How many roof tiles do I need? Using the calculator
- 1Measure the roof: slope length (eaves to ridge) × eaves length for each slope, or the plan area and pitch, or type the roof area.
- 2Choose your tile or slate. Its gauge, cover width, ridge tile, fixings and weight fill in from the maker's figures.
- 3For slates, check the headlap against the maker's table for your pitch and exposure; for interlocking tiles, check the gauge range.
- 4Enter the ridge and hip length and keep about 5 % spare (10–15 % for hips and valleys).
- 5Read the tiles to buy, the battens, ridge tiles, fixings and the weight on the roof.
How to calculate roof tiles and slates (formula)
- 01Slates and plain tiles (double lap): gauge = (length − headlap) ÷ 2
- 02Interlocking tiles (single lap): gauge = length − headlap, within the maker's range
- 03Tiles per m² = 1 ÷ (gauge × cover width)
- 04Tiles = roof area × tiles per m² × (1 + spare %), rounded up
- 05Battens = roof area ÷ gauge × (1 + spare %)
- 06Ridge tiles = ridge and hip length ÷ length one ridge tile covers, rounded up
- 07Roof area = slope length × eaves length × slopes, or plan area ÷ cos(pitch)
Every course of tiles hangs on one batten, so the batten gauge — the distance between battens — is how much of each tile you see up the slope. Interlocking tiles overlap the course below once: their gauge is the tile length minus the headlap. Slates and plain tiles are double-lapped: each one also covers the head of the one two courses down, so the gauge is (length − headlap) ÷ 2. A 500 mm slate with a 100 mm headlap has a 200 mm gauge.
One tile then covers the gauge × its cover width, and the number per m² is one over that. A 500 × 250 mm slate at a 200 mm gauge covers 0.2 × 0.25 = 0.05 m², so 20 slates make a square metre — Cupa Pizarras' table gives the same 20.0. Every metre of slope needs one batten per course, so battens in metres = roof area ÷ gauge.
Interlocking tiles come with a gauge range that depends on the pitch, and the roofer divides each slope into equal courses within it. The presets use the tile length minus 100 mm, the low-pitch end, which gives the maker's highest figure per m² — for Marley Modern 10.7 per m², against 9.9 at the longest gauge.
Roof tile calculation example
Two slopes of 5 m × 10 m make 100 m² of roof.
Redland 49 tiles at a 282 mm gauge and 200 mm cover width give 17.73 per m², so with 5% spare you buy 1,862 tiles, hung on 372.3 m of battens.
A 10 m ridge takes 23 ridge tiles of 450 mm, the tiles need 1,862 nails or clips, and the covering weighs 5.1 t.
Tips before you buy
- Measure the slope from the bottom edge of the eaves to the ridge and split it into equal courses: the gauge must stay within the maker's range for your pitch.
- Buy all tiles from the same batch and mix them from several pallets as you lay, so colour differences spread evenly.
- Check the headlap for your exposure: in exposed coastal or upland areas, slates need a longer headlap — and more slates per m².
- Keep some spare tiles or slates after the job: an exact match is hard to find years later.
- A heavier covering (clay or slate instead of concrete or composite) may need stronger rafters — have the roof checked before you change.
Frequently asked questions
How many roof tiles do I need per m²?
It depends on the tile and the gauge: large interlocking concrete tiles take about 10 per m² (Marley Modern 9.9–10.7), small ones like Redland 49 about 16–18, plain tiles 60, and natural slates 13 (600 × 300 mm) to 20 (500 × 250 mm). Multiply by the roof area and add about 5 % spare.
How do I work out the batten gauge for slates?
Take the headlap from the slate length and halve it: (500 − 100) ÷ 2 = 200 mm for a 500 × 250 mm slate with a 100 mm headlap. The headlap comes from the maker's table for your pitch and exposure, for example 80–100 mm at 30°.
How many slates per m²?
Divide 1 by the gauge times the slate width, both in metres. 500 × 250 mm at 100 mm headlap: 1 ÷ (0.2 × 0.25) = 20 per m². 600 × 300 mm at 100 mm headlap: 1 ÷ (0.25 × 0.3) = 13.3. Welsh Slate adds a 5 mm gap to the width, which gives 19.6 and 13.1.
How many Redland 49 tiles per m²?
Redland's data sheet gives 16.3 per m² at the longest gauge of 307 mm and 17.7 per m² at 282 mm, with a cover width of 200 mm. This calculator uses 282 mm so a low-pitched roof does not run short; type your own gauge if you know it.
How many composite slates do I need?
For DaVinci Single-Width slates (12 × 18 in) at a 7½ in exposure, about 7.1 bundles of 22 per roofing square — roughly 156 slates per 100 ft². The calculator works it out from the size, a 3 in headlap and the 3/8 in gap; add about 5–10 % for cutting.
How many ridge tiles per metre?
Divide the ridge length by the length one ridge tile covers: 450 mm ridge tiles take about 2.2 per metre, and Braas and Edilians count 2.5 per metre. Add the hips if they are covered with ridge tiles too.
How heavy are roof tiles per m²?
Roughly: composite slate 14 kg/m², fibre-cement slate 20, natural slate 33–38, concrete interlocking tiles 45–54, clay plain tiles 64–77. Use the maker's figure when you check whether the rafters can carry a new covering.
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Sources
- Welsh Slate – slate coverage formula (1,000,000 ÷ ((length − headlap) ÷ 2 × (width + 5 mm)))
- Welsh Slate – minimum headlap by slate size, pitch and exposure
- Cupa Pizarras – batten gauge for slates: (length − lap) ÷ 2
- Cupa Pizarras – slates per m² and weight per slate (500 × 250: 1.889 kg; 600 × 300: 2.495 kg)
- Marley – setting out tile battens (gauge = tile length − headlap)
- Marley Modern – 420 × 330 mm, cover width 292 mm, 9.9 / 10.7 tiles per m²
- Redland 49 technical data sheet – 382 × 226 mm, cover width 200 mm, 16.3–17.7 per m², 47–51 kg/m²
- Marley ridge and hip tiles – 450 mm (clay) and 457 mm (concrete)
- BS 5534 fixing requirements, summarised by Marley (Barbour)
- Cedral Thrutone fibre-cement slate – 600 × 300 mm at 100 mm lap: 250 mm gauge, 13.4 per m², 20.4 kg/m²
- National Slate Association – estimating slate quantities (sold by the square at a 3 in headlap)
- North Country Slate – standard sizes and slates per square (18 × 10 in: 192)
- DaVinci Roofscapes technical sheet – Single-Width slate bundles, pieces and weight per square
Formulas last reviewed: September 30, 2026 ·How we check our formulas