Bad weather is coming. Very bad weather. Winds, driving rain—the stuff that makes us pause and lok up at the roof. How does slate hold up compared to other material choices? The American Society of Testing on Material has established a standard specification for roofing slate, C-406, which tests the slate for three physical requirements that the slate must pass to be classified as an S-1 to grade material:
ASTM C 120 - Modulus of Rupture … This test determines the breaking load, modulus of rupture and modulus of elasticity of slate. The modulus of elasticity is not overly important for roofing slates, but should not be entirely discounted.
ASTM C 121 - Water Absorption of Slate … Porosity of the material is tested by submerging the slate samples in water for defined periods and then gauging absorption.
ASTM C 217 - Weather Resistance … This standard defines the depth of softening as an expression of weather resistance of a slate. The depth is determined by a shear/scratch tester or a hand scraping tool.
After being crushed by hurricanes in the early 1990’s and seeing staggering losses from sub-standard building practices and materials, the government in Miami, Florida, decided to do something about it. New standards, testing and licensing were created by legislators that raised the bar higher than anywhere else in the United States. In order to have your product “Miami-Dade-County “ approved the product must pass many stringent tests.
This includes the wind up-lift test where the slate was tested on a roof and subjected to 110 mph wind with gusts up to 140 mph with no movement detected. Slate was also subjected to a wind-driven rain test in which 90 to 110 mph winds were blown against a 2:12 pitch roof while 8.8 inches per hour of rain fell. The result? No water penetration was detected underneath afterward. This testing is by far the most severe for roofing products to pass and only clay tile materials could come close to the performance of slate.