Size, Headlap and Coverage Table
In one line: for a 500×250 mm slate at 75 mm headlap, the gauge (exposure) is 212.5 mm and coverage is 18.8 slates per m² (budget on 19) — or about 175 slates per square. Change the headlap to 100 mm and the same slate covers 20.0/m².
Roofing slate is sold by dimension, fixed by headlap, and ordered by coverage. This page is the reference that ties all three together — the table is the table, the two formulas are the only maths, and the worked examples show how to read any row in both metric and imperial.
Every figure below uses the same method that a slater works with on site. Gauge equals (slate length − headlap) ÷ 2; slates per m² equals 1,000,000 ÷ (gauge × width). The rest is reading a row.
The Coverage Table at a Glance
Gauge is the exposed length of each slate, the distance from the bottom edge of one course to the bottom edge of the course above. Headlap is the overlap a slate makes over the slate two courses below. Both are millimetres, both are set before the first batten goes up.
| Slate size (mm) | Headlap (mm) | Gauge / exposure (mm) | Slates per m² | Per square (approx) |
|---|---|---|---|---|
| 500×300 | 100 | 200.0 | 16.7 | 155 |
| 600×300 | 75 | 262.5 | 12.7 | 118 |
| 500×250 | 75 | 212.5 | 18.8 | 175 |
| 500×250 | 100 | 200.0 | 20.0 | 186 |
| 450×250 | 75 | 187.5 | 21.3 | 198 |
| 400×250 | 75 | 162.5 | 24.6 | 229 |
| 400×200 | 75 | 162.5 | 30.8 | 286 |
| 300×200 | 75 | 112.5 | 44.4 | 413 |
| 300×150 | 75 | 112.5 | 59.3 | 551 |
All figures at 75 mm or 100 mm headlap, the two most common fixing specs in the UK and Europe. The "per square" column uses the trade square of 100 ft² (≈ 9.29 m²), rounded to the nearest whole slate.
Why Headlap Is the First Thing to Set
Headlap is not a style choice — it is the overlap that keeps wind-driven rain, snow and debris out of the roof void. Too little headlap and the roof leaks under a driving storm; too much and the exposed gauge shrinks, so every m² needs more slates. The number is set by the roof pitch, the site exposure and the slate length, and usually fixed by the roofer or the local building regs before a single batten is cut.
The two rules that drive the table:
- Gauge = (slate length − headlap) ÷ 2. The slate is long enough to cover two courses, so the visible part is half of what is left after the overlap.
- Slates per m² = 1,000,000 ÷ (gauge × width). The gauge and width multiply to the area one slate covers, and a square metre is 1,000,000 mm².
That is the whole calculation. Every figure in the table above comes from these two lines, and every supplier quote should agree with them — if a quote shows a different gauge or a different slates-per-m² for the same size and headlap, the quote is wrong or the headlap is different.
How to Read a Row: Two Worked Examples
Example 1 — 500×250 mm at 75 mm headlap
Gauge: (500 − 75) ÷ 2 = 212.5 mm. Each course shows 212.5 mm of slate.
Coverage: 1,000,000 ÷ (212.5 × 250) = 18.82 → 18.8 per m². Round to 19 for ordering.
Per square: 18.8 × 9.29 ≈ 175 slates per 100 ft².
Example 2 — 600×300 mm at 75 mm headlap
Gauge: (600 − 75) ÷ 2 = 262.5 mm.
Coverage: 1,000,000 ÷ (262.5 × 300) = 12.70 per m² — 12.7. Budget on 13.
Per square: ≈ 118.
UK & European Sizes at 100 mm Headlap
100 mm is the typical spec for exposed and low-pitch sites in the UK. The same size covers less, so the numbers change — worth bookmarking because it changes the whole order quantity:
| Slate size (mm) | Headlap (mm) | Gauge (mm) | Slates per m² | Per square |
|---|---|---|---|---|
| 500×300 | 100 | 200.0 | 16.7 | 155 |
| 500×250 | 100 | 200.0 | 20.0 | 186 |
| 400×250 | 100 | 150.0 | 26.7 | 248 |
| 450×225 | 100 | 175.0 | 25.4 | 236 |
| 300×200 | 100 | 100.0 | 50.0 | 464 |
Note how a jump from 75 to 100 mm headlap moves 500×250 from 18.8 to 20.0 per m² — over a square metre on a large roof. Confirming the headlap with the roofer before ordering is the single cheapest check in the whole process.
Coverage in a Real Order
Take a 79.2 m² slope in 400×250 mm at 75 mm headlap. From the table: gauge 162.5 mm, 24.6 per m².
Base count: 79.2 × 24.6 = 1,949 slates. Add 10% waste (cutting, hips, inspection rejects, transport): 1,949 × 1.10 ≈ 2,143 slates.
For 500×250 at 75 mm the same 79.2 m² takes 1,489 before waste, about 1,638 after 10%. Every row in the table feeds the same contract arithmetic — the size and headlap determine the count, and the count determines the crates, the container and the freight.
Weight That Comes with Coverage
Coverage sets the count, and the count sets the weight — the structural check that most importers forget until the roof is designed. With 18.8–20.0 slates per m² for 500×250:
- 4–5 mm slate: roughly 26 kg/m²
- 6 mm slate: roughly 38 kg/m²
Both before battens, underlayment and fixings. It is the same reason a container of slate usually maxes out by weight before volume — the crate is full, but the box is under-loaded. Always run the total against the roof structure and the transport weight limit, and let the packing list show the m² per pallet before you book.
Reading a Row Without a Calculator
On site, no one carries the table. Two rules of thumb that survive in the pocket:
- The 212 rule. A 500×250 at 75 head gives a 212.5 gauge — remember 19 slates per m² and you're close on any 500mm slate.
- The size rule. Halve the length, subtract the head, use the table once — on the next roof the same slates will behave the same way.
For the full list of standard dimensions and how size drives cost, the roofing slate sizes page covers the whole range from 300×200 up to 600×400. For the per-size coverage in one glance, the slates per square metre reference is the quick sheet our sales team quotes from.
Frequently Asked Questions
What does headlap mean in roofing slate?
Headlap is the overlap between the top of a slate and the tail of the slate two courses below. It is the distance the slate sits over the slate below, typically 75 mm in the UK, and it is what keeps wind-driven rain and snow out of the roof. A bigger headlap reduces the gauge (visible exposure) and increases the number of slates per m².
What is the exposure or gauge of slate?
Gauge, also called exposure, is the visible length of each slate: (slate length − headlap) ÷ 2. A 500×250 slate at 75 mm head shows 212.5 mm of slate per course. It is the number that decides how many slates per m² and how the battens are spaced.
How many slates do I need per m² for 500×250 slate?
At 75 mm headlap, about 18.8 per m² (round up to 19). At 100 mm headlap, 20.0 per m². For 400×250 at 75 mm, it is 24.6; for 300×200, 44.4. Always confirm the headlap before ordering.
What is a square in slate roofing?
A square is a roofing unit of 100 ft², about 9.29 m². A 500×250 slate at 75 mm head covers about 175 slates per square. It is still the unit used in US quoting and on some UK contracts.
Does a bigger slate cost less per m²?
In material terms yes — bigger slate means fewer slates and fewer overlaps, so lower coverage count. But larger slates are heavier per m² and cost more per slate, so the per-m² price depends on thickness and the slate price per ton. The coverage table tells you the count; the quote tells you the cost.
Want coverage numbers to line up before you order?
We calculate gauge, exposure and slates per m² on every quote — then verify against the batch. Browse our roofing slate products or send your roof measurements and get the count back.