Reviewed, sourced guidance
Calculate sloped roof area by plane
Direct answer
For one uniform-pitch rectangular roof, multiply plan length by plan width and divide by cos(pitch). For mixed pitches or an irregular outline, calculate each rectangular plane as eaves length × horizontal run ÷ cos(pitch), then add the results. Use the roofing calculator for computed areas and product quantities; use this guide to follow the method.
This guide keeps the manual measurement method visible: eaves length, horizontal run and pitch for each plane. Use it when you want to follow the arithmetic; the roofing calculator runs the figures and any product quantities you enter.
Key takeaways
- Footprint mode requires one rectangular outline and a uniform pitch.
- Use the horizontal run for a plane, not its sloping length.
- Calculate mixed pitches separately instead of using one average angle.
- Sum measured planes and avoid counting any surface twice.
- Area alone does not give a material order quantity.
Convert run into slope length
For one roof plane, slope length is horizontal run divided by cosine of the pitch angle. Multiply it by the plane’s eaves length.
For one rectangular footprint with a uniform pitch, divide plan length × plan width by cosine of the pitch. The footprint dimensions must include the outer covering edges.
For example, an 8 m eaves length and 4 m run at 30 degrees gives about 37 m² for that plane.
- At zero degrees, slope length equals horizontal run.
- A steeper pitch increases slope length for the same run.
- Round only for display; retain measured precision while calculating.
Sources for this section: Cushty, Roof Area Calculator UK (observed 2026-09-25)
Measure all planes consistently
Different pitches create different conversion factors. Calculate each one from its own run and angle.
Record whether the eaves measurement includes the overhang. Use the same edge definition on every plane.
- Separate mixed-pitch or irregular roofs into planes that can be measured as rectangles.
- Mark hips, valleys, dormers and openings for a separate take-off.
- Do not add the footprint to the sloped area.
Sources for this section: Marley, How to calculate roof pitch (observed 2026-09-25)
Area is not the final order
Tile coverage changes by product and laying details. Membrane rolls also lose usable area to laps and overlaps.
Use the selected system’s documented coverage and include fittings as separate measured items.
- Keep net area separate from a cut or breakage allowance.
- Record the source and date for every product rate.
- Have the final take-off checked before materials are ordered.
Sources for this section: Marley, Duo Modern product data (observed 2026-09-25)
Next steps
- Use footprint mode only for one rectangular roof with a uniform pitch.
- Use separate planes for mixed pitches and check product rates independently.