Cambrian and Ordovician Welsh slate masonry

Reading the stone

Slate began as mud on an ancient sea floor. More than four hundred million years ago the Caledonian mountain-building squeezed that mud, and the clay minerals in it turned into thin parallel plates of mica and chlorite. Those plates are the cleavage. It runs through the rock independently of the original bedding, which is why a mason must read both before lifting a tool.

Penrhyn slate, quarried near Bethesda, is Cambrian, around five hundred million years old, and carries a heather-blue to purple cast from fine iron. Ffestiniog slate from the Blaenau valleys is Ordovician, somewhat younger, a cooler blue-grey with a famously long, flat, even cleavage. We buy from both, and we keep them apart on the racks because they weather in different tones.

Splitting by hand

Blocks reach us damp from the quarry, and we keep them under wet sacking, because dry slate fights you. The mason sights along the grain, sets the broad-bladed chisel in the edge and taps it with a mallet. In the workshop we call that chisel the rhiniog, borrowing the Welsh word for a threshold: it marks the line where a block crosses over into slate.

The block parts. Bryn halves it again, and again, until the pieces are five to eight mm thick for roofing, or twenty-five to forty for hearths and cills. A riven face keeps the faint ripple of the cleavage plane, and it catches low light in a way no saw can imitate. Sawn and honed faces have their place, usually in kitchens, and we cut them too.

Hearths, cills and thresholds

Slate shrugs off heat, damp and frost, which is why it ends up in the places that suffer all three. A hearth is cut oversize, riven on its visible face and bedded on lime mortar. We set to the clearances in Approved Document J, and we ask your stove installer to confirm the depth.

A window cill needs a fall of five to ten degrees, a throating groove under the nose to stop water creeping back to the wall, and stooled ends so the mason can build the jambs onto it. A threshold is the simplest job we do and the one visitors remember longest.

A hundred years without delamination

Good Welsh slate is specified under BS EN 12326-1, with the test methods in part 2. Its classes include water absorption below 0.6 per cent (W1), thermal cycling (T1) and sulphur dioxide exposure (S1). Water absorption that low is the quiet reason freeze and thaw barely trouble it: very little water gets in to freeze. Quarry suppliers declare a service life of a hundred years or more, and roofs across Gwynedd are still shedding rain after a century and a half.

We ask every supplier for the declaration of performance and keep it with the job file.

Building with slate

Slate is heavy, about 2.8 tonnes a cubic metre, so a forty mm slab weighs over a hundredweight a square metre and the structure under it must be checked. We bed it in lime rather than cement, because lime lets walls move and breathe, and a cement-bound cill cracks the first hard winter.

The table above uses the same indicative figures as the calculator on our home page. For a real roof, specify it with your roofer against BS 5534 and BS 8000-6 and the supplier’s data. To commission slate, start with our commissioning guide.