How do I calculate roof area from plan dimensions?

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.

What is the difference between plan area and roof area?

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.

How does pitch affect roof area?

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.

How do I measure my roof pitch if I do not know it?

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.

Is hip roof area the same as gable roof area?

No — a hip roof typically has about 5% more roof area than a gable roof of the same plan size, because the hip ends are sloped triangles rather than vertical gable walls. The hip slopes are calculated as triangles, which adds slightly more surface area. Our calculator applies a 1.05 adjustment factor for hip roofs.

Roof Area Calculator — Plan Area to Actual Sloping Roof Area in m2

How We Calculate This

Our roof area calculator converts the plan (horizontal) area of your building into the actual sloping roof area. This is essential for ordering the correct quantity of tiles, felt, battens and other roofing materials.

The formula

Actual roof area = Plan area ÷ cos(pitch angle)

For a gable roof, this gives the total area of both slopes. For a hip roof, we apply an additional 5% factor to account for the triangular hip slopes.

Roof type adjustments

Gable: Plan area ÷ cos(pitch) — two equal rectangular slopes
Hip: (Plan area ÷ cos(pitch)) × 1.05 — four slopes including triangular ends
Mono-Pitch: Plan area ÷ cos(pitch) — single slope
Flat: Plan area (no pitch adjustment needed)

Common UK pitch angles

15-22.5°: Low pitch — interlocking tiles, single-lap slates
22.5-30°: Moderate pitch — most tile types
30-45°: Standard UK pitch — all tile types including plain tiles
45-60°: Steep pitch — decorative, traditional buildings

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Roof Area Calculator — Plan Area to Actual Roof Area