Roofing Quantity Calculator
Convert a roof plan area into true slope area using the pitch, then work out tiles or sheets needed and the material cost.
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A roof is always larger than the building beneath it, and the difference grows with the pitch. Measuring the footprint and ordering that many tiles is the classic error — at 35 degrees the actual roof surface is over twenty percent larger than the plan area.
The conversion is one trigonometric step: slope area equals plan area divided by the cosine of the pitch. Everything else is coverage rate and waste.
The geometry
Slope area = plan area ÷ cos(pitch). At 30 degrees that multiplies the plan area by 1.155, at 35 degrees by 1.221 and at 45 degrees by 1.414. Add a waste allowance for cuts at hips, valleys and verges, then multiply by the coverage rate.
Worked example
A building 10 by 8 has an 80 plan area. At a 35 degree pitch the true roof surface is 80 ÷ 0.8192 = 97.7. With 5% for cuts that is 102.5, and at 12 tiles per unit area you need 1,231 tiles costing 1,477 at 1.20 each.
Complex roofs need more waste
Five percent suits a simple gable. Hips, valleys, dormers and rooflights all generate cut tiles that cannot be reused, and a complicated roof can easily need ten to fifteen percent. Battens, underlay, ridge and fixings are all separate from the tile count.
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Create your free property pageFrequently asked questions
Why is the roof bigger than the building?
Because a pitched surface is the hypotenuse of a triangle whose base is the plan. The steeper the pitch, the longer that surface — at 45 degrees the roof is over 41% larger than the footprint.
How do I find the pitch if I do not know it?
Measure the rise and the horizontal run of the slope and take the inverse tangent of rise divided by run. A rise of 3.5 over a run of 5 is about 35 degrees, which is the most common domestic pitch in many regions.
What else do I need besides tiles?
Underlay, battens, ridge and hip tiles, valley trays, fixings, ventilation and often new flashings. These commonly add a third or more to the tile cost, and none of them are in the tile count.
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