Slate Roof System Fundamentals and Materials
Domain 1 - Slate Roof System Fundamentals and Materials
This domain tests whether you can identify, grade and specify roofing slate, and prove the numbers behind a slate layout.
What roofing slate is
Slate is a low-grade metamorphic rock formed from shale; recrystallized mica and chlorite give it a directional grain (slaty cleavage). Every slate must hang with its length parallel to the grain - the long dimension running down the slope. A slate set cross-grain can split along the grain and delaminate in service. The grain shows as a faint directional sheen (as 'ribbon' in Vermont slate) and is one of the fastest ways to sort a delivery.
Commercial types
Common commercial slates include unfading gray-black and sea green (Vermont/New York), purple and red (New York/Vermont), hard black (Pennsylvania) and mottled. Weathering grades shift color over decades; unfading grades hold theirs. Hard slates are dense and wear slowly; soft or ribbon slates show strong color banding and need those bands kept back from corners and edges.
Grades and testing
ASTM C406 defines roofing slate grades S1 (the most demanding, used for permanent roofing), S2 and S3, with the looser grades restricted to steep slopes and non-critical work. Reject slates with knots, ribbons at the edge, spalls (hard lumps that chip under a hammer), curl, warp, chipped corners or broken nail holes. ASTM C120 tests flexure (modulus of rupture) and ASTM C217 tests freeze-thaw weather resistance.
Size, thickness, coverage
Slate is sold by size (12x20, 16x24, 18x24 in., etc.), thickness (3/16, 1/4, 3/8, 1/2 in.) and by the 100 sq ft square. Weight climbs with thickness, so verify deck and framing before promising a slate.
Worked example
Specify coverage for a 16x24 in. slate with a 3 in. headlap. Exposure = (length - headlap) / 2 = (16 - 3) / 2 = 6.5 in. Exposed area per slate = 24 x 6.5 = 156 sq in. Slates per square = 14,400 / 156 = 92.3, so order 93 per square plus 5-10% waste.
Traps
Using length minus headlap without halving it; mixing weathering and unfading slate in one field; accepting S2/S3 for permanent work; laying slate cross-grain.