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What is an S-1 Slate?

ASTM C 217 -09- Weather Resistance

This test method is useful in indicating the differences in weather resistance between various slates. This test method also provides one element in the comparison of slates. The difference between the longevity of S-1, S-2 & S-3 slates is significant and this is an excepted method of that determination and should be a major consideration in the selection of a premium roofing slate.

The C 217 standard defines the depth of softening as an expression of weather resistance of a slate. This depth is determined by a shear/scratch tester or a hand scraping tool.

The idea is that slates contain pyrite, calcite and carbon and that during weathering calcite turns to gypsum, leading to a bulking. This bulking results in a softening of a slate and thus, the depth of softening correlates somehow with the weather resistance.

At least three specimens have to be prepared, each specimen is 2 by 4 in. (50 by 100 mm) to determine the depth of softening in a 1 % solution of sulfuric acid. The surface of the specimens has to be polished and finished with No. 80 abrasive.

On the fresh specimens the depth of a groove is measured. The groove is made by a shear/sratch tester. After this same specimen is soaked for 7 days in 1 % sulfuric acid a new groove is made and the depth is measured. The deeper the groove the lesser weather resistant is a slate. In other words, the depth of the resulting groove is an expression of the intensity of softening and allows statements about the weather resistance relative to this result.

All procedures are exactly defined in the standard so that the results are reconstructable and comparable with other slates and follow excepted scientific methods.

The classification of the roofing slates is as follows:


max depth of

softening [in./mm]

Grade S1


Grade S2


Grade S3


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