Terminology

• Slaty cleavage: the planes along which slate splits into sheets
• Roofing slate: slate split and cut for roof shingles, graded under ASTM C406
• Slate dimension stone: slate for flooring, steps, and cladding, covered by ASTM C629 (roofing slate is excluded from C629)
• Slate Valley: the Vermont and New York slate district

History and demand

First uses in construction

• Slate was used in building by the Roman period: the Roman fort at Segontium (Caernarfon) was first roofed with tiles, but its later levels contain many slates used for roofing and flooring
• Cilgwyn quarry in the Nantlle Valley, dating from the 12th century, is thought to be the oldest slate quarry in Wales
• Penrhyn quarry opened in 1782 and employed 500 men by 1792, producing 15,000 tons a year; Dinorwig output reached about 80,000 tons a year by the late 1860s
• Slate tiles were used for non-roof building work in 19th-century Britain, and Europe had a slate boom from the 1870s until World War I

Sourcing

• Spain is the largest producer and exporter of natural slate and supplies about 90 percent of Europe's natural roofing slate
• Brazil is second, with Papagaios in Minas Gerais yielding about 95 percent of Brazil's slate; China has large deposits and rising exports
• Wales is known for purple and green slate from North Wales
• United States: Pennsylvania, Virginia (Buckingham County), and the Vermont and New York Slate Valley; Monson, Maine, was a major source in the late 19th and early 20th centuries

Demand trend

• Welsh slate production peaked at over 500,000 tons in 1898, fell to about 70,000 tons by 1945 and about 22,000 tons by 1970 as tile competition, tariffs, and imports grew; Penrhyn still produced in 2022 at much reduced capacity

Grades and types

• Roofing slate (ASTM C406): grades S1, S2, and S3
• Dimension slate (ASTM C629): classed for exterior or interior use
• By color: gray, black, green, purple, and mottled slates from different sources

Properties

Physical

• Specific gravity: 2.7 to 2.8 (about 168 to 175 lb/ft3)
• Composition: mainly quartz, illite, and chlorite, up to about 95 percent of the rock

Thermal and moisture

• Water absorption: less than 0.4 percent, which gives frost resistance

Durability

• Service life: durable, potentially lasting several hundred years; ASTM C406 puts S1 roofing slate at over 75 years
• Chemically inert, and fireproof

Standards

Material specifications

• ASTM C406: roofing slate, grades S1, S2, and S3
• ASTM C629: slate dimension stone, with exterior and interior classes covering absorption, abrasion, acid resistance, and modulus of rupture

Test methods

• ASTM C97: absorption and density
• ASTM C99: modulus of rupture
• ASTM C241: abrasion resistance under foot traffic

Uses in buildings

Building elements

• Slate Roofing: shingles on solid sheathing
• Wall Cladding: cladding panels
• Countertops: laboratory and kitchen tops
• Site Paving: flooring tiles and steps

Pros and cons

Pros

• Very low absorption and long service life
• Splits into thin, flat sheets
• Fireproof and chemically inert

Cons

• Heavy roofs need strong framing
• Cost and weight limit use outside roofing and flooring

Manufacturing and cost

Main manufacturers and suppliers

• Buckingham Slate

Trade association: Natural Stone Institute.

Sources

• Wikipedia: Slate, https://en.wikipedia.org/wiki/Slate
• Wikipedia: Welsh slate, https://en.wikipedia.org/wiki/Welsh_slate
• IIBEC: dimension stone, https://iibec.org/wp-content/uploads/2024/10/2024-10-dimension-stone.pdf
• iTeh Standards: ASTM C406 roofing slate, https://standards.iteh.ai/catalog/standards/astm/1b104d49-443c-4b5b-80c0-c9897e302c7b/astm-c406-c406m-22
• ASTM International: C629 slate dimension stone, https://webstore.ansi.org/standards/astm/astmc629c629m22
• Image: Slate roof Switzerland.jpg, Michael C. Rygel, CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Slate_roof_Switzerland.jpg