History of Natural Slate Roofing: From Roman Forts to Your Roof

This is the story I retell in my own quarry office, because it explains why a Chinese slate factory like ours — Jiujiang, Jiangxi, exporting to 30+ countries — is really the same trade that once shipped Welsh slate from Penrhyn to Hamburg. The rock, the split, the roof. Everything else is detail.

When you are about to spend money on a slate roof — or an entire container of it — you should know what you are buying. Not just the S1 grade certificate. The why. Why is slate the only roof covering in history that comes with a 100-year warranty by default? Why do old church roofs leak less than new ones? And why does your supplier's test report look the way it does? All of it grew out of this history.

1. The First Slate Roofs: Stone Age to Romans

Slate is one of the oldest roof types in existence — older than fired clay tiles, older than timber shingles, older than the wheel. Before there was a word for it, people were roofing with flat, split pieces of the local rock: prehistoric communities in Europe and Asia found that a thin slab of fissile stone sheds rain better than anything else at hand, and the archaeological record includes split-slab roofs from deep before written history. What no one disputes: splitting rock along its natural grain into roof tiles is a very old trick, and the rock itself is old enough to have been waiting — the best roofing slates come from shale formed four to five hundred million years ago in the Cambrian and Ordovician seas, then squeezed and cooked by mountain-building into the cleavage that lets it split so cleanly.

And then the Romans arrived. In Roman Britain, the forts of the garrisons were roofed with something remarkable — locally quarried stone slabs, split thin, laid in a double course so that each joint was covered by the course above. The Roman army took slate seriously enough to use it for the roofs of its military buildings: at Segontium (modern Caernarfon) in North Wales, and across the north-west of the province, the evidence of local slate working survives in the archaeology. A Roman fort in Wales did not look like a red-tiled Italian villa; it looked like grey slabs from the nearest quarry. Which is exactly the point: slate is not a luxury material waiting to be discovered. It is the working material that every frontier builder since the Romans discovered already split.

In mainland Europe, the same thing. The Romans quarried slate in the Auvergne region of France and in Galicia, Spain, for both roofing and floor. The first Spanish roofers were Roman legionnaires' suppliers. The material crossed the Pyrenees before the modern borders did.

The Middle Ages: the Church, the Castle and the Village Roof

Medieval Britain was a wood-and-thatch world — until the king needed a roof that would not burn, or a stone that could shrug off a season of rain without being re-laid every spring. The proof is still standing: the castles Edward I built in North Wales in the 1280s — Conwy, Caernarfon, Beaumaris — were roofed with slate from the same valleys, and roofers still replace their tiles with Welsh slate today. The big houses and cathedrals used slate for the important slopes, while the villages used thatch, oak shingles and whatever else was at hand.

The key invention that medieval Europe gave the trade was the double-lap system — the way a slate is covered at the head by the course above and at the tail by the one below. Two layers of stone over every joint. It was not "invented" by one person; it emerged because a roof that is not double-lapped leaks. And it survives today unchanged: when you are reading our installation guide, the headlap and the double lap are medieval decisions that you are still making.

1666: the Fire That Made Slate the Default Roof

For a short time, slate was a rich man's material. But on 2 September 1666, a baker in London left a fire burning in Pudding Lane. The Great Fire of London burned for four days, destroyed about 13,000 houses, and left maybe 70,000 people homeless. The ruins were roofed mostly with thatch and wooden shingles — which is exactly what the fire ate in hours.

The rebuilding law that followed changed the London roof forever. The authorities banned flammable roofing in the rebuilt city, and the practical answers were tiles, stone, or — where the geology allowed — slate. The effect was not overnight: it took a generation, but the market for slate exploded. By the early 1700s Welsh slate was already shipping from Welsh ports to England in volume, and a century later it was the waterproof skin of British industrial cities. That is when "slate" stopped being a regional word and became a national one.

Why a 17th-century fire matters to your 21st-century roof: the same logic — the insurance industry. The London fire created the first fire insurance companies, who wanted roofs that did not burn and did not cost them. That is the true origin of the "slate = premium roof" reputation: a fire risk, a law, and an industry, not an aesthetic.

The Golden Age: Wales 1750–1900

This is the century where the story becomes industrial. By the early 1700s slate was already shipping from Welsh ports to England. But the scale changed with transport: first the canals of the late 1700s, then the railways. The Ffestiniog Railway, opened in 1836, was built for exactly one cargo — slate from the Blaenau Ffestiniog quarries, running 13 miles down to the harbour at Porthmadog. A railway built for a single rock is the moment slate became big business.

The place that says everything is Penrhyn, near Bethesda, North Wales. At its peak it was the world's largest slate quarry. A slate from Penrhyn, roof of the British Museum, the Albert Hall and whole districts of the northern cities, travelled further than any building stone of its era. Why Wales? Because the Welsh Cambrian and Ordovician slate beds were both thick (deep enough for 500×250 mm and 600×300 mm tiles), and uniform enough to split to a few millimetres thickness. That is geology and skill combined.

The Welsh industrial revolution produced the physical things you still use: square-dressed slate with a nib — the small ledge at the head of a tile that rests it on the batten and shares the load with the nails, a Welsh idea that is still moulded into slate tiles today. The medieval double-lap was standard, and the edges were dressed square to stop water creeping sideways. By 1880, Welsh slate had conquered the UK and Europe, and "Welsh" had become a shorthand for the best natural roof stone in the world.

The American Side: Vermont and Pennsylvania Slate

Across the Atlantic, the story restarts with emigrants. Slate began in North America in the 1700s as a product of the Welsh and Cornish diaspora. The first US quarry is debated, but the states that made it are not: Vermont and Pennsylvania.

Vermont slate was opened around 1840s by Welsh immigrants; the deep green and the grey-green, some of the "Vermont slate" that architects still spec by name, was taken from the old mill towns — the reason the whole state is still dotted with slate roofs. Pennsylvania's Peach Bottom slate district, along the Maryland border, started around the same era and became the main source for the US East Coast until the 20th century.

Why did American roofing skip the 1666-fire effect? Because the US had its own boom — the railroad. Slate from the quarries went out by rail to the new cities, where fire codes were catching up. The material that was a rich man's luxury in England was a railway-freight product in America, and it spread quickly. The result is the classic American slate roof: the blue-grey, the blue-black, the multicolour, laid with a square edge — a hybrid of the Welsh method and the local slate character.

The New Quarries: Spain and the World Market

Welsh slate ruled the western world until the 20th century. Then two things happened.

First, Spain's quarries in Galicia and León became a serious competitor from around the 1960s. The Spanish Cambrian and Ordovician slate was cheaper to split and process than the Welsh, its blocks were deeper, and shipping from Spanish ports to the rest of Europe was cheaper than from the Welsh coast. Today Spain and China are the two biggest exporting countries, with a small but celebrated Welsh industry remaining.

Second, the price of a quality slate tile stopped being a "craft" figure and started being a global commodity. When you import roofing slate from a Chinese quarry — like ours in Jiujiang — the process is the same as the Spanish and Welsh: quarry the block, split it along the cleavage, trim to a tolerance, grade it, pack it, ship it. The slate you put on a house in Cornwall in 1880 was Welsh. The slate you put on a house in Cornwall in 2026 is probably Spanish or Chinese — and the why is economics, not the stone.

Standards, Test Reports and the Modern Buying Age

The last chapter of the history is the shortest but the most important for you if you are buying today.

For most of history, a slate roof was judged by whether it leaked in the first winter. You cannot test a 100-year roof in one season, so the old markets used a rough rule of thumb: if the slate rings, it is good — the "ring test", or the similar "slate must be no softer than a good coin" folklore. The 19th-century slater judged flexural strength by ear and by hand — a sound tile rang, a weak one thudded — and the old standards were thickness, size, soundness and colour.

Then came the standards. The UK wrote its own BS 680 for slate; Europe followed with EN 12326 (developed through the 1990s–2000s and harmonised), and the US uses ASTM C406. The EN 12326 replaced the old local rules with a test that measures water absorption, flexural strength, freeze–thaw cycling and carbonate content. It sorted the market into grades — S1, S2, S3 — and made a certificate a requirement, not a word. The US ASTM C406 grades slate along the same lines.

If you want to know what all of that means for your purchase, our grades guide walks you through it — or you can read our own published EN 12326 / ASTM test results, which we publish because the history taught us that a stone with hidden cracks fails in 20 years, not 120.

Why Old Slate Roofs Still Cover Europe

So after all this history, the question that matters most: why is an old slate roof still a good roof?

Four reasons, all historical:

1. The double-lap system — medieval, still the best way to keep water out. It is the difference between a slate roof that leaks after 30 years and one that leaks after 300.
2. The nails. Copper or stainless (used for 200 years) do not rust and swell and crack the slate. Nail the slate with the wrong nail and the roof kills itself from inside.
3. The 100+ year service life. It is a promise, not a hope — written in the stone and proven by every 1700s house in the UK and Vermont that is still covered in its original slate.
4. The material. Slate resists fire, frost, and most animals. No chemical or coating can match a rock that is already a rock.

That is why a slate roof is expensive — and why it is cheap in the long run: it is a one-time contract with a century of service.

What This History Means When You Buy Slate Today

For buyers, the 2,000-year history has three practical conclusions:

• Standard size and grade matter more than the name. "Welsh" slate, "Spanish" slate, "Chinese" slate — each is a quarry, not a brand. In 1880 you had to trust the quarryman; in 2026 you read the EN 12326 / ASTM C406 certificate. The grades guide shows how to read it.
• The method is the heritage. Double-lap, headlap, batten gauge — these are 500-year-old techniques that still decide whether your roof works. See the installation guide.
• Check the water absorption, not the price. A slate that absorbs over 0.5% water will, over decades, puff in the freeze — the classic slate failure that the 19th-century trade knew by the "sound" of a tile, and the 21st century knows by a lab number. Read our test results.

Frequently Asked Questions

Related Reading

• What Is Natural Roofing Slate? — how the rock forms, why it splits so well, and what makes it a roof material at all.
• How Long Does a Natural Slate Roof Last? — the 75–150 year promise, and the factors that decide where in that range your roof lands.
• Roofing Slate Grades: S1, S2 & T1–T3 Explained — how EN 12326 and ASTM C406 turned a 2,000-year craft into a measurable standard.
• How Is Roofing Slate Made? — from quarry block to finished tile, the process that has changed less than you think.
• Roofing Slate Installation Guide — the medieval double-lap system, step by step, on a modern roof.