Roofing calculator: roof area, allowance and covering units
For a rectangular roof with a common slope, surface area = horizontal projection area / cos(slope). A 10 × 8 m projection at 30° has about 92.38 m² of roof surface. An assumed 10% cutting allowance gives 101.61 m² to plan; at 0.30 m² effective coverage per unit, round up to 339 units.
Calculate with your assumptions
Measure projection and slope
Measure the full horizontal footprint of the roof, including the horizontal overhangs. Use the slope angle in degrees, not a percentage. This model works for a flat roof or roof planes with one common pitch whose horizontal projections completely cover the rectangle without overlapping. A uniform-pitch gable or hip roof can meet this model; unequal pitches, dormers and complex geometry require separate plane calculations.
Apply the surface factor once
The slope factor is 1/cos(angle), with degrees converted to radians for the calculation. If you have already measured the surface along the slope, do not apply the factor again. At 0° the surface equals its horizontal projection. Angles up to 80° are supported here; larger values are excluded from this planning model.
Allowance and effective coverage
Allowance depends on the layout, cuts and product; the example 10% is an assumption you can change. Effective coverage means the installed covering area after laps or overlaps, not necessarily the gross area of a sheet or tile. Required units = ceiling(surface area × (1 + allowance/100) / effective unit coverage).
Finish the takeoff
Pack quantities, ridge and hip tiles, valleys, flashing, battens, fasteners, underlayment laps and penetrations require separate quantities and manufacturer data. Small openings do not necessarily save a whole covering unit. This calculation does not design the structure, drainage or a waterproofing system. Use the linked roof tools for the remaining purchase steps.
Examples with stated assumptions
| Inputs and method | Result |
|---|---|
| 80 m² / cos(30°) | 92.38 m² |
| 92.376 m² × 1.10 | 101.61 m² |
| 101.614 m² / 0.30 m², round up | 339 units |
Result scope
This is a geometric and quantity calculation from your dimensions. The chosen pitch, allowance and coverage are inputs, not product recommendations or a structural assessment.