Natural Slate

Natural slate is a fine-grained metamorphic rock that can be split into thin, flat sheets and used as a roof covering. It forms when shale, mudstone or volcanic ash is compressed and mildly heated deep underground. The pressure aligns flat mineral grains so that the rock develops a strong cleavage, a set of planes along which it splits cleanly. That property, combined with very low water absorption, makes slate one of the most durable natural roofing materials.

Composition and properties

Quartz, illite and chlorite typically make up most of the rock. Accessory minerals can include iron sulphides such as pyrite, iron oxides and carbonates. These minor minerals matter: pyrite can oxidise and rust-stain or weaken a slate, and carbonates can dissolve in acidic rain. Good roofing slate absorbs less than about 0.4% water by weight, which gives strong frost resistance, and it is non-combustible.

Quarrying and production

Slate is extracted in blocks from open pits or underground workings, cut into manageable pieces, and then split by hand along the cleavage using a chisel. The split sheets are trimmed ("dressed") to rectangular or patterned shapes and, for nail fixing, holed. Splitting remains a skilled manual job even in highly mechanised modern quarries.

Producing regions

Historic and current sources include Wales, Cornwall and Cumbria in Britain; Galicia in Spain; the Anjou region of France; Belgium, Germany, Italy and Portugal; Vermont, New York, Pennsylvania, Virginia and Maine in the United States; Newfoundland in Canada; Minas Gerais in Brazil; China; and parts of Australia. Spain is the largest producer and exporter today and supplies about 90% of Europe's natural roofing slate.

Standards and quality

• Europe: EN 12326 sets test methods and classifies slate for water absorption (A1/A2), sulphur-dioxide exposure (S1-S3) and thermal cycling (T1-T3). UK house-builder guidance has asked for A1, S1 and T1 slates.
• North America: ASTM C406 grades slate S1, S2 or S3 by expected service life, with S1 rated at over 75 years; the International Building Code requires roofing slate to comply with it.

On the roof

US model codes set a minimum slope of 4:12 (about 18°) for slate, with a headlap of 2-4 in (51-102 mm) depending on pitch. Standard slate weighs about 700-900 lb per square (roughly 34-44 kg/m²), and heavy architectural slate can exceed 1,200 lb per square, so roof framing must be checked. Slates are fixed with copper or stainless-steel nails, or with metal hooks.

Lifespan

Properly installed nailed or hooked slate roofs are commonly expected to last about 60-125 years, and the best slates can last several centuries. Failure usually comes from corroded nails ("nail sickness"), damaged flashings or deteriorating battens rather than from the stone itself.