Roof area calculator
A pitched roof is always larger than the footprint of the house: the steeper the pitch, the bigger the difference. Roofing materials, underlay and battens are all ordered from this actual roof area.
Your measurements
Result
- Plan area (footprint)
- 80.00 m²
- Actual roof area
- 88.27 m²
Eaves overhangs are not included: add twice the overhang to the length and the width.
How the calculation works
- Plan area = building length × width (the footprint seen from above).
- Actual roof area = plan area ÷ cosine of the pitch: ×1.04 at 15°, ×1.10 at 25°, ×1.22 at 35° and ×1.41 at 45°.
Worked example
A 10 × 8 m house has 80 m² in plan. At a 25° pitch the actual roof area is 80 ÷ cos 25° = 80 ÷ 0.906 = 88.3 m².
What the calculator assumes
- The formula holds for any roof whose slopes all have the same pitch — gable or hipped.
- The calculation uses the building dimensions; eaves overhangs are not included.
When to adjust the result
- Add the eaves: length and width of the building + twice the overhang (usually 40–60 cm each side).
- Roofs with different pitches, mansards, dormers and multiple ridges are calculated slope by slope.
- The result is the covering area; for tiles add waste in the roof tile calculator.
Measuring a roof without climbing it
You can work from the ground: measure the footprint of the house including overhangs, then find the pitch from drawings, an app or by measuring the gable. Plan area ÷ cosine of the pitch gives the real roof area, because every slope is longer than its shadow on the ground.
Hips and valleys do not change the total area of a roof with equal pitches, but they increase waste, ridge length and the number of special tiles. Dormers, chimneys and roof windows are subtracted for coverings and added back as flashings and edge details.
Frequently asked questions
How do I calculate roof area from the floor plan?
Divide the footprint area by the cosine of the roof pitch. At 30°, for example, multiply the footprint by about 1.15.
Does a hipped roof have more area than a gable roof?
At the same pitch and footprint, no — the area is the same; a hipped roof has more ridges and hips, which add waste and accessories.
How do I find the roof pitch?
From the drawings, by measuring rise and run at the gable (pitch = arctan(rise ÷ run)), or with a phone inclinometer app on a rafter in the loft.
Does a hipped roof have more area than a gable roof?
At the same pitch and footprint the slope area is similar, but hips add cutting waste, ridge and hip tiles.