ARCAT.com is the most widely used building product information website and contains an up-to-date library of our slate specifications in a variety of formats.
California Slate is a proud member of the Slate Roofing Contractors Association of North America, Inc. (SRCA) which publishes “Section 07310 — Slate Shingles” setting forth architectural specifications for a slate roofing project. The following in this section is from that document.
The American Society for Testing and Materials (ASTM) is an international standards organization that develops technical standards for a wide range of materials. They developed three tests for roofing slate to determine the water absorption, the fracture strength, and the resistance to corrosion. Their highest rating of S1 would be awarded to slate with low water absorption, high fracture strength, and substantial resistance to corrosion. Slate with an S1 rating has a projected life span in excess of 75 years. S2 is rated for 40 to 75 years. S3 is rated for 20 to 40 years.
The maximum absorption test measures how much water the slate rock will absorb. It’s a simple test: a piece of slate rock is weighed dry and then submerged in water for a certain time period. Then the rock is weighed again. Higher quality rock will have absorbed less water and will have gained less weight after being soaked than lower quality rock.
The “Modulus of Rupture” test is designed to determine a slate’s fracture strength – the load required to break the stone. A piece of slate is subjected to an ever-increasing load while bridging two points of a specified distance. The heavier the load required to break the slate, the higher the quality of the slate.
A piece of slate’s resistance to corrosion is tested by submerging the slate in a weak solution of acid for a particular amount of time and then examining the surface of the slate. The deeper the acid solution was able to penetrate the rock, the lower the quality of slate. Generally, harder rock is more resistant to corrosion than softer rock.
These tests are fairly straightforward for a single piece of slate. But unlike the uniformity found in a manufactured product, slate is a natural product and each individual piece is idiosyncratic with lots of variability—particularly in fracture strength. A random sample of poor quality slate could pass tests required of higher quality slate, or vice versa. To combat the variability, the tests are often repeated on about 20 individual pieces of each slate being tested—which can become expensive.
These tests do reveal something about the quality of slate produced by a particular quarry. If slate samples from a particular quarry consistently rate high, then the quarry itself contains at least some good stone. But provenance isn’t enough. The quality of stone in a particular quarry can change depending on the level being mined, among other factors.
If a particular load of slate is going to be installed in a climate which has extreme freeze and thaw weather cycles, it is important to ensure that it has an extremely low rate of absorption – otherwise it is certain to fail over time.
Conformité Européenne (or European Conformity, CE) is a marking for products sold within the European Economic Area (EEA). It is a manufacturer’s declaration that the products are in conformity with applicable European directives—for slate roofing material, that directive is “EN 12326-1: 2004 Slate and stone elements for roofing” (the “European Norm”). The European Norm classifies slate products using product definition categories: the regularity of thickness tolerance, uniformity, and dimensions; and three different classifying measures: water absorption, risk of rust (called “termic impacts resistance”), and resistance to polluted environments (called “exposition to sulphur’s dioxide”).
Each crate of slate with the CE marking should contain three grades:
Here are some examples of how to decipher the label on the crates:
Non porous slate but sometimes it can have superficial rust with or without runs.
Slate is sometimes porous and can have rust. Don’t ever fix in aggressive atmospheres.
The Solar Reflectance Index (SRI) is a measure of the roof’s ability to reject solar heat, as shown by a small temperature rise. It is defined so that a standard black (reflectance 0.05, emittance 0.90) is 0 and a standard white (reflectance 0.80, emittance 0.90) is 100.
In California, the Energy Commission’s Title 24 Standard is the statewide building code regulating energy efficiency standards. Coolroofs.org, maintained by the Cool Roof Rating Council, has a document with more information: “Title 24 Update: Summary of 2013 Changes to California’s Cool Roof Requirements.”