Calculate roof area, sheets or tiles, trusses, ridge capping, fascia and total cost — for gable, hip, mono-pitch or flat roofs.
Roof area = (Length + 2×overhang) × (Width + 2×overhang) × pitch correction.
Estimates total linear metres of rafters, purlins and bracing as a multiple of roof area. Default 3.5 m/m² is a reasonable rule of thumb — adjust for unusually simple or complex framing.
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Roof area starts from your building's plan dimensions, extended by the eaves overhang, then corrected for pitch using 1 ÷ cos(pitch) — a steeper roof has more actual sheet area than its flat plan footprint suggests. A hip roof adds an extra factor for the additional hip and valley intersections. A flat roof skips the pitch correction entirely and instead needs a screed laid to falls plus a waterproofing membrane, with an optional parapet wall priced the same way as a block wall. Trusses and purlins are estimated as a multiple of roof area — a practical rule of thumb rather than full structural design — using your chosen material's current rate. All prices are pulled from the same rates your admin manages on the Material Rates page, so this calculator stays in sync with the rest of the site.
Use our complete Estimator for a full cost breakdown and itemised Bill of Quantities — not just roofing.