Reviewed, sourced guidance
Roofing calculator questions answered
Direct answer
Roof area is eaves length times horizontal run divided by cos(pitch) per plane; tile counts multiply that area by the chosen product rate, such as Marley’s 9.9 or 10.7 tiles per square metre at 75 mm or 100 mm headlap; no waste percentage is universal.
The measurement and quantity questions behind most roofing calculations, answered within the site’s scope and cited manufacturer rates.
Key takeaways
- Pitch turns footprint measurements into sloped area.
- Tile rates are product-specific and headlap-dependent.
- Waste allowances are entered, not universal.
- Batten and membrane quantities follow the same measured area.
How do I work out roof area from pitch?
For one uniform-pitch rectangular roof, multiply plan length by plan width and divide by cos(pitch).
For mixed pitches, measure each plane as eaves length times horizontal run, divide by cos(pitch), then add the planes.
The footprint alone is not the covering area; the slope always makes the roof larger than the plan.
Sources for this section: Marley, How to calculate roof pitch (observed 2026-09-25)
How many tiles do I need per square metre?
Tile quantity equals the sloped area multiplied by the coverage rate for the chosen product and laying specification.
One cited Marley tile page lists 9.9 tiles per square metre at 75 mm headlap and 10.7 at 100 mm headlap.
Rates differ by product and headlap, so use the rate published for the exact tile you are specifying.
Sources for this section: Marley, Duo Modern product data (observed 2026-09-25)
What waste allowance should I add for a roof?
No single percentage fits every roof; cut-up roofs and simple gables produce different offcut patterns.
UK calculator competitors suggest different starting ranges, which is why this site keeps waste as an entered allowance.
Decide the figure against the roof shape and the product’s pack sizes, then record it with the order.
How do I estimate battens and membrane?
Both estimates start from the same measured sloped area, then apply the product’s coverage or gauge rate.
Batten quantity follows the chosen gauge for the tile and headlap; membrane follows its roll coverage and laps.
Enter the exact product rate or a supported Marley preset so the arithmetic stays tied to published data.
Sources for this section: Marley, technical services for roof specification (observed 2026-09-25)
Can I measure or work on the roof myself?
Measurement from the ground or from plans avoids work at height; HSE guidance treats roof work as high risk.
Fragile surfaces, edges and weather make access decisions a planning matter, not an afterthought.
This site estimates quantities; it does not assess whether any roof is safe to walk on.
Sources for this section: Health and Safety Executive, Roof work (page updated 2026-06-05; observed 2026-09-25)
Next steps
- Measure each plane, then run the roofing calculator with your product rate.
- Record the entered waste allowance with the order so quotes stay comparable.