I’ve been asked to examine slate roof installations all across the U.S., and one of the most disheartening things I’ve observed is how often incorrect side laps are used. For example, the photo shows a slate roof that was installed less than one year ago and already has more than a dozen leaks. Why? Among one of the most basic problems is the side lap.The side-lap detail drawing that appears on this page is from the National Slate Association’s Slate Roofs: Design and Installation Manual, page 86, Detail 5-B. The side lap also is referred to as a side joint, vertical joint, keyway, bond line or rain course. As defined in the glossary of the NSA manual, it’s “the longitudinal joint between two slate shingles”.
Whenever one is installing a slate roof, it is absolutely imperative the side lap is a minimum of 3 inches. As seen in the detail, if the lap is less than 3 inches, water will flow in between the two slates and leak into the building. When I’m asked to inspect a problematic new slate roof, I find the side and head laps are the problems on a majority of all cases across the country.
If you’re installing a single-sized slate, such as 20 by 12 inches, the slates should be installed so the joints are exactly split in two; the side laps on the whole roof should be 6 inches. Similarly, if the roof has 10-inch-wide slate, the side laps should be 5 inches. Whatever the width, the side lap should always be one-half the width on a single-sized slate.
It gets a little trickier on a random-width slate roof. Slate widths can be as narrow as 6 inches or as wide as 20 inches or more. When dealing with 6-inch slates, the joint obviously must be split exactly in the center, so there are 3 inches on each side. If you question the width, pull out a tape measure; this will save you and the building owner lots of money and headaches. When there are inadequate side laps, inevitably, the owner, architect or general contractor gets concerned, and then I get a phone call to do a full roof survey on the slate roof.
As slaters become more advanced, they are able to eyeball 3 inches extremely well, but until that point, installers should use a tape measure, or they should stick with using single-sized slates. It might seem too easy, but this is one of the most common errors I encounter. If a slater studies Detail 5-B and adheres to it, he or she will avoid having this problem with slate roof installations.
Too often, slate is given a bad name because of poor installation. Hopefully, this article and detail will resolve that problem.