How to Calculate Roofing Slate Headlap: 5 Steps From Pitch to Millimetres

Quick answer: roofing slate headlap is calculated in five steps — (1) start from a 75 mm baseline for a standard double-lap roof, (2) add 25 mm for every 5° the pitch drops below 25°, (3) add 10–30 mm for exposed sites, (4) check it against the slate's nail-hole position, and (5) verify the visible joint on site. For a 500×250 mm slate on a 20° roof in a normal site, that works out to 100 mm of head — and the gauge becomes (500 − 100) ÷ 2 = 200 mm.

I am a slater. Not a designer, not a spec-writer — I am the one who stands on a ladder at 7 a.m. with a roll of battens and a roof pitch that the architect "estimated". After twenty-plus years of doing this across the UK, I can tell you exactly what happens when headlap is treated as a rumour instead of a number: the roof leaks, the client phones, and the first question is always "who set the headlap?" Nobody did. That is what this page is about — turning headlap from folklore into a calculation you can defend.

The Calculation You Can Defend

Here is the whole method as five steps. Work through them top to bottom and you never have to argue about headlap again:

1. Fix the baseline. Start at 75 mm for a normal double-lap slate roof. This is the classic British minimum, and it is still the right number for a 25°–35° roof on a normal site.
2. Read the pitch. The flatter the roof, the more headlap it needs. Under 20°, add 25 mm; under 15°, the roof stops being a double-lap design and needs a different conversation (and often a shorter slate).
3. Read the site. Coast, hilltop, open field — add 10–30 mm on exposed sites, more if the building stands alone.
4. Check the nail position. The headlap can never be bigger than the distance from the nail holes to the head edge of the slate. If your slate is punched for a 70 mm maximum, a 100 mm headlap is not possible — the nail would fall outside the batten above. This check is the one everyone forgets.
5. Verify on site. Once the first courses are up, measure the visible joint between two slates. It should be the headlap plus a couple of millimetres of tolerance — if it drifts, the battens are wrong, and the whole roof repeats the error.

That is the calculation. Everything below is the detail of steps 2, 3 and 4 — because those are the ones that actually change your number.

Step 2 in detail: the pitch table that actually matters

Pitch is the biggest lever on headlap, and it is also the most commonly ignored. Most slate quotes are written for a 30° roof that the building never had. Here is the rule I use on every job, and it has never once let me down:

Roof pitch	Headlap to set	Why
35° and above	75 mm (baseline)	Steep roof, fast run-off, standard double-lap defence
25°–35°	75 mm	The classic band, most UK and European roofs
20°–25°	100 mm	Water starts to crawl up the lap under wind; the extra 25 is the cheap insurance
15°–20°	110 mm minimum	Driving rain pushes further; consider a shorter slate and a membrane underlay
Below 15°	Not a slate roof	Redesign: single-lap tile, or drop to a 400×250 mm slate at maximum headlap

This table is the practical version of what BS 5534 and EN 12326 imply: the minimum is 75 mm, and anything below 20° starts paying in millimetres. The moment I say "your roof is 19°, you are not on a 75 anymore", the conversation stops being about tiles and becomes about maths. And it is maths — you can hold the number in your hand and check it against a roof.

A good habit from the site: measure the pitch yourself on the day. Paperwork says 25°, the roof says 22°, the difference is 25 mm of headlap — and the roof will leak where the paper was generous. A level and a digital angle finder cost less than one hour of my labour, and they end 80% of headlap disputes before they start.

Step 4 in detail: the nail-hole check nobody does

Here is the step that saves you from ordering the wrong slate. The headlap number is not free — it is limited by where the factory punched the nail holes. A standard slate is punched for a 70–75 mm headlap: two holes centred about 70–75 mm from the head edge (and about 80–90 mm from the tail). That is why 75 mm is not just a habit — it is the geometry of the common hole pattern.

When a roof wants 100 mm of headlap, you need a slate whose holes sit at least 100 mm from the head, or a centre-nail fixing where the hole is at the centre of the board and the batten spacing is set to match. A 500×250 mm slate has room for a 125 mm maximum; a 400×250 mm slate maxes out lower. Always ask the supplier for the exact hole positions before you write "100 mm headlap" into a spec — otherwise you are ordering a roof that cannot be fixed.

The full worked example, with the maths in one place

Let me give you the entire calculation on one roof, the way I actually do it. A client has a gable roof, 24° pitch measured on site, a bit exposed — the house sits on a ridgeline with open fields to the west. He wants 500×250 mm slate. Here is the number:

1. Baseline — 75 mm.
2. Pitch correction — 24° is inside the 20°–25° band, so the head jumps to 100 mm.
3. Site correction — ridgeline, west fields: add 20 mm → 120 mm. If it were a valley site, we would keep 100 mm. If it were a coastal farmhouse, 130 mm.
4. Nail check — the slate head punched holes at 70 mm. A 120 mm headlap cannot be fixed with that hole pattern. So the fixing method becomes centre-nailed, or we switch to a slate with a longer distance between the holes and the head edge. This is where most specs silently change to centre-nail.
5. Gauge — (500 − 120) ÷ 2 = 190 mm. The roof needs 1,000,000 ÷ (190 × 250) = 21.1 slates per m².

That is the whole defence: one roof, one headlap, one number. If the same roof had been calculated as a standard 75 mm, the gauge would be 212.5 mm, the count 18.8/m², and the wind-driven rain would have found the gap by the third winter.

How to verify headlap on a finished roof

You do not need to trust the calculation — you can check it in two minutes with a tape. On a fixed slate roof, the visible joint between two adjacent slates should be roughly headlap + 2–3 mm tolerance. So for 100 mm of headlap you should see a joint around 102–103 mm.

On a course at mid-roof, measure the gap between the bottom edge of one slate and the top edge of the one two courses below it — that is the actual headlap in place. If it reads 95 mm on a roof that wanted 100 mm, the battens were spaced to the wrong gauge. Fix it before the weather does: pull the courses, re-set the batten spacing, and re-fix. It costs more to leave it wrong than to do it twice.

A quick field trick I use: mark the batten at the first course with the calculated gauge, then every course after that is the same number. If the roof is true, the joints hold the gauge across the whole slope. A 2 mm drift per course is invisible at ground level and a disaster at the ridge — the calculation only works if the spacing stays locked.

The slate sizes and the headlap they can carry

Headlap is not a free variable — the slate size limits it. The head of a slate must reach the batten of the course above, and the nail holes must sit over that batten. The bigger the headlap, the more the slate nose must hang over the batten below — and there is a structural limit.

Slate size (mm)	Practical max headlap (mm)	Note
600×300	125–140	Long boards carry the most head; usually centre-nailed at high head
500×250	100–120	The workhorse; 100 mm needs a careful hole pattern
450×250	100–110	Medium; common on 20° roofs
400×250	90–100	Short board, used on low pitches with heavy head
400×200	90	Small format; each board covers less area

Notice what happens when you force a small slate to carry a big head: the visible exposure shrinks, and the roof turns into a comb — a hundred fine courses. That is the correct trade on a low roof, but it is not the economical one. On a 18° roof the honest answer is usually a 400×250 mm slate at 100 mm head, not a 500 mm slate fighting the pitch.

One table for your estimator: headlap by size and pitch

For the day you need a quick number — or to double-check a supplier quote — here is the headlap map for common slate sizes and the roof bands they support:

Slate size (mm)	30°+	22°–30°	18°–22°	Under 18°
600×300	75 mm	75–100 mm	100 mm	not ideal
500×250	75 mm	75–100 mm	100 mm	100–120 mm (heavy)
450×250	75 mm	75–100 mm	100 mm	100–110 mm
400×250	75 mm	75–100 mm	100 mm	100 mm
400×200	75 mm	75–90 mm	90–100 mm	90–100 mm

These are working numbers, not a table of standard law — the final headlap must always be confirmed with your roofer and the local building inspector. They are the range a roof is designed in, and the reason the calculation is a calculation.

BS 5534 and the hidden 75 mm

A quick note for spec writers. The 75 mm baseline is enshrined in BS 5534, the British Standard for slating and tiling, as the minimum — not the default. The standard assumes a double-lap roof on a pitch where the run-off works, and the 75 mm is the floor. On a low-pitch or exposed roof, the standard itself says the headlap should be increased — it is the site that decides, not the slate.

The other standard in the room is EN 12326 for slate material itself — the stone quality, not the installation. A slate that passes EN 12326 gives the roof its durability, but the headlap is a geometry decision, not a stone test. When a quote says "EN 12326 tested", it tells you the stone is sound; it tells you nothing about the headlap. The number that makes the roof weather-proof is the one on the drawing, not on the certificate. (If you want the stone side of the story, our test results are public; the headlap is decided here, on this page.)

When "more headlap" is not the answer

Every slater has seen the fix that was not: the owner adds 25 mm to the headlap to cure a windy roof, and the roof gets heavier, denser, and still leaks — because the problem was not the lap. The three things that mimic a headlap problem:

• Wrong batten spacing. If the courses are not to gauge, the head drifts by the first course. 5 mm across 20 courses is 100 mm of accumulated error.
• Missing underlay. A slate roof is a two-layer defence: the slate and the felt below. If the underlay has a tear at the ridge, no amount of headlap fixes it.
• The nails backing out. A nail pulled through the top course breaks the whole lap. Headlap number is good, the fixing is not.

So the calculation is the beginning, not the whole story. It gives you the number to fix to; the roof is made weather-proof by the man who fixes to it — the batten, the nail, the underlay, all holding the same number.

Frequently Asked Questions

Related Reading

• Understanding Slate Headlap, Gauge and Exposure — what the three numbers mean, with the visual diagram and a worked order example.
• Size, Headlap and Coverage Table — the reference table tying size, head, gauge and slates per m².
• How Roof Pitch Affects Slate Selection — how pitch moves the whole slate spec, not just the head.
• Roofing Slate Quantity Calculator — the full count maths with waste allowance.
• Minimum Roof Pitch for Roofing Slate — why very low slopes are a different system entirely.