How to calculate roof area in square metres (UK pitched and hip roofs)
Roof area = plan area x the pitch factor, and the factor is 1 / cos(pitch): 1.221 at 35°, so a 12 x 9 m footprint (108 m² on the plan) is 131.8 m² of roof, 119.2 m² at 25° and 152.7 m² at 45°. Measure the plan on the ground, walls plus overhang, in metres; the pitch in degrees comes off the drawings or a phone inclinometer. A hip roof of the same footprint and pitch has the same area as a gable, only more ridge and hip tiles. The charts give 8 x 6 to 15 x 10 m footprints at 25° to 45°, gable against hip, and the tiles for the example roof.
The formula
Roof area (m²) = length x width of the plan, overhang included, x the pitch factor. The factor is geometry, not a published figure: with the roof rising at an angle θ, each metre of plan is 1 / cos θ metres of slope, so 1.221 at 35°. In US terms the same factor is the square root of 1 + (rise / 12)², which is why the roof pitch calculator shows both. The plan area is the footprint of the roof, not the floor area of the house: a two-storey house of 120 m² has a footprint nearer 60 m².
| Pitch | x:12 | Factor | Roof per 100 m² of plan |
|---|---|---|---|
| 25° | 5.6:12 | 1.103 | 110.3 m² |
| 30° | 6.93:12 | 1.155 | 115.5 m² |
| 35° | 8.4:12 | 1.221 | 122.1 m² |
| 40° | 10.07:12 | 1.305 | 130.5 m² |
| 45° | 12:12 | 1.414 | 141.4 m² |
Roof area chart: common footprints at 25° to 45°
Length along the ridge x width eave to eave, in metres, no overhang (add yours to every side first). The area holds for a gable and a hip of the same footprint and pitch.
| Footprint (m) | Plan (m²) | 25° | 30° | 35° | 40° | 45° |
|---|---|---|---|---|---|---|
| 8 x 6 | 48 | 53 | 55.4 | 58.6 | 62.7 | 67.9 |
| 10 x 7 | 70 | 77.2 | 80.8 | 85.5 | 91.4 | 99 |
| 12 x 8 | 96 | 105.9 | 110.9 | 117.2 | 125.3 | 135.8 |
| 12 x 9 | 108 | 119.2 | 124.7 | 131.8 | 141 | 152.7 |
| 15 x 10 | 150 | 165.5 | 173.2 | 183.1 | 195.8 | 212.1 |
Gable against hip on the same footprint
The 12 x 9 m footprint at 35° as a gable and as a hip roof: same area, different edges, so a different count of ridge and hip tiles and more cutting on the hips. Tiles are Marley Modern (concrete interlocking) at 9.9 per m² (Marley, Modern interlocking concrete roof tile, technical specification, read 2026-09-20) with 10% waste (Rule of thumb, not a tile manufacturer figure, read 2026-09-20); battens at 1,000 / 345 mm gauge per m²; ridge and hip tiles at 457 mm.
| Shape | Roof (m²) | Ridge (m) | Hips (m) | Eaves (m) | Verge (m) | Tiles | Batten (m) | Ridge and hip tiles |
|---|---|---|---|---|---|---|---|---|
| Gable | 131.8 | 12 | 0 | 24 | 22 | 1,436 | 421 | 27 |
| Hip | 131.8 | 3 | 28.4 | 42 | 0 | 1,436 | 421 | 69 |
From area to tiles and cost
Once you have the m², the roof tile calculator turns it into tiles for the type you choose (Marley, Redland, plain clay, slate), metres of batten from the gauge, ridge tiles, the tile cost at your price per tile and the supplied-and-fitted range per m². The UK tiling cost guide has that range for 50, 100 and 150 m² by material, and the roof pitch guide the degrees-to-x:12 chart for other angles.
Frequently asked questions
- How do I calculate the roof area of my house in square metres?
- Measure the length and width of the building on the ground, add the overhang on every side, multiply for the plan area, then multiply by the pitch factor: 1 / cos(pitch in degrees). A 12 x 9 m house at 35°: 108 m² x 1.221 = 131.8 m². An L-shaped house is two rectangles added together; the roof tile calculator takes up to 12 sections in metres and does the rest.
- What is the pitch factor for a 30 degree roof?
- 1.155: 1 / cos(30°). In US terms 30° is 6.93:12. The factors for 25°, 30°, 35°, 40°, 45° are in the first chart; at 45° it is 1.414, the square root of 2, because the rise equals the run. The roof pitch calculator converts any angle.
- Is the roof area the same for a hip roof and a gable?
- Yes, when every slope has the same pitch: both are plan area x the factor, 131.8 m² on the 12 x 9 m footprint at 35°. What changes is the edges: the gable has 12 m of ridge and 22 m of verge (the two gable ends along the slope); the hip has 3 m of ridge plus 28.4 m of hips and eaves all round (42 m), so 69 ridge and hip tiles against 27 at 457 mm each (Marley, Ridge and Hip Tiles, read 2026-09-20). Hips cut more tiles than a gable; the chart applies the same 10% to both, and the figure it is taken from runs to 15% for hips and valleys (IKO, Blueprint for Roofing, read 2026-09-20).
- How many roof tiles do I need for 100 m²?
- Tiles = roof area x tiles per m² x (1 + waste), rounded up: 9.9 per m² for Marley Modern (concrete interlocking) at 345 mm gauge (Marley, Modern interlocking concrete roof tile, technical specification, read 2026-09-20), so 100 m² is 990 tiles laid and 1,089 to order with 10% waste. Plain clay tiles at 100 mm gauge are 60 per m², 6.1 times as many; the UK tiling cost guide has every type.
- What does a roof of that size cost to tile?
- Supplied and fitted, MyJobQuote, "Roof Tile Cost Guide", last updated 30 July 2026, cost per m² supplied and fitted, read 2026-09-20 gives £80 to £150 per m² for concrete tiles, so 131.8 m² of roof is £10,547 to £19,777. Clay and slate are dearer per m²; the UK cost guide has all three for 50, 100 and 150 m².
- How do I measure the pitch of a UK roof in degrees?
- From the drawings (the section shows the angle), from a phone inclinometer laid flat on a tile or on the underside of a rafter in the loft, or from a rise and run: measure how much the rafter rises over a horizontal metre and the roof pitch calculator turns it into degrees and the factor. The tile data sheet gives the minimum pitch for the tile you choose.
More guides
- How much does it cost to tile a roof in the UK? (per m² and per roof)
Supplied-and-fitted cost per m² for concrete, clay and slate (MyJobQuote 2026) for 50 to 150 m² of roof; tiles, battens and tile cost per type at 10% waste.
- How many shingles do I need for a shed? (8x8 to 12x16 chart)
Bundles, underlayment, ridge cap, nails and drip edge for 8x8 to 12x16 ft gable sheds at 4:12 and 6:12 (Owens Corning, IKO); a 10x10 shed takes 4 bundles.
- How much does a metal roof cost? (per square foot and by house size)
$7 to $13 per sq ft of roof installed (HomeAdvisor 2026): $15,652 to $29,069 for a 2,000 sq ft footprint at 6:12. Charts by size, pitch and material.
- How to calculate roof pitch (chart: x:12, degrees, multiplier)
Rise over run: 6:12 is 26.6° and multiplies the footprint by 1.118. Charts for 1:12 to 12:12, degrees to x:12 (UK, metric), rise and rafter length per 12 ft of run.
Last updated 2026-09-22. Planning quantities from manufacturer coverage figures and published cost ranges, each with its source and date: measure the roof and follow the product data sheet. Not a structural assessment.