Roof Area Calculator — Plan Area to Actual Roof Area

Divide the plan area by the cosine of the pitch angle. For a gable roof: Actual area = Plan area / cos(pitch). For example, a 10m × 8m plan area with a 30° pitch: 80 / cos(30°) = 80 / 0.866 = 92.4 m². This accounts for the extra area created by the slope of the roof.

Plan area is the horizontal footprint of the building (the bird's-eye view). Roof area is the actual sloping surface area you need to cover with tiles, felt and battens. The steeper the pitch, the greater the difference. A 45° pitch increases the area by about 41% compared to the plan area. Once you have the actual area, use our Roof Tile Calculator and Felt and Batten Calculator to order materials.

The steeper the pitch, the larger the actual roof area compared to the plan area. At 15° the increase is about 3.5%, at 30° it is about 15.5%, at 40° about 30.5%, and at 45° about 41.4%. This is why it is important to measure or calculate the true roof area before ordering materials.

You can measure pitch from inside the loft. Place a spirit level horizontally from a rafter and measure 1 metre along it. Then measure vertically down from the 1m mark to the rafter — this gives the rise per metre run. Use our Roof Pitch Calculator to convert rise and run to degrees. Alternatively, a digital inclinometer app on a smartphone placed on a rafter gives a quick reading.

Yes — for an equal-pitch hip roof, the total sloped area is exactly the same as a gable roof of the same plan footprint and pitch: Plan area / cos(pitch). The hip ends simply replace the vertical gable-wall triangles with sloped triangles over the same horizontal footprint, so no extra surface area is added. (Worked example: a 10m × 8m footprint at 30° gives two main trapezoids of 55.43 m² plus two hip-end triangles of 36.95 m², totalling 92.38 m² — identical to 80 / cos(30°) = 92.4 m².) Ridges, hips and valleys are lines where planes meet, and lines have length, not area, so they never change the surface area. You should still add a separate cutting/waste allowance — typically about 5-10% on tiles or slates for the extra hip and valley cuts per BS 5534 take-off practice — but that is waste, not extra surface area.