A new slate roof is a big investment. Whether you are reslating an addition, or installing slate on your custom home, you want your new slate roof to last its intended life span. There is only one way to install a slate roof – the correct way. Installing slate correctly only comes from experience.
This home in the Guilford neighborhood of Baltimore City, demonstrates the need for quality workmanship. A lack of a consistant 3" headlap, shody workmanship, and missing lathe strip show how going with the low bid can be a very costly mistake. Usually, we might see one important element of a slate roof being violated, but this roofer violated all of the rules of a quality slate roof. Unevenly spaced and crooked rows in the curved section of roof demonstrate poor workmanship. It was revealed to me that this roof was installed for $35,000 and we can see why. This roof, if installed by a professional slate roofer would have cost over $50,000. But it will have to be redone.
Experienced slate roofers know how to drive the nail in to hold the slate, what type of nail to use, and how to lay out the roof and starter coarse. The fasteners holding the slate on the roof have to be set correctly, not driven too loosely and not driven too tightly. The slate should just hang on the nail, with the head of the nail driven just into the recess of the pre punched nail hole. If the nail is driven too tightly, the slate wont hang loosely, and will break with the change of temperature, load, and other forces acting on the roof. Each slate must move independently of every other slate on the roof. If the nail is driven too loosely, the nail head will break the slate above it.
Using the wrong fastener is detrimental to a new slate roof. Many inexperienced slaters are quick to reach for the wrong nail when applying slate. A large head copper slating nail should be used in the correct length to match the thickness of the slate. A non-corrosive metal is a must. Copper should be used at all times, especially on low sloped roofs where water tends to shed slowly. I have seen a few roofs both repaired with the wrong fastener and installed with the wrong fastener. Corrosive fasteners disintegrate to the point where virtually nothing holds the slate in place - and the slate fall out!
Going back to the Guilford house, this picture shows a lack of headlap. Even though this section of roof is steeply sloped, adequate headlap should be used throughout the roof. Two inches might be acceptable on this slope, but no headlap is not acceptable.
Finally, among the most frequent problems encountered with new slate installation are inadequate side and headlap and the failure to install lathe strips under the starter equal to the thickness of the slate.
Slate is sold by the square. One square equals one hundred square feet of coverage using a 3" headlap. Shortening the headlap or trying to "stretch" the slate out over the roof using less than a 3" headlap is an unacceptable practice sometimes used to cover more square footage with fewer slate.
One of the most detrimental omissions on a new slate roof, however, is the failure to install a lathe strip under the starter course of slate. Absence of the lathe strip will result in a condition where all of the slate above the starter slate will be cantilevered across the slate below it, or rather the exposed part of the slate will be unsupported by the slate below. This condition will reveal itself after the roof has been installed after a heavy snow, or storm. The result will be a small percentage of slate will crack down the middle and slide down the roof.
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