Roof Area and Pitch Calculator

How is roof area calculated?

The surface of a pitched roof is larger than the plan area seen from above. When every face has the same pitch the calculation is one step: roof area = plan area / cos θ. The same relation holds for gable, mono pitch and hip roofs. The eaves overhangs are included in the plan area.

Pitch as an angle or a percentage

Pitch is quoted in two ways. The angle is measured between the roof surface and the horizontal. The slope percentage is the rise per 100 units of horizontal run, so slope percent = tan θ × 100. A 100 percent slope is 45 degrees and a 33 percent slope is about 18.3 degrees. Whichever you enter, the calculator shows the other. If you know the ridge height it can work out the pitch from that as well.

Rafter length

The rafter runs from the eaves edge up to the ridge. On gable and hip roofs it is (width / 2 + overhang) / cos θ, and on a mono pitch roof (width + 2 × overhang) / cos θ.

Example

A 10 m × 8 m building with 50 cm eaves has a plan area of 11 × 9 = 99 m². At a 30 degree pitch the roof area is 99 / 0.866 = 114.3 m², the rafter length is 4.5 / 0.866 = 5.20 m and the ridge height is 4 × tan 30° = 2.31 m.

Limits

Split roofs with different pitches, steps or an L shaped plan into parts and calculate each one. Laps and waste are added separately when ordering the covering.