Roofing Slate Terminology: A Practical Glossary (25+ Terms You'll Actually Hear)

If you have ever read a slate quote and felt the document was written in another language — "headlap 75 mm, gauge 212, S1 grade, cleft finish, nib fixing" — you are not alone. I run a slate quarry and factory in Jiujiang, China, and I have shipped natural roofing slate to Europe for over thirty years. Almost every importer, architect and new slater starts the same way: the words look familiar, but they mean something different from what the dictionary says.

So here is the glossary I wish someone had handed me when I started. It is not alphabetical for decoration; the terms are ordered the way you will meet them on a project — from the rock to the roof, from the quarry to the invoice. Each term comes with the number or habit that separates a professional from a guess.

1. The Slate Rock Itself

Cleavage (and "cleft")

What it means: The natural plane along which slate splits into thin, flat tiles. It is what makes slate slate and not just grey stone. "Cleft" is the finished surface you get when a block is split along this plane — the slightly rippled, natural face.

Why it matters: A slate must be split from the block with the cleavage, not against it. That is how a single block becomes a whole pile of tiles — and why a slate roof catches water and slides snow with the same neatness. The texture you feel on the face is the cleft.

Riving / Splitting

What it is: the act of splitting the raw block along the cleavage by hand or machine to make slate tiles. "Rived" is the older word for "split". On a job site, riving means the way the surface looks and the way the tile behaves when nailed.

Gauge — and the Gauge of a Quarry

Two different "gauges". In the quarry, "gauge" used to mean the thickness of the slate — in inches. In the old slate counties, you would say "a five-eighth gauge" to mean a ~16 mm slate. On the roof, gauge is the exposure — the height of the visible face of each slate. I cover both below. Always ask which gauge a contractor means.

Thickness (in millimetres)

Modern slate is specified in millimetres: typically 5 mm, 6 mm, or 8 mm for roofing, sometimes up to 10 mm for heavy or commercial work. The thickness decides weight, strength and how it is fixed — see the full thickness guide. A "5 mm" roof is noticeably lighter than an "8 mm" one, and each fits a different roof structure and climate.

2. Measuring a Slate Roof

Headlap

What it is: the overlap of one course of slates over the next — the distance a slate's top laps up the underside of the slate above it. In British practice, the minimum is usually 75 mm; windy or exposed areas use 100–110 mm.

Why it matters: headlap is the waterproofing seam. Wind-driven rain slides up under a tile, and the headlap is what stops it crossing the joint. Set it too short and the roof leaks; set it too long and you shrink the covered area per m² — paying for slate you cannot see. For a 500×250 mm slate at 75 mm headlap, the gauge works out to 212 mm, and you need about 18.8 tiles per square metre. See the installation guide for the working method.

Gauge (on the roof)

This is the vertical spacing between courses — the visible exposed height of each tile, also called the "exposure" or "margin" in some regions. Formula: gauge = (slate length − headlap) ÷ 2. For a 500 mm slate with 75 mm headlap: (500 − 75) ÷ 2 = 212 mm. That single number, with the width, tells you how many tiles you need — see our coverage calculator.

Exposure

Used loosely to mean the same as gauge — how much of each slate is left visible to the weather. It is also used about the direction the roof faces (north-facing, south-facing). When a slater says "the exposure of this roof", he usually means the weather the slope gets, not the millimetres.

Margin

In British roofing, the margin is the exposed height of a course — the same as gauge. In some trades it is used more loosely for the visible edge of a slate along the verge. If a quote uses "margin" and "gauge" as the same number, that is a good sign the writer knows the trade.

Slate lengths and widths

Slate is sold in standard sizes — 300×200, 400×250, 500×250, 600×300, 600×400 mm and more. The size guide lists the full chart. The length runs up the roof, the width across. On a random-width roof, the length stays the same and the width varies from tile to tile — that is the "random" of random-width slate.

3. Fixing, Laying and the Roof System

Batten / Battening

Wooden strips fixed to the roof deck, running horizontally, onto which the slates hang. Battens are spaced by the gauge — the vertical distance between each course. The batten size and spacing must match the slate length, the headlap and the roof pitch. Battening is a detail that costs pennies and controls the whole roof: if it is laid too wide or crooked, the courses will not run straight and the headlap will drift.

Underlay (and underlayment)

The waterproof membrane between the battens and the rafters (or deck) — the second line of defence if wind lifts a slate. In exposed or cold climates, good underlay, correctly lapped at the eaves, is as important as the slate. In the UK, a breather membrane is now common under slate.

Nailing / Nib fixing

Slate is fixed by driving nails near the top edge of the tile, hidden under the slate above, or by nibs — small hooks at the top of the slate that hang on the batten. "Nib fixing" is a hook-and-ledge fixing, mostly used on re-slating work and for slate that is hard to nail. The head of the nail must be sunk in the slate, not proud.

Copper / Stainless nails

Nails must be non-corroding — copper or stainless steel. Use of the right metal is the difference between a roof that lasts a century and one where the nails rust, swell and crack the slates in thirty years. The full method is in our installation guide.

Course

One horizontal row of slates. The courses run from eaves to ridge; each course laps the one below. The "first course" (the one at the eaves) is usually a double course, so that the first exposed row has a full slate under it.

Eaves, ridge, hip, valley, verge

• Eaves: the lower edge of the roof where water runs off.
• Ridge: the horizontal top line where the two slopes meet.
• Hip: the external angle where two slopes meet going up.
• Valley: the internal angle where two slopes meet going down — the spot that collects water.
• Verge: the slanted edge of a slope at the gable, where the roof meets the wall.

Water rarely goes through slate; it goes through the details. A valley and a verge done badly will leak no matter how good the tile is.

4. Quality, Grades and the Paperwork

Grade (S1 / S2 / S3, T1–T3)

Under EN 12326, slate is graded by flexural strength and water absorption. S1 is the highest grade (low absorption, high strength), down to S3. T1–T3 are thickness tolerances (T1 is the thinnest). "S1/A1/F1" on a certificate means the slate passes the top quality and thickness class. It is the difference between a 60 MPa flexural and a weaker slate that may crumble at the nails. A grade is not a "rating" a trader invents — it is a measured number in the lab, and a serious supplier publishes the report. See the grades guide for how to read it.

Water absorption

How much water the stone takes in, measured as % of weight. Under 0.5% is typical of true slate; the lower the number, the better the freeze–thaw resistance. Any figure above 1% should make you ask why. In our test results we publish the real numbers from every batch.

Flexural strength

The force a tile resists before snapping, in megapascals (MPa). 60 MPa is a good floor; premium slate exceeds 100 MPa. This number decides whether the tile can be nailed without cracking and stand on a roof for a century. Test method EN 12326-1 / EN 12326-2, or ASTM C406.

Freeze–thaw test

The cycle of freezing and thawing that a slate must survive without crumbling. For cold climates, the report must show it passed a large number of cycles — the figure that matters is on every honest test report. There is more in our grades guide.

5. Buying, Shipping and the Yard

Nominal vs actual size

A slate listed as "500×250" will be cut to a tolerance. A serious supplier works to a small tolerance (a few millimetres); a cheap one may cut 495×245 and call it "500×250" on the invoice. The difference changes how the courses line up and how many you get per square metre. Ask for the tolerance in writing.

MOQ / Container

Minimum order quantity, usually expressed per container. A 20-ft container of slate carries roughly 22–24 tonnes (the weight limit, not the volume); a 40-ft container is normally capped around 26–28 tonnes, and some countries' road limits push it lower. So "how many square metres per container" is a freight question, not a stone question. Our buying guide covers the whole process.

Breakage allowance

Slate travels in crates; a small percentage of broken or chipped tiles in transit is normal. The industry standard allowance is usually 2–5%, and a good contract states who pays and how it is replaced. That is why "breakage allowance" is a line in the terms, not a handshake.

Sorting and mixing

Before a roof is laid, the slates are shuffled — so the colour and grain variation of a natural roof is spread evenly, not patchy. The factory sorts by grade, the roofer mixes at the roof. A roof with one row dark and one row light means the mixing was skipped.

Scrap / waste

The cut-offs and chipped pieces that do not fit a course. Budget 5–10% waste on a slating job — for cutting at the edges and the broken ones. Our quantity calculator shows how to add it.

6. Terms You'll See on an Invoice or a Plan

Sewing slate / sewing tile

Not something to sew. "Sewing" (sometimes "seamed") slate means a tile that has been drilled for an invisible fixing — used on vertical or decorative work where you do not want visible nails. A specialist item, usually at a premium.

Nail-bound / nail-splitting

When a nail is driven too tight — or the slate has a weakness near the hole — the tile splits around the nail. That is "nail-bound". It is the classic sign of a bad slate, a too-tight fixing, or a nail hole drilled across the cleavage. A quality slate at 60 MPa will not do this if it is nailed right.

Puff / crumbling edge

A tile that spalls along the edge — a sign of weathering, often where the stone has a slightly open lamination. In a pallet, look for edges that look "crumbly" and reject the piece.

Random width vs. graduated

Random-width slate uses a single length and widths that vary tile to tile — the "random" roof. Graduated (or "diminished") slate tapers the length in courses, an old roofing style. Both change the look, and both change the quantity per m². There is a full guide in slate types.

Double lap vs single lap

British and Irish slating is a double lap system — every slate is covered at the head by the course above and at the tail by the one below, so water has two layers of stone to cross. Single lap is used with big heavy slates that sit almost edge to edge. If a quote says "single lap", the maths and the underlay change — ask why.

7. The Slate You'll Feel and See

Rustic / naturally cleft

A "rustic" or "natural cleft" slate keeps the raw cleft face — the texture of the split surface, with slight ripples. It is the classic heritage look and the standard for traditional British roofing. It hides small variations and weathers gently.

Dressed / sanded / honed

A slate that has been machine- or hand-dressed to a flat, uniform face. It is crisper, flatter, easier to read — and it shows every deviation. Used for modern and precision work. The choice between dressed and cleft is the choice between a razor edge and a natural edge.

8. The Only Three Terms That Decide a Contract

If you are on the buying side, you can skip the poetry and watch for three words:

1. Gauge — because it decides how many tiles you need and how the courses line.
2. Headlap — because it decides whether the roof leaks in wind-driven rain.
3. Grade — because it decides whether your tile is a rock or a brick, measured, not claimed.

Master those three and you can read any slate invoice in the language of the roofer, not the salesman.

Frequently Asked Questions

What is the most important term in roofing slate?

Headlap — it controls whether a slate roof stays waterproof. Second is grade, because it controls whether the tile itself lasts. Together they are 80% of the difference between a 20-year roof and a 150-year roof.

What is the difference between headlap, gauge and exposure?

Headlap is the overlap of the tile over the one below. Gauge (or exposure) is the visible height of a course — the measurement that decides how many tiles you need per square metre. For a 500×250 tile at 75 mm headlap: gauge = (500 − 75) ÷ 2 = 212 mm, and you need about 18.8 tiles per m².

What does "cleft" mean in slate?

Cleft (or natural cleft) is the surface you get when a slate is split along its natural cleavage — the slightly rippled, un-dressed face that is the classic traditional slate look. It holds water in tiny pockets, which is part of why a slate roof sheds snow and water so well.

What does S1 grade mean on a slate test report?

S1 is the highest flexural grade under EN 12326 — the slate has high strength and low water absorption. S2 and S3 are lower grades. T1–T3 describe thickness tolerance. An "S1 / A1 / F1" certification means the slate passed the top quality class.

How much headlap does a slate roof need?

In British practice the minimum is usually 75 mm; exposed or high-wind roofs need 100–110 mm. More headlap means fewer tiles cover per square metre — for a 500×250 tile, 75 mm vs 100 mm changes the count from 18.8 to 20 per m².

What is a nib on a slate?

A nib is the small hook or ledge at the top of a slate used for the slate to hang on the batten instead of nailing. It is the "nib fixing" method, common on re-slating and on short slates.

Related Reading

• Advantages and Disadvantages of Slate Roofing — the honest trade-offs of cost, weight and beauty, in the real numbers.
• Roofing Slate Sizes & Standard Dimensions — the full size chart, effective coverage and pieces per square metre.
• Roofing Slate Grades: S1, S2 & T1–T3 Explained — how to read a grade before you buy.
• Roofing Slate Installation Guide: Step by Step — how headlap, gauge and fixing play out on the roof.
• Roofing Slate Quantity Calculator — how many tiles your roof needs, with waste allowance.