Not all metamorphosed ocean sediments are capable of protecting a building for 100+ years. It takes a special combination of chemistry and geologic metamorphosis to create a slate durable enough to withstand centuries of exposure on a roof. Slate testing is important to determine quality.
The US slate industry has identified average breaking load, water absorption and depth of softening as the three physical properties of slate that relates most directly to the life expectancy of slate on a roof. These three properties are commonly evaluated using ASTM C406 Standard Specification for Roofing Slate. Based on the outcomes of three independent tests, the slate in question is assigned a grade. Each grade corresponds to an expected service life.
We specialize in Grade S-1, ASTM tested product that is never broken up, is handled meticulously, has all four full corners and maintains consistent color and thickness from pallet to pallet. Our quality control gives you slate that produces a solid ring and has a flat bottom and 100+ years guarantee.
This page lists the ASTM C406-10 specifications for Grade S1 slate roofing material (testing was performed by Froehling & Robertson, Inc. on April 24, 2015).