Slate

A fine-grained metamorphic rock that splits cleanly into thin, flat sheets along its natural cleavage, quarried and worked chiefly as a roofing material.

Historical usage

Slate has been used for roofing in Britain since Roman times, with documented use from at least the 13th century, but large-scale quarrying only began in the 18th and 19th centuries. North Wales — above all the Penrhyn, Dinorwig and Ffestiniog quarries in Snowdonia — became the heart of the industry: by the late 1890s Wales was producing around 485,000 tons of finished slate a year, employing some 17,000 workers and supplying roughly a third of the world's roofing slate.

How it worked

At quarries like Penrhyn, men worked open galleries and terraces, sometimes hanging from ropes tied round their waists, using hand tools and gunpowder-packed boreholes to break slate rock from the face. The critical skill came afterwards: splitting the rock down its natural grain into thin sheets using a mallet, chisel and a splitting knife called a whittle — a practised splitter could get four roof slates from a block barely an inch thick. Sawing, planing and other dressing was mechanised in slate mills from around 1840-1860, but splitting itself resisted mechanisation and stayed a hand craft throughout the industry's life.

Why it was used

Slate's natural cleavage lets it be split into sheets thin enough to be light on a roof yet strong and completely waterproof, and — unlike thatch or timber shingle — effectively fireproof and durable for well over a century with no maintenance.

Value over time

Welsh slate exports peaked in 1889 at almost 80,000 tons, with Germany alone taking around 39,000 tons a year by the mid-1890s (worth some £226,000 in 1876 terms). Cyclical trade depressions from the 1880s, a severe downturn as Victorian building activity slowed, and growing competition from cheaper imported slate then eroded the industry through the early 20th century — German imports alone fell from 21,000 tons in 1901 to just 11,000 tons by 1910 — and Welsh production drifted into near-total inactivity by the 1970s, a striking fall from supplying a third of the world's roofing slate within living memory of its collapse.

Hazards & challenges

Splitting and dressing slate generated fine, constant rock dust in the sheds where that work was done, and slate quarrymen suffered heavily from it: an official inquiry (the Wardell inquiry) found excess mortality among slate and stone quarrymen specifically attributable to lung disease, years before the mechanism was well understood. Silicosis was formally recognised for compensation in Britain only in 1918, and slate dust is now also linked to reduced lung function and COPD in quarrymen independently of silicosis itself. Rock-face work added a second, more immediate danger: at quarries like Penrhyn, men worked exposed open galleries, sometimes hanging from ropes tied round their waists to reach the rock face at all.

Risks & limitations

Splitting slate was skilled, labour-intensive work that never mechanised, keeping production costs comparatively high; once cheaper slate (and, later, other roofing materials entirely) became widely available, an industry built on skilled hand labour had little room to compete on price.

Regional variation

North Wales was Britain's dominant slate-producing region by far, but the material and technique are far from unique to it — the Lake District, Cornwall and Scotland's Slate Islands (including Easdale, on this site) all worked their own slate on a smaller scale, and colour and texture vary noticeably by source: Welsh slate is typically blue-grey to purple, while, for example, Lake District slate runs distinctly green.

Images

Mines associated with this term

Included only where MineArchive's own research gives a reasonable evidential basis — not every mine where this might plausibly apply.

• Cwmorthin Slate Mine — Gwynedd (well documented example)
  A working example of the underground (rather than open-pit) slate mining common in the Ffestiniog district.
• Dinorwig Slate Quarry — Gwynedd (well documented example)
  One of the two largest Welsh slate quarries, alongside Penrhyn, at the heart of the industry's Victorian peak.
• Easdale Slate Quarries — Argyll and Bute (well documented example)
  A smaller, geographically distinct slate industry on Scotland's Slate Islands, ended abruptly by the 1881 flood — see this site's own account.

Sources

1. (secondary) Chronic obstructive pulmonary disease in Welsh slate miners — Occupational Medicine (Oxford Academic)
2. (secondary) DUST TO DUST: Silicosis cases expose government complacency on deadly engineered stone — Hazards magazine
3. (secondary) Historical aspects of the Welsh slate industry — D. Dylan Pritchard, via slateroof.co.uk
4. (secondary) Historical aspects of the Welsh slate industry: causes of contraction — D. Dylan Pritchard, via slateroof.co.uk
5. (secondary) Slate Quarrying Techniques - Working the Slate — Llechi Cymru / Welsh Slate
6. (secondary) Welsh slate: chiselling through time — The Past

Record created: 20 August 2026 · Last researched: 20 August 2026