The formula, your substituted values, and the evaluated result are shown below.
This is a planning estimate. Confirm dimensions, allowances, material specifications, applicable codes, and supplier requirements before purchasing or construction.
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How the Roofing Calculator works
Calculate the project area for roofing.
Before using the result
This is a planning estimate. Confirm dimensions, allowances, material specifications, applicable codes, and supplier requirements before purchasing or construction.
This calculator gives the plan area, the area of the roof as seen from directly above:
Plan area (m²) = length (m) × width (m)
Measure the building footprint including the eaves overhang. A sloping roof has more actual surface than its plan area, so multiply the result by a pitch factor to get the true roof area.
Pitch factors
Roof pitch
Pitch factor (1 ÷ cos pitch)
5°
1.004
15°
1.035
22.5°
1.082
25°
1.103
30°
1.155
35°
1.221
45°
1.414
Worked example: a 16 m × 10 m house with a 22.5° roof
Footprint including 450 mm eaves: 16 × 10 = 160 m² plan area
True roof area: 160 × 1.082 = 173.2 m²
Add waste: about 5% for simple gables, 10–15% for hips and valleys: 173.2 × 1.10 ≈ 190 m² to order
This works for gable and hip roofs because every plane shares the same pitch. For roofs with mixed pitches, work out each section separately. If you don't know the pitch, the roof pitch calculator will find it from the rise and run.
Turning area into materials
Metal sheeting: order by sheet length and cover width. For example, a 762 mm cover sheet covers 0.762 m² per lineal metre.
Tiles: concrete tiles are often roughly 10–11 per m², depending on profile and batten gauge. Use the manufacturer's figure.
Sarking and underlay: use the true roof area plus the laps specified by the manufacturer.
Ridge, hips and valleys: measure these lengths separately; they aren't in the area figure.
Common mistakes
Measuring the wall footprint and forgetting the eaves.
Ordering by plan area, which undercounts by 8% at a 22.5° pitch and by 15% at 30°.
Forgetting ridge caps, flashings and barge cappings.
How do I calculate roof area from the floor plan?
Multiply the footprint including eaves by the pitch factor, 1 ÷ cos(pitch). For a 25° roof, that factor is about 1.10.
How many roof tiles per square metre?
Typically around 10–11 concrete tiles per m², but it varies by tile, so use the manufacturer's coverage.
How much waste should I allow for roofing?
About 5% for simple gables, and 10–15% for roofs with hips, valleys and dormers.
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