ASTM C 121 - Water Absorption

At least 6 specimens has to be tested with a thickness between 3/16 to 5/16 in. (5-8 mm) and not less thean 4 in. (100 mm) on any side.

The specimens has to be dried in a ventilated oven until the mass doesn't change anymore. After drying, the specimens has to be cooled for 15 min and then to be weighed. The weight has to determined to the nearest 0.0005 oz (0.01g).
Afterwards, the specimens has to be completely immersed in filtered or distilled water at approximately 68 ± 9 °F (0 ± 5 °C) for 48 h. After removing from water and wiping the surface the specimens has to be weighed to the nearest 0.0005 oz (0.01g).

The absorption is calculated in percentage of absorption as follows:
Absorption [%] = [(W₂ - W₁/W₁] x 100

where:
W₁ = weight of dried specimen
W₂ = weight of specimen after immersion

The classification of the roofing slates is as follows: