How it is calculated
Each slope is its eaves length times its slope length. The slope length is the horizontal run plus overhang divided by the cosine of the pitch. For a hipped roof with equal pitches the area equals the plan area with overhangs divided by the cosine of the pitch. Sheet materials are counted in whole sheets per slope using the effective width.
Slope length = (house width ÷ 2 + overhang) ÷ cos αGable roof area = 2 × (house length + 2 × overhang) × slope lengthSheets per slope = (house length + 2 × overhang) ÷ effective sheet widthExample
A gable roof on a 10 × 8 m house with a 30° pitch and 0.5 m overhang covers 114.3 m². With metal tile sheets 1.1 m wide you need 20 sheets 5.2 m long, 6 ridge pieces, about 915 screws and 2 membrane rolls of 75 m². The ridge sits 2.3 m above the walls.
Tips from practice
- Order metal sheets to the length of the slope; slopes longer than 6–7 m need two rows with an overlap.
- Metal roofs need a breathable membrane and a ventilated gap, otherwise condensation forms under the sheets.
- Bitumen shingles need a solid deck of OSB or plywood, which the calculator includes.
Questions and answers
How do I find the roof pitch?
Divide the ridge height above the walls by half the house width and take the arctangent. For example 2.3 m ÷ 4 m = 0.58, which is about 30°.
What is the effective sheet width?
It is the width a sheet covers after overlapping the next one. Metal tile sheets are about 1.18 m wide overall and 1.1 m effective.
How many roof tiles per square metre?
Between 10 and 15 depending on the model. The manufacturer’s catalogue gives the exact figure.
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