Roof Area Calculator — Plan Area to Actual Roof Area

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 an equal-pitch hip roof the sloped area is the sameas a gable of the same footprint — Plan area ÷ cos(pitch) — because the four hip triangles project onto exactly the same horizontal area as the two vertical gable-wall triangles they replace. No extra factor is geometrically correct; hips and ridges are lines, not area. Add a separate cutting/waste allowance (typically about 5-10% on tiles or slates) for the extra hip and valley cuts.

Roof type adjustments

• Gable: Plan area ÷ cos(pitch) — two equal rectangular slopes
• Hip: Plan area ÷ cos(pitch) — four slopes (two trapezoids + two triangular ends) covering the same footprint as a gable, so the same area
• 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