Roof Area Calculator
Calculate the total area of any pitched roof — including both slopes, hip ends, and overhangs. Essential for calculating roofing tiles, felt underlay, and insulation.
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Roofing Guide
What's the key thing to understand about Calculating Rafter Length?
Rafter length (horizontal span from ridge to eave) depends on the pitch angle: rafter length = (half-span) ÷ cos(pitch angle). For a 10m-wide building (5m half-span) at 35° pitch: rafter length = 5 ÷ cos(35°) = 5 ÷ 0.819 = 6.1m. The actual cut rafter adds the eave overhang to this measurement. At 45° pitch, rafter length = half-span × √2 = 1.414 × half-span. Common UK roof pitches: 30–40° for interlocking concrete/clay tiles; minimum 12.5° for corrugated sheets; minimum 22.5° for single-lap clay tiles, with steeper pitches generally needed in areas with heavier rainfall or snowfall to shed water and snow more effectively.
What's the key thing to understand about Tile Coverage Rates?
Coverage varies significantly by tile type: Standard concrete interlocking tiles (Marley Mendip, Redland Renown): approximately 10 tiles per m². Traditional plain clay tiles (hand-made): approximately 60 tiles per m² (much smaller). Slate (600×300mm): approximately 16–18 slates per m². Spanish round tiles (Roman/Mediterranean): approximately 15 tiles per m². Always refer to the manufacturer's technical data for specific tiles as coverage varies by pitch, exposure, and headlap. Add 10-15% for wastage from cutting, breakages, and the need to match tiles across hips, valleys, and edges.
What's the difference between Flat and Pitched Roof Area?
The roof area is always larger than the floor area beneath it because the slope increases surface area. The multiplier = 1 ÷ cos(pitch angle). At 30° pitch: 1 ÷ cos(30°) = 1.155 (roof area is 15.5% larger than floor area). At 45°: 1.414 (41% larger). At 60°: 2.0 (twice the floor area). This is why roofing costs significantly more than flooring for the same building footprint — more material, more difficulty of working at height, plus the cost of scaffold access.
When to Call a Professional?
UK law requires scaffolding for working above 2m on roofs — DIY tiling without scaffold access is both dangerous and illegal on public buildings. For domestic roof repairs, a licensed roofer is strongly advised for any work above first-floor level. When getting quotes, check the roofer is a NFRC (National Federation of Roofing Contractors) member for quality assurance. For heritage or listed buildings, clay tile or natural slate matching the original specification may be required by planning conditions, so always check with your local planning authority before assuming a modern equivalent will be accepted.