How to Identify Different Types of Slate Roofing

By The Heritage of Welsh Slate Quarrying
Comments 35
11 Sep 2026

How to Identify Different Types of Slate Roofing

Standing at the foot of a slate roof, most people just see grey. Look a little closer, though, and the slate starts talking: where it was quarried, roughly how old it is, how much life it has left, and — most importantly — what you can and cannot do when it begins to fail. A 5mm Welsh slate and a 25mm Westmorland slate are doing the same job, but they behave very differently under a hammer. You cannot simply swap one for the other.

Here is how to tell the main types apart, working from the smooth and slender through to the rough and chunky.

Welsh Slate: Thin, Hard and Blue-Grey

Welsh slate is by far the most common on British roofs, quarried in the slate valleys around Blaenau Ffestiniog, Bethesda and Corris. If your roof was laid between roughly 1850 and 1980 and you are not in the South West or the Lake District, this is most likely what you have.

Colour: dark blue-grey to charcoal, sometimes with a faint purple or heather tint (the prized Penrhyn purple), or a greenish cast on some beds.
Texture: smooth and flat, with an even, almost glassy surface. Run your hand across it and you feel very little.
Thickness: typically 4–6mm on a good-quality slate, though cheaper "seconds" and "thirds" run thinner and have a shorter life.
Sound and edge: tap it and it rings cleanly. Edges are straight and usually dressed square, and the slate splits neatly along the grain.

Sizes are traditionally imperial — 20 x 10, 18 x 9, 16 x 8, 14 x 8, down to 12 x 8 and 10 x 6 — and a Welsh roof often uses diminishing courses, with the largest slates at the eaves.

Cornish Slate: Softer, Thicker and More Riven

Cornish slate comes from Devonian stone, famously at Delabole, and it has a completely different character. It is the slate you find across Devon and Cornwall, and it looks and feels older than Welsh material even when it is not.

Colour: paler grey, often silvery, with green or brownish tinges and a slightly mottled appearance.
Texture: rougher and more uneven, with visible riven lines across the face and a slightly sandy feel.
Thickness: noticeably chunkier, commonly 6–12mm, and the thickness varies from slate to slate.
Edge and sound: edges are often rough and irregular rather than neatly trimmed, and the tap is duller, with less ring.

Cornish slate is softer and more porous than Welsh, so it weathers faster, but it also takes a lovely silver-grey patina as it ages. Handle it gently — it chips more readily.

Westmorland Slate: Green-Grey and Built for Weather

Westmorland slate — Borrowdale volcanic stone from the Lake District — is the heavyweight of the family, and unmistakable once you have seen it.

Colour: distinctive green-grey, sage or blue-green, sometimes with a light grey cast. It almost never looks plain blue.
Texture: strongly riven with visible grain and a slightly wavy, handmade face.
Thickness: thick and often tapered, anywhere from 12mm to 30mm, with a heavy, solid feel.
Size: frequently random widths and lengths, and often very large — "tall slates" of 24 to 36 inches are common on older Lakeland roofs.

This slate was built for driving rain and gales, and a well-maintained Westmorland roof can last well over a century. Because it is so heavy, it is usually fixed with copper nails or oak pegs, and the roof structure beneath is built accordingly.

Reading the Details: Size, Thickness, Sound and Fixings

If you are still unsure, a few simple checks will settle it.

Measure a slate. Metric sizes such as 300 x 200mm suggest modern imported or Spanish slate; imperial sizes point to traditional British stone.
Check the thickness with a ruler at the edge of a lifted slate. Under 6mm, think Welsh; over 10mm with a rough face, think Cornish or Westmorland.
Look at the underside from inside a loft. The back of a Westmorland slate is obviously chunky and irregular; Welsh backs are smooth and uniform.
Note the fixings. Oak pegs, copper nails and torched mortar beds all suggest a roof of real age.
Watch for replacements. Concrete tiles or thin modern slate mixed in with older stone are a strong hint that repairs have already been done — often badly.

Matching Slate for Repairs and Replacements

Once you know what you have, matching becomes much easier. Reclaimed Welsh slate of the same size and colour is widely available, and a good roofer will blend new slates in, moving them around the roof so any difference in tone is spread out rather than concentrated in one patch.

Cornish and Westmorland slate are harder to match. Reclaimed Westmorland green is scarce and expensive, so repairs are often done with the best available match, and any sound originals should be saved and reused. If your property is listed or sits in a conservation area, you will usually be expected to use natural slate of a matching type, colour and size — so identifying the original correctly at the outset saves time, money and a great deal of paperwork.

Get the identification right, and every decision that follows — from a handful of replacement slates to a full re-roof — becomes far more straightforward.

Roofing

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5 Comments

Robert Sonny says Jul 21, 2024 at 10:00am

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Reply
- John Abraham says Jul 21, 2024 at 10:00am
  
  I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
  Reply
  - Robert Sonnysays Jul 21, 2024 at 10:00am
    
    I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
    Reply
Robert Sonny says Jul 21, 2024 at 10:00am

I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
Reply

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