Estimate the surface area and tile quantity for a simple rectangular gable roof.
Estimate for a simple symmetrical gable roof without overhangs. Valleys, hips, dormers, rooflights, laps and manufacturer specifications can change the required quantity.
How to use the Roofing Calculator
Enter the building length, total width, roof pitch, waste allowance and installed tile coverage. The calculator converts the horizontal footprint into the sloping area for both sides of a simple gable roof and rounds the tile quantity up.
Roof area formula
| Result | Formula |
|---|---|
| One slope length | Half building width ÷ cosine of pitch |
| Total roof area | Two slopes × slope length × building length |
| Area including waste | Roof area × (1 + waste percentage) |
| Tile quantity | Area including waste ÷ installed tile coverage, rounded up |
Measure the roof carefully
This tool assumes a rectangular footprint and a symmetrical gable. Add the roof overhang to the length and width before entering measurements if it must be covered. For hips, valleys, dormers and extensions, divide the roof into measurable sections or ask a roofing professional to complete a take-off.
Coverage and waste allowance
Use installed coverage rather than the tile’s overall dimensions because headlap and sidelap reduce effective coverage. Manufacturer fixing instructions take priority. A simple roof may need less waste than a roof with valleys, hips or many cuts.
Permissions and building regulations
Roof replacement work may require building-regulations approval. GOV.UK lists replacement roof coverings on pitched and flat roofs among work that can require approval. Check the official building regulations guidance and contact the relevant local authority before work begins.
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Frequently asked questions
How do I calculate the area of a pitched roof?
Divide half the building width by the cosine of the pitch to get one slope length, then multiply by the building length and by two.
Does the calculator include roof overhangs?
No. Add overhang dimensions to the building measurements before calculating.
How much roofing waste should I allow?
The suitable allowance depends on the material and roof complexity. Confirm it with the supplier or installer.
What does tile coverage mean?
It is the surface area one installed tile covers after allowing for the required overlaps.
Can I use this calculator for a hip roof?
Not as a final take-off. Hip roofs need separate measurements for their roof planes and additional allowance for cuts and fittings.