Slate Roof Leaks:
How To Find & Fix the Leak


Slate roof leaks are beginning to show up more and more lately, due mainly to the fact that most slate roofs were installed in the late nineteenth century and the early twentieth century and they are reaching the end of their estimated 60 to 125 year life span. They are considered to be an important part of historic construction and should be repaired rather than replaced whenever possible.

The key questions to ask concerning a slate roof:

  • Is the slate “hard” slate or “soft” slate?
  • How much life remains? (There is a church in England whose roof is 1100+ years old)
  • What will it take to repair my slate roof leak?
leaks in slate roofs

Tracking Down The Leak

One of the first places to check for slate roof leaks is near valleys, areas of roofing where two rooflines come together. Small openings or gaps could be allowing water to flow in and then under the valley flashing. Another concern near valleys is if there have been previous repairs, the felt underlayment used on slate roofs may have been torn during an earlier repair effort. Many times the underlayment will not have been repaired so look for leaks below older repairs.

90 percent of the time broken or missing slate tiles are visible from the ground. You are looking for a slate that that is missing, a tile that has slipped and is no longer in line with its neighbors, or a cracked slate that has a vertical crack. It is these types of issues that lead to slate roof leaks.

Ice dams are also a common cause for leaks in slate roofs. Usually a heavier grade of felt is used near the eave edges but every nail used to secure the roof also punctures the felt underlay, making these areas more susceptible to leaks. Also the felt will age much faster than the slate, often drying out or just plain disintegrating. This is especially prevalent in homes that have had their attic spaces converted into living spaces. The new ceilings can trap moisture against the underside of the roof, adding to the problem. The solution in this case is to make sure there is adequate venting to allow heat and moisture to be released and not trapped against the slate roof. Slate roofs require proper ventilation to equalize humidity and temperature conditions between the inside of the house and outside environment. Mineral deposits on the inside of slate roofs, even when they can’t be seen outside, are often a indicator that ventilation is a problem.

Faulty flashings are another area of concern that can cause slate roof leaks. The flashings will be located around chimneys, dormers, or vents. These metal flashing will fail long before the actual slate tiles, so they should always be inspected first if a leak occurs.

slate roof leaksOld slate roof

A rather obvious cause of slate roof leaks would be missing or cracked tiles. Eliminating anything that could fall onto the slate roof, such as heavier tree branches, should be considered for removal. Periodic roof inspections by qualified roof slate installers are recommended to inspect for this type of damage.

Moss can damage slate roofs and cause leaks. Moss holds moisture that is not good for slate, and its roots can eventually penetrate between the tiles themselves. This could lead to cracking or chipping individual tiles. In addition, this creates an entry point for water to affect the home. A note of caution here, be careful about using chemicals to remove the moss as using certain chemicals could cause further damage to the slate tiles themselves.

Repairing Slate Roofs to Prevent Leaks

If repairs are not carried out promptly, water damage can occur that may affect not only your home’s interior, but also accelerate structural damage to the roof itself.

When replacing a damaged slate tile with a new one, a copper or galvanized nail should always be used and a small copper or lead strip of metal should be placed under the joint between the two tiles directly above the replacement tile. Be careful not to hammer the nail too tightly as this can pinch the tile and cause it to crack. This strip for metal can then be bent upward to act as a “clip” to help hold the replaced tile aligned in its proper position. You should never use a sealant or other type of roof mastic as they will, with age, eventually crack and make future repairs even more difficult. Also, the sealant itself will inhibit the natural flow of water downward and cause the water to “back up” and then flow sideways, creating a leak.

Prevent Slate Roof Leaks Through Timely Maintenance

Every slate roof should be inspected annually and especially after any severe storm event. It is recommended that you use binoculars from the ground or a cherry picker to view the roof, and avoid actually walking on the roof. Roof traffic is a major threat to slate roofs. If you must actually get on the roof, you should use a ladder to stand on, and avoid standing directly on the slate tiles.

You should also inspect for slate roof leaks in the attic. Look at the rafters and underside of any wood sheathing or lattices, inspecting for water stains. When you see a stain on any wood, the stained area should be checked with a moisture meter to determine if it is a new problem or staining from an older repair. The areas where problems are most likely to occur are near valleys and near the roof’s peak.

A slate roof is said to be one of the best roofs you can buy. The cost of a slate roof is somewhere around seven and a half times as much as a asphalt shingled roof. However, where a shingled roof is good for 15 to 30 years, a slate roof will last for up to 125 years, or even 200. If you have a slate roof that is leaking and you would like a pre-qualified expert’s opinion on what needs to be done to stop the leak, call 877-960-0491. You can talk to a local professional, 24 hours a day, who can answer any questions you may have.




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Written by Mark Huey.