Terminology

• Roofing slate: natural slate split and cut for roofing
• Grade S1, S2, S3: ASTM C406 grades by breaking load, absorption, and depth of softening
• Headlap: overlap of the course above on the course two below
• Shingle: single piece of slate in a course
• Slating nail: corrosion-resistant nail for slate; copper nails are common

Standards

ASTM C406 grades

• S1: service life over 75 years; water absorption 0.25 percent maximum
• S2: 40 to 75 years; absorption 0.36 percent maximum
• S3: 20 to 40 years; absorption 0.45 percent maximum
• Modern US slate is almost entirely S1 or S2, mostly S1

Application

• Slate is fastened to a solidly sheathed roof, two fasteners per slate
• Headlap varies with slope; one adopted code lists 4 in. for slopes from 4:12 to 8:12
• Check the manufacturer and code for the minimum slope before specifying

Materials

• Natural roofing slate: quarried and split in several sizes and colors
• Copper or other corrosion-resistant nails and flashing: match the long life of the slate
• Underlayment and solid sheathing: deck for the slate

Detailing

• Design the structure for the weight of slate
• Provide solid sheathing and underlayment per the code reference
• Use two fasteners per slate and the headlap for the slope
• Flash valleys, hips, ridges, and penetrations in compatible metal
• Specify the grade, color range, and thickness in the submittal

Uses

Specific uses

• Historic and high-end houses
• Churches, schools, and civic buildings
• Roof restoration and replacement
• Long-life roofs where weight is acceptable

Pros and cons

Pros

• Very long service life for S1 slate
• Distinctive natural appearance

Cons

• Heavy and needs strong framing
• High material and labor cost
• Needs skilled slaters

Manufacturing and cost

Construction

• Slate is quarried, split into thin pieces, cut to size, and punched or drilled for nails; installers lay courses from eave to ridge with the required headlap

Main manufacturers and suppliers

• Buckingham Slate

Sources

• Buildipedia: 07 31 26 slate shingles, https://buildipedia.com/knowledgebase/division-07-thermal-and-moisture-protection/07-30-00-steep-slope-roofing/07-31-26-slate-shingles/07-31-26-slate-shingles
• iTeh Standards: ASTM C406 roofing slate, https://standards.iteh.ai/catalog/standards/astm/1b104d49-443c-4b5b-80c0-c9897e302c7b/astm-c406-c406m-22
• Roof Observations: slate roofing introduction, https://roofobservations.com/slate-roofing-introduction/
• Roof Observations: minimum roof pitch for slate roofs, https://roofobservations.com/minimum-roof-pitch-for-slate-roofs/
• Massachusetts 780 CMR 1507.7 slate shingles, https://regulations.justia.com/states/massachusetts/780-cmr/chapter-15/section-1507/subsection-1507-7
• WJE: ASTM C406 testing standards, https://www.wje.com/expertise/laboratories/testing-standards/astm-c406