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Roofing calculator
For a standard dual-pitch (gable) roof. Enter the span, length and pitch to get the rafter length and roof area, then the tiles, battens, membrane and ridge tiles to order.
- Roof area (both slopes)
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- Rafter length
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- Ridge height above plate
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- Tiles / slates
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- Tile battens
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- Membrane (50 m² rolls)
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- Ridge tiles
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How the roof area is worked out
Each slope of a gable roof is a rectangle. Its width is the roof length along the ridge, and its height is the rafter length. The rafter length comes from the pitch: half the span (plus the eaves overhang) divided by the cosine of the pitch. For an 8 m span at 35°, half the span is 4 m and 4 ÷ cos 35° = 4.88 m. Two slopes of 4.88 × 10 m give 97.7 m².
Always work from the sloping area, not the plan area. At 35° the roof is 22% larger than the floor below it; at 45° it is 41% larger.
Tiles, slates and batten gauge
| Covering | Per m² | Batten gauge | Usual minimum pitch |
|---|---|---|---|
| Plain tiles 265 × 165 mm | about 60 | 100 mm | 35° |
| Natural slate 500 × 250 mm, 100 mm lap | about 20 | 200 mm | about 25° |
| Standard interlocking tiles | about 16 | about 310 mm | about 22.5° |
| Large-format interlocking tiles | about 10 | about 345 mm | about 17.5° |
These are typical figures. Every tile has its own data sheet with the exact coverage, gauge and minimum pitch, and in exposed areas the headlap often has to increase, which means more tiles. Use the data sheet figures for your order.
Battens per m² are simply 1 ÷ gauge: at a 100 mm gauge you need 10 m of batten per m² of roof, at 345 mm only about 2.9 m.
Worked example
A 10 m long house with an 8 m span at 35°, a 300 mm overhang and interlocking tiles: the rafter is about 5.25 m including the overhang, so the roof is about 105 m². With 10% waste that is about 1,850 tiles, 375 m of batten, 3 rolls of membrane and 23 ridge tiles.
Things the calculator does not include
- Hips and valleys: add hip or valley tiles and extra cutting waste (use 15% waste on hipped roofs).
- Verge tiles or dry-verge kits, eaves comb filler, fixings and clips.
- Counter battens on warm roofs, and lead or GRP flashings around chimneys and windows.
- Rafters and structural timbers: those need span tables or an engineer.
Re-roofing a whole roof usually needs Building Regulations approval, especially if the new covering is heavier than the old one. Speak to Building Control or use a competent-person scheme roofer.
Questions
How do I measure roof pitch?
Use a pitch gauge or a phone angle app on a rafter in the loft, or measure the rise and half-span: pitch = arctan(rise ÷ half-span).
How many roof tiles do I need per m²?
About 60 plain tiles, 20 natural slates (500 × 250), 16 standard interlocking or 10 large-format interlocking tiles per m², before waste.
How much waste should I allow?
About 10% on a simple gable roof, 15% or more on hipped roofs, valleys and dormers where many tiles are cut.
What size is a roll of breathable membrane?
Commonly 1 m × 50 m (50 m²) or 1.5 m × 50 m. Allow about 15% extra for horizontal and vertical laps.
Results are estimates for planning and ordering. Check quantities with your supplier and product data sheets, and anything structural or regulated with a qualified professional. See our terms.