How Many Slates Do You Need per Square Metre?
Every week, an enquiry lands in our inbox that reads the same way: roof area, a slate size, and the question — “how many pieces do I order?” It sounds like a maths problem, and that is exactly where the trouble starts, because people solve it like a maths problem and the answer comes out wrong.
This page is written for buyers, estimators and contractors who want the right number the first time. No lecture, just the three mistakes that cause 80% of under-orders we see, the formula that fixes them, and a coverage table you can quote from today.
The First Mistake: Dividing Roof Area by Tile Area
The most common wrong answer goes like this: “The roof is 100 m², the slate is 0.125 m² each, so I need 800 slates.”
Sound familiar? The math is fine and the result is useless, because a slate roof is not a jigsaw where tiles sit edge to edge. Every row overlaps the row below it by a strip called the headlap — the distance the upper tile covers the lower one. That overlap is what keeps rain out, and it means part of every tile is doing nothing for coverage. Under the simple maths, a 500 × 250 mm slate looks like it should cover 0.125 m² (8 tiles/m²). In reality, with a 75 mm headlap, the same tile is doing 19 tiles per m². More than double. Order at 8 per m² and you are short by a full pallet.
Here is the exact number that surprises most people: at 500 × 250 mm with a 75 mm headlap, the effective visible area of each tile is about 0.053 m², not 0.125 m². The rest is hiding under the tile above it.
The Mistake: Believing There Is One Answer per Size
Ask two slaters “how many 500 × 250 slates per m²?” and you might get 18, 19 or 20 — all correct. The size is the same; the headlap differs. On a 30° roof, 75 mm headlap is standard and 500 × 250 works out at 18.8 tiles/m². On a shallower roof where the headlap must rise to 100 mm, the same tile needs 20 tiles/m². That is why this page and every supplier’s quote should ask one thing back: what is your headlap? And headlap comes from pitch — the relationship is covered in detail in how roof pitch affects slate selection, but the short version: lower pitch, bigger headlap, more tiles.
So the correct way to think about any slate size is not “how big is one tile” but “how much of the roof does one tile show after overlap” — the technical terms are gauge (the horizontal spacing, row to row) and exposure (the visible length of each tile). For square-butted slates, gauge equals exposure:
Exposure = (slate length − headlap) ÷ 2
For 500 × 250 at 75 mm headlap: (500 − 75) ÷ 2 = 212.5 mm of exposed length. That 212.5 mm, multiplied by the 250 mm width, is what you really cover per tile.
Mistake 3: Measuring the Floor Area Instead of the Roof Area
People measure under the roof, not the roof. A 100 m² house footprint with a 35° gable roof is not a 100 m² slate job — the sloped planes add up to about 20–25% more surface area than the floor plan. If you are working from a floor area or a plan drawing, multiply by the slope factor:
- 25° pitch: roof area ≈ floor area × 1.10
- 30° pitch: × 1.15
- 35° pitch: × 1.22
- 45° pitch: × 1.41
And that is before you touch hips, valleys, verges and ridges, where cut tiles go to waste. Rule of thumb used across the industry — and by our own dispatch office — is add 5–10% on top of the calculated count, more for complex roofs. A roof with three hips and two valleys eats more than a plain gable; take the higher end there.
Coverage Table: Slates per Square Metre, Common Sizes
Below is the table we actually quote from in Jiujiang. It assumes square-butted slates, nailed fixing, and standard 75 mm headlap unless otherwise noted — the most common configuration for roofs at 25–35°. The final column is what you order per m² of roof surface:
| Slate size (mm) | Headlap (mm) | Exposure (mm) | Tiles per m² | Notes |
|---|---|---|---|---|
| 300 × 200 | 75 | 112.5 | ≈ 44 | Traditional small format, steep pitches |
| 300 × 250 | 75 | 112.5 | ≈ 36 | Common on low-pitch work |
| 400 × 250 | 75 | 162.5 | ≈ 25 | Popular mid-size |
| 400 × 250 | 100 | 150 | ≈ 27 | At lower pitch (20–25°) |
| 500 × 250 | 75 | 212.5 | ≈ 19 | The export default — most orders |
| 500 × 250 | 100 | 200 | ≈ 20 | At low pitch, roughly 1 extra tile/m² |
| 600 × 300 | 75 | 262.5 | ≈ 13 | Large format, fast to lay |
| 600 × 400 | 75 | 262.5 | ≈ 9.5 | Very large format |
This assumes regular (equal-width) slates with square edges. If you are working with random-width or textured roofing slates, the formula still works per tile, but you calculate with the actual width of each piece instead of a fixed one — which is exactly why random-width roofs are quoted by weight and square footage, not by piece count.
The full size range with more dimension options is on our roofing slate sizes & standard dimensions page, including the 400 × 400 and 500 × 300 options some markets use.
Worked Example: A Real Roof, Calculated from Start to Finish
Let’s put the three steps together on a concrete job so you can copy the method.
Step 1 — find the roof surface area. A house with a footprint of 80 m² and a 35° pitched gable roof: 80 × 1.22 ≈ 98 m² of actual slate surface.
Step 2 — pick the slate and read the coverage. The spec calls for 500 × 250 mm at 75 mm headlap (fine for 35°): 19 tiles/m². 98 × 19 = 1,862 slates.
Step 3 — add waste. A straightforward gable roof with two verges: 5% is honest. 1,862 × 1.05 ≈ 1,955. Round up to a round quantity: order ~2,000 slates.
Had you divided roof area by tile area and ordered 800, you would be standing on a half-finished roof asking for a second shipment — and paying freight twice. The price of doing this on paper first is ten minutes.
If you want to skip the arithmetic altogether, our sales team runs this exact calculation for every enquiry — send the roof drawings or dimensions through the contact form and the quote comes back with the piece count and the packed square metres stated on it.
Ordering Tips That Save Real Money
Four habits that separate smooth jobs from “the container is short” calls:
- Order by piece count and by square. A proper quote states both. If a supplier only gives “per m²” without the headlap it assumes, ask the question — that is where misunderstandings start.
- Book the same batch for the whole roof. Natural slate varies slightly from batch to batch; two orders six months apart can sit side by side looking different. One order, one batch, one roof. If you need a small future repair pack, take a few pieces from the same batch and set them aside in the loft.
- Ask about cutting waste on the side before ordering. Tiles for hips, valleys, eaves and chimneys arrive whole — the cutting happens on site, and the offcuts are yours to pay for. The 5–10% allowance covers exactly this; roofs with a lot of detailing should lean toward 10%.
- Know what “per m²” means to the supplier. On 500 × 250 mm there is a real difference between 18.8 tiles (75 mm headlap) and 20 tiles (100 mm headlap). One extra tile per m² on a 300 m² roof is 300 tiles — a whole extra pallet of material you did not plan to pay freight on.
How Much Does That Work Out to in Weight?
Because someone always asks: 500 × 250 mm slate at 5 mm thickness weighs about 1.6–1.7 kg per tile, or roughly 31–32 kg per m² laid at standard exposure. Step up to 6 mm and you are near 38 kg/m². That is well within what most trusses are designed to hold, but if you are replacing a lighter covering or pushing headlap up on a low roof, it is worth confirming the structure before the tiles arrive. More detail lives on our slate weight guide and the thickness page.
Frequently Asked Questions
How many roof slates are there per square metre?
It depends on the size and the headlap. As a practical guide: 300 × 200 mm runs about 44 tiles/m², 400 × 250 mm about 25, 500 × 250 mm about 19, and 600 × 300 mm about 13 — at a standard 75 mm headlap. Raising the headlap to 100 mm adds roughly one tile per m² on the 500 × 250 size.
How do I calculate how many slates I need for my roof?
Step 1: roof surface area (roof plans usually give the slope area; if you only have the footprint, multiply it by the slope factor, about 1.15 at 30° pitch). Step 2: divide 1,000,000 by (exposure × slate width) in millimetres, where exposure = (slate length − headlap) ÷ 2. Step 3: multiply area × tiles per m² and add 5–10% waste for cutting.
How many 500 x 250 slates do I need per square metre?
About 19 tiles/m² at 75 mm headlap, or 20 at 100 mm headlap. If your roof pitch is 25–35°, plan on 19; if it is around 20–25° and the roofer has asked for a bigger headlap, use 20.
How many slates do I need for a 100 m² roof?
Assuming a simple gable roof of 100 m² of actual slate surface: roughly 1,900 tiles of 500 × 250 mm (at 19/m²), plus 5–10% waste, so order around 2,000–2,100 pieces. Larger 600 × 300 mm format drops that to about 1,300–1,400 pieces; small 400 × 250 mm raises it to about 2,600–2,800.
What percentage of extra slates should I order?
5–10% is the usual allowance. Plain gable roofs with few details sit at the 5% end; roofs with hips, valleys, chimneys and dormers should budget 10% because every cut produces an unusable offcut. If the roof has unusual detailing or you plan future repairs from the same batch, order a few extra tiles and keep them in storage.
Related Reading
- Roofing Slate Sizes & Standard Dimensions — the full size chart with coverage and pieces per square metre.
- How Roof Pitch Affects Slate Selection — where headlap numbers come from and how pitch changes your count.
- Roofing Slate Weight Guide — per-tile and per-m² weights to check before the container ships.
- Roofing Slate Installation Guide — batten spacing and fixing pattern behind the numbers.
- Roofing Slate Thickness Explained — when 5 mm is fine and when you need 6 mm+.
Send us your roof drawings and we’ll return the piece count, not a guess.
We produce EN 12326 / ASTM C406 tested roofing slate in Jiujiang, China, cut on CNC to tight tolerances and every tile inspected before packing. Tell us the roof area, pitch and your preferred size — we’ll confirm the number of pieces, packed m² and freight roughly in one reply.
Request a quote or samples →