Slate Headlap, Gauge and Exposure: The Three Numbers That Decide Your Roof

Quick answer: headlap is the overlap that stops wind-driven rain (75 mm standard, up to 100 mm+ on low or exposed roofs), gauge is the distance between course bottoms — the number that sets batten spacing — and exposure is simply the visible part of each slate, equal to gauge on a plain-edge roof. For a 500×250 mm slate at 75 mm headlap, gauge and exposure both read 212.5 mm, and you need about 18.8 tiles per square metre. Lower the pitch and all three change together.

I design slate roofs for a living — not the stone, the geometry of it. When a client sends me a photo of a beautiful new slate roof and asks why it leaks, I rarely find a broken tile. I find a number that was never written down. Someone ordered slate by size, the roofer fixed it by habit, and nobody agreed on the three measurements that actually define a slate roof. This page is the conversation I have in my office every week, written down so you can have it with your own team.

Start with the biggest source of confusion: how the three numbers relate

Here is the mental model that makes everything else click. Take a single slate on a finished roof and divide it into three zones:

• Headlap — the part you cannot see, hidden under the slate two courses above.
• Exposure — the part you can see, the face of the roof.
• Gauge — the measurement between course lines, which happens to equal the exposure.

Roofers use gauge when they talk about batten spacing and quantities; exposure when they talk about what the roof looks like; headlap when they talk about keeping water out. Same roof, same three numbers, three different jobs. The diagram below is worth a thousand words:

Headlap: the measurement that does the waterproofing

Headlap is the overlap between a slate and the one two courses below it. It is the hidden part of the tile — the part that sits under the next course up — and it is the only overlap in the whole system that actually decides whether water gets into your roof. Why "two courses below"? Because of the next section's geometry: each slate covers one tile below it, and in turn is covered by the tile above; the head of the tile meets the tail of the tile two courses down. That's where the wind-driven rain has to cross two layers of stone, which is the whole point of a slate roof.

The classic number in British practice is 75 mm. It has been the standard headlap for a century, and it is what most size-and-coverage tables assume. But 75 mm is a minimum on a standard roof, not a universal law. The conditions that push it up:

• Low pitch. The flatter the roof, the slower the run-off and the more wind-driven rain can be forced up the lap. Below 20° most specifiers go to 100 mm, and on really shallow slopes, to 110 mm.
• Exposed sites. Coastlines, hilltops, open fields — anywhere the wind has a run at the roof. A 75 mm lap that works in a sheltered valley will fail in the same valley if the house sits on a ridgeline.
• Heavy snow. Snow holds water at the laps for days. The bigger the head, the more chance the water drains before it finds a seam.

Here is the trade nobody writes on the quote: headlap is paid for in tiles. For a 500×250 mm slate, moving from 75 mm to 100 mm headlap changes the count from about 18.8 to 20.0 per m² — one extra tile per square metre. On a 200 m² roof that is 240 tiles, roughly half a pallet, shipped from the other side of the world. That is not a reason to cheat the headlap; it is the reason to agree the headlap before the order, not after the roof leaks.

Gauge: the number you set on site

Gauge is the vertical distance between two course lines — in practice, the distance between the top edges of two adjacent courses, which is also the spacing between your battens. It is the measurement the slater works with every day, because it is the measurement he can actually adjust on the roof.

Headlap is decided by the roof geometry and the weather. Gauge is decided by the slater, from the slate and headlap:

gauge = (slate length − headlap) ÷ 2

That is the formula that builds every coverage table in the trade. A 500 mm slate at 75 mm headlap:

(500 − 75) ÷ 2 = 212.5 mm gauge

On site, this number becomes the batten spacing. The first batten at the eaves is set slightly differently (the eaves course is doubled), but from the second course up, every batten goes in at 212.5 mm centres. If the roofer skips the gauge and spaces by eye — and they do, often — the courses drift, the headlap changes tile by tile, and the roof's defence against wind-driven rain stops being a number and becomes a hope.

Which is why the gauge is also the number I watch when a roof is finished. Get a tape measure on a finished slope: the vertical distance between the bottom edges of two adjacent courses should be dead consistent across the whole roof. On a 212 mm gauge it should hold within a couple of millimetres. If it drifts, the battening was careless — and the slates that fixed it will be the first to leak.

Exposure: the visible face, and the name everyone over-looks

Exposure is simply the part of the slate that shows. On a plain rectangular slate it equals the gauge — the same 212.5 mm — which is why the two words get used interchangeably and why this page treats them together. But it is worth knowing the difference in emphasis:

• Gauge is a construction number. It sets battens, courses and quantities.
• Exposure is a visual number. It is what the roof looks like from the street — the rhythm of the courses.

If you are an architect, the exposure is what you see. If you are a slater, the gauge is what you set. They are the same distance, seen by two different people, and that is the entire confusion.

One place they genuinely diverge is worth knowing: exposure changes with the slate size even when the gauge stays the same. On a random-width roof, the length of each tile is fixed but the widths vary — so the course lines stay straight while the vertical joints wander. The gauge is still one number for the whole slope; the exposure is still that number. The terms only split when you talk about vertical joints vs horizontal ones. Keep it simple: on a standard roof, gauge = exposure, and the word you use says which craft you come from.

How the three numbers flow into an order (with real numbers)

Let me take a real project to show the flow. A gable roof in the north of England, 150 m² per slope — call it 300 m² total, measured on the slope. The roof reads about 28° at the eave. The architect wants 500×250 mm slate because that is the classic, and the slate will run 5–6 mm thick.

1. Set the headlap first. At 28°, sheltered valley, 75 mm is correct. It keeps the tile count honest and the roof light. (If it were 20 °, we would jump to 100 mm and the whole count changes.)
2. Then the gauge. (500 − 75) ÷ 2 = 212.5 mm.
3. Then the batten spacing. Battens run at 212.5 mm centres from the first full course. The eaves course gets a double under-course so the first exposed row has a full slate beneath it.
4. Then the count. 1,000,000 ÷ (212.5 × 250) = 18.82, so 18.8 tiles per m²; with waste on top — cutting at hips and verges, handling breakage, the inevitable odd tile — we order 300 × 18.8 = 5,640, plus 10% waste, call it 6,200 tiles.

Every one of those numbers comes from the three measurements. Change the headlap to 100 mm and you are ordering 20.0 per m² — 300 extra tiles on the same roof, about a third of a ton more stone, and a heavier roof to support. That is why the headlap is not a decorative choice: it moves the order, the weight, the freight and the structure, all from one number.

Low roof, recombined geometry — and the mistake I see most

On a low roof — below 20°, say 18° — the same 500×250 mm slate rearranges itself:

Roof condition	Headlap	Gauge / exposure	Slates per m²	What it means
Standard 28°–35°	75 mm	212.5 mm	18.8	The classic, most economical setup
Low-pitch 20°–25°	100 mm	200.0 mm	20.0	Water drains slower; the overlap gets the extra millimetres
Exposed / coastal / severe	100–110 mm	195–200 mm	20.0–20.5	The weather, not the look, decides the number

Watch what happens: the headlap grows, so the gauge shrinks, so the count rises. Three numbers, one movement. That's the whole relationship, and it is exactly the kind of configuration the architectural fix ends up drawing wrong. The single biggest mistake I see on this: the slate size is chosen first and the roof pitch is assumed standard. The architect draws a 500×250 tile, the client approves it, the slater takes one look at the 18° slope, and the whole thing has to be re-specified — new headlap, new gauge, new batten spacing, new tile count, and often a smaller tile. If the pitch is measured on day one and the headlap written into the spec on day one, the entire downstream stays calm.

The honest fix for a low roof is often smaller than 500 mm. A 400×250 mm slate at 100 mm headlab gives a gauge of 150 mm and needs about 26.7 per m² — more tiles, more laps, more defence. That is the classic answer on a low pitch: shorter slate, more courses, more waterproofing. It costs more per square metre and it looks finer, and it is the right engineering trade.

The two-lap principle: why the geometry works in the first place

There is a quiet reason the three numbers work the way they do, and it is worth one paragraph because it stops you over-lapping forever. British and Irish slating is a double lap system. Each tile is covered at its head by the course above and at its tail by the course below — so water that tries to go straight down meets two layers of stone before it reaches the battens. The headlap is exactly the length of that second layer: the bigger it is, the deeper the water has to travel sideways and up against gravity before it can get through.

That is why gauge = (length − headlap) ÷ 2 and nothing else. A slate is long enough to cover two courses: one visible, one hidden. Subtract the overlap (the headlap), divide the rest by two (one course down, one course up), and you have the visible face. There is no magic in the formula — it is just the geometry of a tile that must always sit under the one above and over the one below. Once you see that, you stop memorising numbers and start reading every slate roof in your town for free.

Choosing your numbers before you order: the checklist

You do not need to be a slater to walk a slate roof spec. You need four numbers, in this order:

1. Measure the pitch. Level and tape at the eave, or a digital protractor on a rafter. Within half a degree. This is the keystone of everything.
2. Decide the headlap with the installer. 75 mm standard; 100 mm at 20°–25°; 110 mm below that or in exposed sites. Put it in writing.
3. Check the gauge against the size. (length − head) ÷ 2, and make sure the batten spacing matches. It is not an opinion; it is arithmetic.
4. Get the count. 1,000,000 ÷ (gauge × width), round up, add waste. Compare with the supplier's number — if it does not match, the head is not what you agreed.

When these four numbers are written into the spec, the roofer, the architect and the supplier are all speaking the same language, and the tile order arrives ready for the slope it will sit on. When they are not, the roof still gets built — and that is exactly when the gauge drifts, the laps go thin, and the phone rings a few winters later.

If you are sizing the stone itself, one honest note from the factory side: a 500×250 mm tile with a verified thickness tolerance is what lets the gauge hold course across a whole slope. We ship exactly that from Jiujiang — browse the roofing slate range or send the roof measurements and we'll return the three numbers for your slope.

Frequently Asked Questions

Related Reading

• Size, Headlap and Coverage Table — the reference table that links size, headlap, gauge and count, with a worked example.
• How Roof Pitch Affects Slate Selection — the pitch-driven headlap table and how size changes per slope.
• Roofing Slate Installation Guide — where gauge becomes batten spacing on the roof.
• Roofing Slate Terminology — every term in this page, plus the nib, cleft and margin words.
• Roofing Slate Quantity Calculator — full count maths with waste, once the gauge is set.

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