Mastering Roof Inspections: Slate Roofs, Part 2

by Kenton Shepard and Nick Gromicko, CMI®

Slate Grades

• Slate grade S1 has a minimum lifespan of more than 75 years.
• S2 is rated for 40 to 75 years.
• S3 is rated for 20 to 40 years.

Slate can be grade-tested by a laboratory for compliance with ASTM standards, but this won’t tell what the remaining lifespan is with any real accuracy. Slate rated S1 may last anywhere from 75 to 200 years or more.  Almost all slate installed on new roofs in North America is rated S1.

Grading Standards

ASTM C-406 is the standard used to grade slate. It consists of three separate test types:

• C121 for water absorption;
• C217 for weather resistance; and
• C120 for flexure testing.

Complete grade testing requires about 20 samples for each slate being tested, and is somewhat expensive, so testing is seldom done on residential slate roofs.

Slate Lifespan

Although the slate itself may be very durable, the lifespan of a slate roof is limited by its weakest component. The slate may be of a type known to last 200 years, but if the fasteners or the crucial underlayment fails after 50 years, then the lifespan of that slate roof is only 50 years.

Foreign Slate

In addition to being quarried in the U.S., slate is imported from a variety of sources.

Purple and grey-black Welsh slate was imported into cities along the Eastern Seaboard prior to 1840. It generally has an extremely long service life and may still be seen on roofs today.

Large amounts of inexpensive slate have been imported from China since the 1980s. Chinese slate comes in a wide range of colors and quality.  The example below is common.

Looking closely, you can see that the slate has cleaved less evenly than the other samples. The different minerals show up as different colors, and you can see that they’re not very uniform across the face of the tile. The dark area in the upper left quarter is likely to have a high carbon content, eventually appearing as a mineral occlusion and failing sooner than the other portions of the tile. It’s generally brittle, so it’s more easily broken than North American slates. It may also be soft, and the colors may be unstable.

Although they may be seen anywhere in the U.S., Chinese slates are most common in California, in the southern U.S., and in states along the Eastern Seaboard.

Performance problems with Chinese slate are more common in northern areas where snow and ice cause a freeze-thaw cycle that can cause delamination.  Homeowners sometimes insist on having these slates installed on roofs even after being told that they will be damaged by frost.

Inexpensive slate with similar properties also comes from India and Brazil. Low-quality, black Spanish slate has been sold in the U.S. since the 1980s. It has oxidizing iron pyrite that appears as rust stains. Spain does produce some very high-quality slate, but very little of it is sold in North America.