The short answer
A square metre of roof takes roughly 12 to 20 slates. A 600 x 300 mm slate at an 80 mm headlap needs about 13, and a 500 x 250 mm slate at the same lap needs about 19. Larger slates need fewer, a longer lap needs more, and the order rises again for waste, cut slates and the detailing around valleys and hips.
How the number is worked out
Slates are laid in courses, each overlapping the one below. The overlap is the headlap, measured as how far a slate reaches over the one two courses below. A deeper headlap keeps water out in harder weather, but it uses more of each slate.
The part of each slate left showing is the gauge, and it sets the spacing of the battens, which are the timber laths nailed over the rafters. Gauge is found by taking the slate length, subtracting the headlap and halving the result. The number of slates per square metre is then one divided by the gauge multiplied by the slate width, with both in metres.
For a 500 x 250 mm slate with an 80 mm headlap, the gauge is 210 mm, so each slate covers 0.21 by 0.25 metres, or about 0.0525 square metres. One square metre needs about 19 slates. The method is simple arithmetic, and it is the same one a supplier uses.
Worked counts for two common sizes
The table below applies the method to the two sizes for which we hold sourced headlap figures. The headlaps come from a slate manufacturer's guidance for moderate exposure, and they change with pitch.
| Slate size | Pitch and headlap | Gauge | Slates per square metre |
|---|---|---|---|
| 500 x 250 mm | 25 degrees, 95 mm | 202.5 mm | about 20 |
| 500 x 250 mm | 30 degrees, 80 mm | 210 mm | about 19 |
| 500 x 250 mm | 45 degrees, 55 mm | 222.5 mm | about 18 |
| 600 x 300 mm | 30 degrees, 80 mm | 260 mm | about 13 |
| 600 x 300 mm | 45 degrees, 55 mm | 272.5 mm | about 12 |
Notice that a steeper roof needs fewer slates, because the headlap can be shorter. A shallow roof at 25 degrees needs the deepest lap and the highest count. The figures are working numbers, and the slate manufacturer's current guidance and BS 5534 govern the real specification for your exposure.
Why the real order is larger than the sum
The count per square metre covers the plain field of the roof only. The order needs more than that.
- Cut slates. At the edges, hips, valleys and around chimneys and roof windows, slates are cut to shape and the offcuts cannot all be reused.
- Breakage. Slate is a stone that can crack when it is split, carried or nailed, and some slates are rejected for flaws at the roof.
- Courses at the eaves and ridge. At the eaves the bottom course is doubled, and the top course is shortened to suit the ridge, so these rows use slates in a different way from the rest.
- Spares. A few extra slates left with the roof are worth having for later repairs, because the same size and tone may not be easy to find years afterwards.
We do not use a fixed allowance as a rule of thumb. The right figure depends on the number of valleys, hips and openings, and we work it out from the roof drawing and the survey photographs.
What slate size does to the price
Larger slates need fewer to cover the same area, so there are fewer to fix, and fewer nails and fewer courses to lay. That can save time, but it does not decide the price alone. Large slates are also usually dearer each, and the heavier piece needs a sound roof structure and suitable battens.
On an older Bath house, the choice is often not free. The original slates were cut to a particular size, and changing to a larger one alters the lines of the roof and the shadow at each course. Staying with the original size preserves the old courses, and that counts for more on a listed house or inside a conservation area. Our answer on whether natural slate is worth the extra cost looks at the wider value question.
Typical UK ranges reported by MyBuilder (2026) put installed slate roofing at £100 to £180 per square metre, with the origin and grade of the slate among the main drivers. Your fixed quote follows the survey.
Man-made slate counts differently
Fibre-cement slate is made in set sizes, with its own manufacturer's tables for headlap and pitch. A common fibre-cement size is 600 x 300 mm, and the manufacturer's older guidance gives laps such as 100 mm and 110 mm, depending on pitch and exposure. Applying the same method to a 600 x 300 mm slate at a 100 mm lap gives a gauge of 250 mm and about 13 slates per square metre.
The rules are different from natural slate in other respects too. Fibre-cement slate is usually fixed with rivets and nails, and its minimum pitch differs from that of natural slate. Our fibre-cement slates page goes further. Always use the manufacturer's current table for the product and the exposure, because older fitting guides can change.
Using the count on your own roof
Measure the area of each slope, not the footprint of the house. A pitched roof is bigger than the floor beneath it, and the difference grows as the pitch steepens. Add the slopes, then multiply by the slates per square metre for the size and lap you have chosen.
Treat the result as a first estimate rather than an order. The lap changes with pitch and exposure, the cut slates change the total, and a roof that is being re-slated may reuse a share of the existing slate. We take measurements and photographs of your own roof at the free survey, and the schedule of slates is part of the fixed price we give in writing.
If you are still deciding how long the roof should last, see how long a slate roof lasts for what usually ends one. The wider slate roofing service page has the rest.
