Roof estimates go wrong in two places: using the plan area instead of the sloping area, and using a product’s overall width instead of the width it covers once lapped. This guide works through concrete tiles and IBR sheeting with the makers’ own figures, and covers corrugated sheeting’s slope rule.
From plan to roof area
A pitched roof is bigger than the floor it covers. For a simple gable roof, each side’s rafter length = (half the span + eaves overhang) ÷ cos(pitch), and the roof area = rafter length × 2 × (building length + both gable overhangs). The roof pitch calculator converts between degrees and rise over run.
Concrete tiles
Marley’s datasheet for its Double Roman concrete tile gives two settings:
| Pitch | Headlap | Tiles per m² | Battens per m² |
|---|---|---|---|
| 17.5° to 25° | 100 mm | 10.42 | 3.13 m |
| 26° and up | 75 mm | 9.66 | 2.9 m |
Marley’s battens are 38 × 38 mm where the trusses or rafters are no more than 760 mm apart, as in SANS 10400-L table 4; wider centres, up to 900 mm (38 × 50 mm battens laid flat) or 1,000 mm (on edge), need engineer-designed trusses. The maximum batten gauges are 320 and 345 mm, the tile covers 301 mm in width, the roof weighs about 44 or 41 kg per m², and the maximum pitch is 45°. Other tiles have their own figures, so use the datasheet for the tile you buy. SANS 10400-L table 2 asks for at least 17° with an approved underlay and 26° without, and a membrane is required in some places; see roof pitch rules and when you need a membrane.
Worked example: 100 m² of Double Roman at 22.5° needs 100 × 10.42 = 1,042 tiles and 313 m of battens, before ridges, hips and waste.
IBR sheeting
Clotan’s IBR datasheet gives:
- cover width 686 mm; the sheet is 740 mm wide overall, the difference being the side lap;
- lengths from 1.5 to 14 m (up to 20 m on request), so most sheets are ordered cut to the rafter length;
- sheets per side = (building length + gable overhangs − 70 mm) ÷ 0.686, rounded up;
- minimum pitch 5° for slopes under 30 m long and 7.5° for longer slopes;
- end laps of 150 mm above 15° and 250 mm at lower pitches, where a run needs more than one sheet.
Clotan’s 5° is lower than the building regulations allow on most houses. SANS 10400-L table 1 puts box-rib sheeting, which includes IBR, at a minimum of 11°, with end laps of 150 mm sealed or 250 mm unsealed. Its clause 4.2.2.1 allows 5° only on a category 1 building (one with no more than 80 m² of floor, among other limits) whose roof has no hips or valleys and whose end laps are all sealed and at least 250 mm. Plan an ordinary house roof at 11° or more, unless your approved plans say otherwise.
SANS 10400-L table 4 puts timber purlins of 50 × 76 mm on edge at no more than 1,200 mm centres under metal or fibre-cement sheets. The sheet itself can span further than that: Clotan gives 0.47 mm G550 IBR a purlin spacing of up to 1.65 m on a single span and 1.95 m over continuous spans. On a timber truss roof, though, the 1,200 mm purlin rule from the trusses governs, so plan purlins to it. Clotan fixes IBR through the crests of alternate flutes with 90 mm screws and 26 mm bonded washers on timber. The flutes are 171.5 mm apart, so there is a fixing every 343 mm: two per 686 mm sheet at every purlin, plus one at the end of each row. Screws = (sheets per side × 2 + 1) × purlin rows × sides.
Worked example: a 10 × 8 m gable roof at 17.5°, with 500 mm eaves and 300 mm gable overhangs, has a rafter length of 4.5 ÷ cos 17.5° = 4.72 m. Sheets per side = (10.6 − 0.07) ÷ 0.686 = 15.4, so 16, and 32 for the roof. With five purlin rows per side, that is 106 m of purlins and (16 × 2 + 1) × 5 × 2 = 330 screws. The roof truss and IBR calculator gives exactly this list.
Corrugated sheeting
SANS 10400-L table 1 gives corrugated and box-rib profiles a minimum slope of 11°, with end laps of 150 mm sealed or 250 mm unsealed, and corrugated fibre-cement 11° with 200 mm sealed laps. Under 4.2.2.1, a sheeted roof without hips and valleys on a category 1 building may go down to 5°, with every end lap sealed and at least 250 mm. Use the maker’s cover width and lap figures for the profile you choose; the roof sheeting calculator takes the cover width as an input.
Waste and extras
- Allow extra tiles for cutting at hips, valleys and verges, and for breakage.
- Count ridge, hip, barge and valley pieces separately from the roof area.
- Order sheets cut to length to avoid end laps where you can.
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.