From the Archives: The 3 Grades of Classification for Roofing Slates

This blog features a Slate Roof Quarterly editorial from June 2000 about the testing and classifying of roofing slates. It was written by our President & founder, Brian Stearns, who, besides being the owner of Alpine SnowGuards, got his start as a slate roofing contractor. He even consulted on slate roofing jobs in Japan! That’s him on the left in the photo below.

Slate Roof Quarterly Editorial by Brian Stearns 
As more and more slate roofs are installed in this country, there are more and more slate roof failures. As a slate roofing consultant, I am continually asked to visit projects where “the slate seems to be just falling off the roof.” It is common to hear, “We have tested the slate to see if it meets ASTM specifications and the results vary from S-2 – S-3. It is obvious that the slate supplied on this job is substandard and should therefore be replaced.” Well, to that I must respond with two questions:
  1. Was the slate tested before it was installed?
  2. How old is the roof?
According to ASTM test C-406, roofing slate falls into three grades of classifications:
  1. S-1 will provide a service life of over 75 years
  2. S-2 will provide a service life of 40-75 years
  3. S-2 will provide a service life of 20-40 years
(Service life is defined as a period of time over which the slate material may be expected to require no repair or replacement due to weathering).
The next question might be: what is this test telling me? Presumably this test is to be performed on new roofing slate. Let’s assume that a new sampling of roofing slate is tested and the results come back showing that it is an S-1 grade. This means that it should provide a service life of “over 75 years.” Now let’s assume that the slate is installed using galvanized steel flashings and 60 years later the flashings need to be replaced. During the flashing evaluation process, the person doing the planning decided to test the slate again to determine how much life there is left in the slate. The test results come back showing the slate is an S-3 grade, meaning that it has a service life of 20-40 years. Considering that the slate has been on the roof 60 years and the slate now tests as S-3, with a service life of 20-40 years, then it would stand to reason that indeed the slate performed for the originally stated “Over 75 years”. However, this previous scenario, when interpreted literally, is creating serious confusion in today’s slate roofing business.
The point is, that if the roof is more than one year old it could test as an S-2 grade now (service life of 40-75 years) and still have been an S-1 quality (over 75 years) when it was new. To take this a step further, if the slate is tested when it is one year old and the results show a slate of S-2 quality, does that mean that 40 years from the date the slate could have reached the end of its service life? This would indicate that S-1 roofing slate, if not tested until one year after it is installed on a roof, could arguably reach the end of its service life at 41 years. If this is the case, then it could also be argued that the S-1 grade slate if not tested for one year after it has been installed, could fall into the S-2 ride (20-40 years).
I have provided this illustration to make a couple of points. It would appear the ASTM testing for roofing slate must be done when the slate is new if it is to be used to determine the quality of the product being provided. Based upon the definitions of service life provided in the ASTM test, S-2 slate and therefore S-3 slate may have been of S-1 quality when it came out of the ground. The ASTM test for slate that is already installed on a roof can only be of value to determine the estimated service life of the material from that point forward. Who can say without proof what the quality of that material was when it came out of the ground? The second point is that of all the slate roofing projects I have looked at, I can think of one that failed due to the quality of the material. All of the other failures were a result of poor workmanship or structural problems.

We hope this is helpful to all who read it. You can check out all of our snow guards that were specifically designed to be installed on slate roofing!

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Alpine SnowGuards designs, engineers, and manufactures snow management systems from our facilities in Morrisville, VT. We work closely with leading roofing contractors, engineering firms, developers, and roofing manufacturers to ensure we deliver quality products that do what we say they’ll do.