TRADITIONAL ROOFING MAGAZINE Iisue 8 — The Incredible Slipping Roofing Slates

The Incredible Slipping Roofing Slates

Joseph Jenkins

People are alarmed about the slipping slates theyre seeing on new slate roof installations.

Being as invested in the slate roofing trade as I am, people are always contacting me with questions about slate roofs. When the same question is asked enough times, I make a note to write an article about the subject in the next TR. That is the genesis of this article which can now be found online to be read by anyone at any time without me having to repeat myself over the phone to another stranger.

Slipping slates or shedding slates are slates that appear to be sliding out of place on a roof, or are missing (Figure 1). There may be a few, there may be hundreds, but in most cases, the defect is associated with new slate roofs. The slates may slide out days or weeks after the roof has been installed, or years later usually within ten years, but most often within the first five. The questions most often asked are, Are some slates supposed to fall out after installation? What is an acceptable number of slipping slates per square? What causes the slates to shed?

Brief answers to these questions are this: no slates are supposed to fall out after installation and there is no acceptable number of slates that can fall out. A new slate roof installation should be 100% intact nothing slipping, sliding, shedding or breaking. If there is a slate or two that looks like its broken or sliding out, the installer should be back there pronto to repair it. This is a good reason, by the way, to not settle for a one year warranty on a new slate roof. Get a five year warranty at least. A good installation contractor will stand behind his work and repair any defect at no charge no matter how much time has elapsed since the installation.

But what causes slates to break and fall out? There are three main reasons:

1) The slates themselves. Of all the slate roofs I have looked at with shedding slates, this is the least likely cause of the problem. However, not all slate is the same. Slate is a natural stone that is hand-split into roofing shingles. Some stone types are more brittle and more irregular than others. For example, we installed two slate roofs, each having about 30 squares of standard-thickness (3/16 — 1/4) slate shingles. One was Vermont sea green slates produced by a conscientious manufacturer. Of these 30 squares, only about 3 slates had to be culled out because they were broken or defective. We never had to go back to this job and repair a single slate. On the other roof, we installed Vermont mottled green and purple slates from a less-than-conscientious manufacturer. We had to cull out nearly a full pallet of defective slates. Five years later, we still have to go back to the purple roof every year and repair some broken slates that seem to be falling apart on their own.

The installation procedures were exactly the same on both roofs. The only difference was the type of slates and the manufacturing quality. This is not to suggest that Vermont mottled green and purple slates should be avoided they should not, as theyre some of the best slates in the world. What matters, however, is manufacturing quality. The sea green slates had been individually hand punched for nail holes. Each slate was handled by various people in the manufacturing process several times splitting, trimming, punching and palleting, with plenty of opportunity to cull out defective slates. The purple slates, on the other hand, had been hand drilled in stacks, like pancakes. That meant that stacks of slates were being rushed through the manufacturing process without the needed scrutiny that would have eliminated a lot of cracked or defective slates. Also, the nail holes must be in the thinner end of a roofing slate. When the slates are individually hand-punched, the worker can make sure the holes are being punched on the thinner end (if there is one). When slates are being stacked and drilled, a number of them will be drilled on the wrong end. These will not lay well on a roof and must be culled out as rejects.

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