Roof Area Calculator
Convert a building footprint into the true sloped roof surface area you actually have to cover.
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Formula and method
Half the building width plus the overhang gives the run, and the rise gives the geometry of one rafter. The slope length is multiplied by the roof length including overhangs to give the surface area, which the waste allowance then increases.
How to measure the inputs
- Measure the building length and width at the wall line, then add the overhang you plan.
- Measure the rise or the pitch from the eaves to the ridge.
- Record the overhang at the eaves and the gable ends separately if they differ.
Worked example
Building length 32.81 ft; Building width 26.25 ft; Roof rise (optional) 6.56 ft; Roof overhang (optional) 11.81 in; Waste 5 % → Roof area with waste: 1,136.29 ft²
Practical tips
- Measure the roof rather than the ceiling wherever it is safe to do so.
- Add dormers, rooflights and valleys as separate areas.
- Order roofing materials after the roof structure is complete and checked.