The title should really be "Lack of Adequate Headlap," because it is a common problem with newly installed slate roofs. Headlap; what is it and why do I want more of it?
Headlap, or the relationship of overlap of one slate to the 2nd slate above it. Headlap is one of the most important technical aspects of slate roofing.
Roof slopes between 5 in 12 and 12 in 12 should have a minimum of 3" of headlap. Steeper sloped roofs can get away with less headlap, but why take a chance? Roofs under 5 in 12 should have 4" headlap.The picture on the left shows a steeper sloped roof with no headlap. This is often done to conserve material. The less headlap used requires fewer slate. This is a shortcut that has had dire consequences for many slate roofs.
Wind driven rain can enter the keyways, and chances are if there is little or no headlap, the rain will find its way into the house as a leak. The photo below is an extreme example of what some roofers will try to get away with. It may seem like a good idea to try to "save" money by having no headlap on a steep roof. This only invites trouble for the homeowner. This is a common occurance associated with accepting the lowest bid on a job. On lower sloped roofs, this is a disaster.