Roof Sheet Calculator — South Africa
Calculate IBR or corrugated roof sheets for any SA roof. Sheet count, ridge lengths and end-lap allowances — IBR 686 and corrugated 762 profiles.
Quick answer: Divide your roof width by the effective cover width of your sheet profile — 0.686m for IBR 686 or 0.762m for corrugated 762 — to get the number of sheet runs needed. Sheet length is then the rafter length plus end-lap allowances of 150mm at both the ridge and the eave (SA roofing standard).
⚠️ For planning purposes only. Always have a registered roofing contractor verify quantities and structural requirements before ordering.
How to Use This Calculator
Enter the roof width (eave to eave, one slope) and rafter length (eave to ridge, measured along the slope). Select the number of slopes for your roof shape — 2 for a standard gable/hip, 4 for a full hip roof — and choose your sheet profile (IBR 686 or Corrugated 762).
Pick the closest sheet length and the correct end lap for your region — 200mm for coastal, high-rainfall, or low-pitch roofs. Results show total sheets (with wastage), roof area, and the ridge cap length in 1.8m pieces.
Roof Sheet Calculations in South Africa — IBR vs Corrugated
IBR (Inverted Box Rib) and corrugated sheeting are the two dominant metal roofing profiles in South Africa, found on millions of residential and commercial buildings. Getting the sheet count right before ordering is critical — ordering too few means a second delivery at extra cost, and trying to match a sheet profile from a different batch or supplier can result in colour and gauge mismatches that are visible from the street.
SA Roof Sheet Profiles — Key Specifications
| Profile | Cover Width | Effective Cover | Min Pitch | Purlin Spacing |
|---|---|---|---|---|
| IBR 686 | 800mm nominal | 686mm | 5° (approx 1:12) | 1.2–1.8m (0.47mm) |
| Corrugated 762 | 900mm nominal | 762mm | 8–10° | 0.9–1.2m (0.47mm) |
| IBR 686 (thicker) | 800mm nominal | 686mm | 3° | Up to 2.4m (0.58mm) |
Roof Sheet Calculation Formula
Sheet runs = CEIL(Roof width ÷ Effective cover width)
Sheets per run = CEIL(Rafter length ÷ (Sheet length − End lap))
Sheets per slope (net) = Sheet runs × Sheets per run
Total sheets = Sheets per slope × Number of slopes × Wastage factor
Roof area (m²) = Roof width × Rafter length × Number of slopes
End Laps — Why They Matter in SA Conditions
The end lap is the horizontal overlap between two sheets along the same rafter run. In South Africa, the minimum end lap for IBR and corrugated sheets is 150mm for inland areas on roofs with a pitch of 15° or more. In coastal regions, high-rainfall areas, or on lower-pitch roofs, this increases to 200mm. An insufficient end lap is one of the most common causes of roof leaks in South African buildings — wind-driven rain can travel up under the lap on low-pitch roofs during severe storms.
Side Laps and Ridge Caps
The side lap (overlap between adjacent sheets widthways) is built into the effective cover width calculation — one rib of the IBR sheet or one corrugation of the corrugated sheet overlaps the adjacent sheet. Ridge caps are ordered by linear metre — measure the total ridge length and add 10% for overlaps between cap pieces. Ridge caps are typically 1.8m long in SA. Valley flashings, barge boards and other accessories are ordered separately based on the specific roof geometry.
Fasteners and Fixing Patterns for SA Roof Sheeting
The correct fastener for IBR and corrugated sheeting in South Africa is a self-drilling roofing screw with a bonded EPDM washer — not a roofing nail, not a tek screw without a washer. Roofing nails allow micro-movement that enlarges the hole and causes leaks; screws without proper washers compress and deteriorate, leaving the hole exposed. The standard fixing pattern is into every third rib at sheet ends and every fifth rib in the field, with additional fixings at ridges and eaves. In coastal and high-wind zones (such as KwaZulu-Natal coastal or the Cape Peninsula), the fixing density must be increased as specified by a structural engineer — under-fixed sheeting is the primary cause of storm damage to metal roofs in South Africa.
Anti-capillary ribs on IBR sheeting prevent water from being drawn upward through the end lap by capillary action. This is a specific feature of IBR profile that corrugated sheeting does not have — one reason IBR is preferred for lower-pitched roofs in South Africa, where the slower water runoff increases the risk of capillary ingress. For roofs with a pitch below 8°, always use IBR sheeting and ensure the end lap is at least 300mm. Corrugated sheeting on a low-pitch roof with insufficient end lap is a guaranteed maintenance problem within a few rainy seasons.
Roof Sheet Colour and Heat Gain in South Africa
Roof sheet colour has a significant impact on indoor temperature in South Africa's high-solar-irradiance climate. Dark colours (charcoal, dark grey, black) absorb substantially more solar radiation than light colours (off-white, light grey, zinc). In regions with hot summers — Gauteng, Limpopo, Free State — a light-coloured or Zincalume finish roof can reduce attic temperatures by 10–15°C compared to a dark colour, reducing cooling loads and making the space more comfortable without mechanical ventilation. If the building has no ceiling insulation, roof sheet colour is one of the most cost-effective ways to reduce heat gain.