Slate is one of the most durable roofing materials in use today. Depending on the type of slate, the pitch of the roof and its installation, a slate roof could last from 60 to more than 200 years before it needs to be replaced.
Since most homeowners with a slate roof did not have it installed themselves, they may be unsure about when it needs to be replaced. There are several ways you can tell if your slate roof needs to be replaced or repaired.
Try to find records from when the roof was installed. This should give you an indication of its age, and possibly its variety. Slate tiles that were quarried from New York may last 175 years, where as tiles quarried in Pennsylvania could last up 200 years if they came from hard veined quarries. Any information about your slate roof could be used to help determine its lifespan.
Examine the slate tiles for flaking, or delaminating. Slate is made of several layers, which have been compressed. These layers will flake or peel away over time unevenly. Look at the surface of slate tiles from several sections of the roof. Check to see if the surface of the tile is uniformly flat, or if it appears to thicker in one area and thinner in another; this is a sign of flaking. Slate tiles with excessive flaking will soften and need to be replaced.
Look for cracked, broken or chipped tiles. Examine the entire tile, not just the edges, as some varieties of slate tiles were manufactured to have a rough or uneven edge. Look for large variation between the tiles in texture, missing pieces from some tiles, or cracks that go through the stones.
Look at the eaves or beams below the slate if possible. Check the wood for water damage, softening or weakening. If the slate tiles are allowing moisture in, the wood could be compromised. Slate tiles rarely have significant, obvious leaks; so look for subtle damage or soft wood.