How the calculation works
The footprint area is length × width. A pitch factor converts horizontal plan area into an approximate sloped surface area. An allowance is then added for cuts, overlaps and waste.
Worked example
A 10 m × 6 m footprint is 60 m². With a 1.12 pitch factor, the estimated sloped area is 67.2 m². Adding 10% gives about 73.92 m² of planning area.
Roofing note: This is a simple estimation tool, not a roof design or take-off. Complex roofs, valleys, hips, penetrations, eaves and manufacturer-specific laps need separate measurements and product rules.
Common mistakes
- Using roof footprint as if it were sloped surface area.
- Forgetting overlaps and ridge/eave details.
- Using one pitch factor for a multi-pitch roof.
FAQ
What pitch factor should I use?
Use a factor calculated from the actual roof geometry or a reliable roofing reference. Do not assume 1.12 applies to every roof.