A pitched roof always covers more area than the building beneath it, because the slope stretches the flat span. Enter your building’s footprint (length × width) and the roof pitch in degrees, and this tool works out the true sloped area — the figure you actually need when ordering tiles, sheeting or membrane. It calculates on your device.
Enter a value.
Enter the footprint and pitch to estimate the roof area.
An estimate for a simple gable roof. Overhang, hips and valleys are not included — split complex roofs into sections and add them.
How it works
The footprint length × width gives the flat plan area. The slope multiplies that: the sloped surface is the plan area divided by the cosine of the pitch angle. At 30° that factor is about 1.15, so an 80 m² footprint becomes roughly 92 m² of roof. A flat (0°) roof has a factor of exactly 1.
This models a simple gable roof where the whole footprint is covered by two equal slopes — the classic house shape. The tool then adds your waste margin for cuts, overlaps and offcuts. It doesn’t add eaves overhang, hips or valleys, so for complex roofs split them into rectangles and add the results.
Practical examples
A standard house roof
10 × 8 m footprint at a 30° pitch gives an 80 m² base and a slope factor of 1.15, so about 92 m² of roof — around 102 m² once you add 10% for waste.
How pitch changes it
Keep the same 80 m² footprint but raise the pitch to 45° and the roof grows to about 113 m². Steeper roofs shed water and snow better but cost more to cover.
A near-flat roof
At a 5° pitch the factor is barely above 1, so the roof area is almost identical to the footprint — useful for low-slope garage or extension roofs.
Frequently asked questions
What is roof pitch?
The angle of the slope from horizontal. This tool takes it in degrees — a common house roof is 30–45°. If yours is given as a ratio (e.g. rise 4 : run 12) or a percentage, convert it to degrees first with the angle converter.
Why is the roof bigger than the floor plan?
Because a sloped surface is longer than the flat distance it spans — think of walking up a hill versus across the map. The steeper the pitch, the bigger the difference, which is exactly what the slope factor captures.
Does this include the eaves overhang?
No. The result covers the footprint only. If your roof projects beyond the walls, add the overhang to the length and width before entering them, or add a little extra waste to compensate.
Can I use it for a hip roof?
For a simple hip roof of uniform pitch the total sloped area is close to the same footprint ÷ cos(pitch), so the estimate is reasonable. For mixed pitches, dormers or valleys, calculate each plane separately and sum them.
How much waste should I allow?
Around 10% is typical for tiles or sheeting on a plain roof; add more for lots of cuts, hips and valleys, or for small-format tiles. The tool applies whatever percentage you enter.
How do I turn area into a number of tiles?
Take the roof area with waste and feed it into the tile calculator with your tile’s coverage — it returns the piece count. Roofing tiles also have a minimum overlap, so use the manufacturer’s "tiles per m²" if given.
What if my building isn’t rectangular?
Break the footprint into rectangles, calculate each roof section with its own pitch, and add them. The area calculator can help with the individual shapes.
Why does it refuse very steep pitches?
Above about 80° the cosine approaches zero and the slope area shoots towards infinity, which is neither realistic nor a normal roof. The tool caps the input to keep results meaningful.
Is my data uploaded?
No. The calculation runs entirely in your browser; nothing is sent anywhere and analytics never receives your figures.
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