Roof area and tile calculator
Turn the plan size and pitch of a roof into its surface area, then into the number of tiles or slates and the metres of batten needed, using manufacturers' published coverage figures. A guide for comparing quotes, not an order list.
Roof surface and materials
Field tiles and battens only. Ridge and hip tiles, verge pieces, underlay, fixings and ventilation are extra, and a roofer will measure the roof itself before ordering.
How the roof area calculation works
A pitched roof is bigger than the footprint it covers, and the steeper the pitch the bigger the difference. The calculator takes the plan area, which is the area seen from directly above, and divides it by the cosine of the pitch. That is simple geometry: a slope rises along the hypotenuse of a right-angled triangle, so its length is the horizontal distance divided by the cosine of the angle. The same factor applies to every slope of the same pitch, so it works for gable, hipped and lean-to roofs alike provided the pitch is the same throughout.
A worked example: a roof covering 8m by 6m has a plan area of 48 m². At a 35° pitch the factor is 1 ÷ cos 35°, which is 1.221, so the roof surface is about 58.6 m². Covered in concrete plain tiles at 60 per m² that is 3,516 tiles, with 586 linear metres of batten at 10.0 per m². Switch to the Marley Modern interlocking tile at 75mm headlap and the same roof needs 9.9 x 58.6, about 581 tiles, which becomes 639 with a 10% wastage allowance added.
| Pitch | Surface area factor |
|---|---|
| 0° | x 1.000 |
| 15° | x 1.035 |
| 22.5° | x 1.082 |
| 25° | x 1.103 |
| 30° | x 1.155 |
| 35° | x 1.221 |
| 40° | x 1.305 |
| 45° | x 1.414 |
| 50° | x 1.556 |
What changes the result
Pitch
Pitch does the heavy lifting. Between 30° and 45° the surface area grows from 1.155 times the plan area to 1.414 times, which is a quarter more tiles for the same footprint. Measure the pitch with an inclinometer app against a rafter in the loft or a straight edge on the tiles rather than guessing from the ground.
Tile format, gauge and headlap
Small plain tiles need many more pieces per square metre than large interlocking tiles because every tile overlaps the two courses below it. The gauge, which is the batten spacing, follows from the tile length and the headlap, and a bigger headlap means a shorter gauge and more tiles. That is why the Marley Modern is listed twice: 9.9 per m² at 75mm headlap and 10.7 per m² at 100mm, the headlap needed for lower pitches. The Cambrian interlocking slate changes gauge at 25°, from 250mm above it to 225mm below.
Roof shape and wastage
Hips and valleys are cut on the diagonal, dormers and chimneys interrupt the courses, and some tiles break in handling. The manufacturer tables the calculator uses are described as approximate values for estimating, with an allowance to be made for cutting and wastage, but none of them gives a percentage. The calculator therefore starts at 0% and lets you enter the figure a roofer suggests for the roof's shape.
Minimum pitch
Every product has a minimum pitch below which it will not keep the rain out. The calculator warns when the pitch entered is below the figure published for the chosen product, but exposure, rafter length and the underlay all affect the real minimum, so a roofer should confirm the specification.
Common mistakes and tips
- Measuring the floor instead of the roof. The roof overhangs the walls at the eaves and often at the verges, so the plan area is larger than the floor plan.
- Mixing up pitch and slope length. If you already know the rafter length, multiply it by the eaves length for each slope instead of using the pitch factor.
- Comparing tiles per m² at different headlaps. Check that quotes use the same gauge, or the tile counts will not match.
- Forgetting the extras. Ridge, hip and verge components, underlay, battens, fixings, ventilation and scaffolding can be a large share of a re-roof.
- Ordering from the calculator. Suppliers sell by the pallet or pack, and a roofer will measure the roof before ordering.
When to get a professional
Replacing a roof covering is a job for a roofer, who will check the battens, underlay, flashings and ventilation at the same time and advise whether the structure can take a heavier tile. Changing from slate to concrete tile, for example, can add weight that the rafters were never designed for. The calculator is most useful for sanity-checking the tile quantities on competing quotes and for understanding why one tile costs more to lay than another.
Re-roofing or repairing? Compare well reviewed local roofers and ask them to measure the roof.
Find a top rated roofer near youHow this is worked out
- Plan area (m²) = plan length x plan width, or entered directly.
- Roof surface (m²) = plan area ÷ cos(pitch). Geometry, not a sourced figure.
- Tiles or slates = roof surface x the manufacturer's tiles per m² for the chosen product, x (1 + wastage % / 100), rounded up.
- Battens (linear m) = roof surface x the manufacturer's battens per m².
| Product | Per m² | Battens per m² | Gauge | Minimum pitch |
|---|---|---|---|---|
| Clay plain tile (265 x 165mm) | 60 | 10.00 m | 100mm gauge, 65mm headlap | 30° |
| Concrete plain tile (267 x 168mm) | 60 | 10.00 m | 100mm gauge, 65mm headlap | 35° |
| Concrete interlocking tile (420 x 330mm), 75mm headlap | 9.9 | 2.90 m | 345mm maximum gauge, 75mm headlap | 22.5° |
| Concrete interlocking tile (420 x 330mm), 100mm headlap | 10.7 | 3.10 m | 100mm headlap | 17.5° |
| Fibre cement slate 500 x 250mm, 100mm lap | 19.6 | 5.00 m | 200mm gauge, 100mm headlap | 23.5° |
| Fibre cement slate 600 x 300mm, 110mm lap | 13.6 | 4.08 m | 245mm gauge, 110mm headlap | 23° |
| Interlocking slate (Cambrian), 250mm gauge, 25° and over | 13.3 | 4.00 m | 250mm gauge | 25° |
| Interlocking slate (Cambrian), 225mm gauge, below 25° | 14.8 | 4.00 m | 225mm gauge | 15° |
- Marley, Acme Single Camber clay plain tile (product page, technical data) (checked 9 November 2026). Size 265 x 165mm; covering capacity 60 tiles/m² at 100mm gauge (roof) with 65mm headlap; battens 10.0 lin.m/m² at 100mm gauge; minimum pitch 30°.
- Marley, Concrete plain roof tile (product page, technical data) (checked 9 November 2026). Size 267 x 168mm; covering capacity 60 tiles/m² at 100mm gauge with 65mm minimum headlap; battens 10.0 lin.m/m²; minimum pitch 35°.
- Marley, Modern concrete interlocking roof tile (product page, technical data) (checked 9 November 2026). Size 420 x 330mm; covering capacity 9.9 tiles/m² at 75mm headlap and 10.7 tiles/m² at 100mm headlap; maximum gauge 345mm; battens 2.9 lin.m/m² at 75mm headlap and 3.1 at 100mm; minimum pitch 22.5° (smooth, 75mm headlap) or 17.5° (smooth, 100mm headlap).
- Marley Eternit (now Cedral), Fibre cement slates fixing guide, Table 1: slate dimensions (June 2013, checked 9 November 2026). 500 x 250mm slate at 100mm lap: 200mm batten gauge, 5.00 battens/m², 19.6 slates/m²; 600 x 300mm at 110mm lap: 245mm gauge, 4.08 battens/m², 13.6 slates/m². The guide describes these as approximate values for estimating and says the minimum pitch for both sizes at 100mm lap is 23.5° (29.5° in severe exposure).
- BMI Redland, Cambrian slate (product page and technical data sheet) (checked 9 November 2026). Interlocking slate, 13.3 slates/m² at 250mm gauge (25° and over) and 14.8 slates/m² at 225mm gauge (below 25°); battens 4.0m/m² at maximum gauge; minimum pitch 15° at 75mm headlap or 25° at 50mm headlap.
Related guides
Roof area and tile questions
How do I calculate the area of a pitched roof?
Measure the plan area the roof covers from above (length by width, including the overhang at the eaves), then divide by the cosine of the pitch. At 35° the cosine is 0.819, so a 48 m² plan area becomes 48 ÷ 0.819, about 58.6 m² of roof surface. The calculator does this for you and shows the factor it used.
How many roof tiles do I need per square metre?
It depends on the tile. Marley's data gives 60 per m² for clay and concrete plain tiles at 100mm gauge, and 9.9 per m² for its Modern interlocking concrete tile at 75mm headlap. For fibre cement slates, the Marley Eternit fixing guide gives 19.6 per m² for 500 x 250mm slates at 100mm lap and 13.6 per m² for 600 x 300mm at 110mm lap. The Redland Cambrian interlocking slate is 13.3 per m² at 250mm gauge.
How much extra should I allow for cuts and wastage?
The manufacturer data this calculator uses gives coverage as approximate values for estimating and says an allowance should be made for cutting and wastage, but does not put a figure on it. Hips, valleys and dormers create more cuts than a plain gable roof. The wastage field is left at 0% so you can set the allowance your roofer suggests for the roof shape.
What is the minimum pitch for roof tiles?
Each product has its own. From the manufacturer pages checked, Marley's clay plain tile is 30° and its concrete plain tile 35°, while the Marley Modern interlocking tile goes down to 22.5° at 75mm headlap and 17.5° at 100mm headlap for the smooth finish. Fibre cement slates at 100mm lap are quoted at 23.5°, and the Cambrian interlocking slate at 15° with a 75mm headlap. The calculator flags a pitch below the product's minimum.
How many battens do I need for a roof?
Batten length per square metre follows the gauge. Marley gives 10.0 linear metres per m² for plain tiles at 100mm gauge and 2.9 per m² for the Modern tile at 75mm headlap. The Marley Eternit guide gives 5.00 per m² for 500 x 250mm slates and 4.08 for 600 x 300mm, and BMI Redland gives 4.0 per m² for the Cambrian at maximum gauge. Multiply by the roof surface area.
Does the calculator include ridge, hip and verge tiles?
No. It counts the main field tiles or slates and the battens. Ridge and hip tiles are priced by the metre, verge details depend on whether dry verge or slate-and-a-half pieces are used, and the underlay, fixings and ventilation are separate. A roofer's quote will cover all of these.
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