring pitch
South African plans and SANS 10400-L state pitch in degrees. If you know the rise and the run, pitch = atan(rise ÷ run). A roof rising 1.5 m over a 4 m run is atan(1.5 ÷ 4) = 20.6°. The roof pitch calculator converts between degrees, ratios and percentages.
Minimum pitch for tiles and slates
SANS 10400-L table 2 sets these minimum pitches:
| Covering | With an approved underlay | Without underlay |
|---|---|---|
| Concrete and clay tiles | 17° | 26° |
| Fibre-cement slates | 11° | 17° |
| Metal tiles | 11° | 15° |
Within these limits, the tile maker’s own instructions still apply. Marley, for example, lays its Double Roman tile with a 100 mm headlap from 17.5° to 25° and a 75 mm headlap from 26°, up to a maximum of 45°.
When a membrane is required
SANS 10400-L 4.2.2.6 requires an undertile membrane in three situations:
- where the pitch is below 26°;
- where the pitch is 45° or steeper; and
- between the coastline and a line 30 km inland, or the top of the escarpment or watershed of the first mountain range inland if that is less than 30 km from the coast.
A note to the clause adds that if the 30 km line cuts through a local authority’s area, the whole of that area counts as coastal. So a house more than 30 km from the sea can still need a membrane because its municipality reaches the coastal strip; check with the municipality if you’re unsure.
At low pitches, wind can drive rain up and under the tiles. At very steep pitches, tiles are more exposed to wind lifting them. Near the coast, wind-driven rain is heavier and more frequent. In each case the membrane catches water that gets past the tiles and leads it out at the eaves.
In practice, this means most tiled roofs in coastal towns and any tiled roof below 26° need a membrane. The roof tile calculator asks whether the house is within 30 km of the coast and adds the membrane when the rules call for it.
Three quick examples
- A tiled roof at 22.5° in Johannesburg: below 26°, so a membrane is required.
- A tiled roof at 30° in Johannesburg: none of the three conditions applies, so 4.2.2.6 doesn’t require a membrane, though you may still choose one.
- A tiled roof at 30° in Durban: within 30 km of the coast, so a membrane is required.
For a feel of the numbers: at 26°, a roof rises about 0.49 m for every metre of run (tan 26° = 0.488), so a 4 m run rises about 1.95 m.
Sheeted roofs
Metal and fibre-cement sheeting can go much flatter than tiles. SANS 10400-L table 1 gives corrugated profiles a minimum slope of 11°, with 150 mm sealed end laps. And 4.2.2.1 allows sheeted roofs without hips or valleys on category 1 buildings (no more than 80 m² of floor, among other limits) to go down to 5°, with every end lap sealed and at least 250 mm. Clotan’s IBR datasheet gives a minimum of 5° for roof slopes under 30 m long and 7.5° for longer ones, with 150 mm end laps above 15° and 250 mm below, but on a house that isn’t a category 1 building the regulations’ 11° applies. See estimating concrete tiles, IBR and corrugated roofs.
Pitch and the roof structure
The covering and the trusses are linked. SANS 10400-L table 4 allows its simple Howe trusses at 17.5° to 35° under tiles and 15° to 30° under sheets or metal tiles. A tiled roof at 15°, or a sheeted roof at 10°, is outside those trusses and needs a designed roof. See roof trusses and SA pine grades.
Practical points
- Fix the pitch at design stage, before trusses are ordered, so the covering, membrane and trusses all agree.
- Lap membranes as the maker specifies and run them into the gutter, not behind the fascia.
- Low-pitched tiled roofs are less forgiving of broken or slipped tiles; inspect them after storms.
- Check the tile maker’s warranty conditions, which often depend on following its pitch and membrane rules.
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.