Roofing Calculator
Sheet area and cost for a metal roof.
Estimate the roof sheeting area and cost from the building's plan area and roof pitch, using SA IBR / Chromadek benchmark rates.
- Roof (sloped) areaplan area × pitch factor126.0 m²
- Sheeting to order138.6 m²+10% overlaps & offcuts
- Estimated installed costR 335/m² (incl. structure)R 46 431
Pitch factor converts plan area to sloped area. Installed-cost rate from NNAQS benchmark data (sheeting incl. supporting structure). Purlins, flashings, and insulation vary by detail.
A worked example
With the calculator’s starting values (plan (footprint) area: 120 m²; roof pitch: Moderate (10–17°); sheet type: IBR / Zincalume), it returns:
- Roof (sloped) area: 126.0 m² (plan area × pitch factor)
- Sheeting to order: 138.6 m² (+10% overlaps & offcuts)
- Estimated installed cost: R 46 431 (R 335/m² (incl. structure))
How the calculator works
It takes the roof's plan area and multiplies it by a pitch factor to get the sloped area, then adds 10% for side laps, end laps and offcuts. A flat roof uses a factor of about 1.0, a moderate pitch about 1.05 and a steep pitch about 1.15. The cost uses a benchmark installed rate for the sheeting and its supporting structure.
Measuring the plan area
Measure the roof as seen from above, including the eaves overhangs, not the floor area of the building. A house with 600 mm overhangs on every side has a noticeably larger roof than its floor plan. For a roof with several levels, measure each section and add them.
The pitch factor is one divided by the cosine of the pitch angle. A 15° roof has a factor of about 1.04, a 30° roof about 1.15 and a 45° roof about 1.41, so steep roofs grow quickly.
Choosing the sheeting
- IBR and corrugated profiles in Zincalume or galvanised steel are the standard choices for houses, sheds and commercial roofs.
- Each profile has a minimum pitch set by its manufacturer. IBR handles lower pitches than corrugated, which is why it is common on low-pitched commercial roofs.
- Pre-painted sheeting such as Chromadek adds colour, and insulated panels add thermal performance in one product.
- On coastal sites, specify a heavier coating or a product rated for marine conditions, and use fixings that match.
What else the roof needs
Ridge capping, flashings, gutters and downpipes are priced separately. SANS 10400-XA requires insulation in the roof of new buildings, with the required performance depending on the climatic zone. Timber trusses should be designed by the truss supplier or an engineer, and many municipalities and the NHBRC expect a certificate for the truss installation. On large roofs and in windy areas, the fixing pattern matters as much as the sheet: under-fixed sheeting is what lifts in a storm.
Questions
- How do I convert plan area to roof area?
- Multiply the footprint by a pitch factor: ~1.0 for flat, ~1.05 for moderate pitch, ~1.15 for steep. A steeper roof has more surface area for the same footprint.
- Is IBR or corrugated cheaper?
- IBR and corrugated Zincalume are similar in price per m². Chromadek and insulated panels cost more but include a colour finish and thermal performance.
- What does the installed cost include?
- Sheeting plus the supporting timber or steel structure. Flashings, ridge caps, insulation, and gutters are detailed separately on a real BoQ.
Compiled by Setout. Figures are planning-stage estimates, not quotes.
Reviewed by Aidan Koetaan MBA PMP, quantity surveyor and construction project manager.