Slate roofs can last more than century – if installed correctly

Posted by on September 3, 2013 in News | 8 comments

Slate roofs can last more than century – if installed correctly By LEIGH PROTIVNAK (originally on

A properly installed slate roof can last as long as 120 years with little to no maintenance. But slate roofing can be a disaster if the installer doesn’t use the proper flashing and overlapping, roofer Barry Smith told attendees at the recent Victorian Architecture Conference in Oil City and Franklin.

Slate roof shingles are made of rock mined or quarried up and down the Appalachians.

Pennsylvania black slate, abundant on area roofs, is a softer slate with a life span of 70 to 130 years. Harder slates that last longer often come from Vermont. Pennsylvania Peach Bottom slate, found in Lancaster and York counties, is considered by some roofers to be the best quality building slate in the world, Smith said.

World War II practically killed the slate roofing industry, and in the late 1980s when thousands of turn of the century slate roofs began to fail, property owners had a difficult time finding roofers to fix them, Smith said.

Smith studied slate roofing with Joseph Jenkins, author of “The Slate Roof Bible,” and owns Smith Slate Roof Restoration in Union City.

The tools used in slate roofing are slate rippers to pull slates off roofs without breaking them, cutters to size slates, hammers, and ladder hooks that don’t damage slates. The holes used for nailing shingles to roofs are cut into the shingles at the quarry.

The tools are simple and the methods are classic, but the work is grueling and anyone who is not properly trained could easily botch a slate roofing job, Smith said.

Felt underlayment is often used during slate roof construction to protect the insides of buildings, but Smith said the cloth – which keeps water out during construction but deteriorates long before the slate tiles do – could hide leaks and other common roof problems. When the felt deteriorates in 10 to 15 years, a problem might surface and the roof might have to be repaired or re-shingled. At that point, the contractor could be difficult to track down, Smith said.

Another common building practice is hanging slates on pegs on lath boards with large gaps between the boards. This practice is common in Europe because lumber is harder to come by there, Smith said.

The best method is to build solid board roofing underneath the shingles.

Some newer homes and buildings are not made to handle the weight of slates and would need to be reinforced before in order to install slate roofing, Smith said.

Smith showed examples of roofs that had been improperly laid and had to be completely replaced, and other roofs that were wrongfully condemned and just needed some minor repairs.

The most common problems in slate roofs are faulty slates, faulty flashings, and bad repairs. Any single old or damaged slate can be removed and replaced without disturbing the rest of the roof, Smith said. He showed slides of improper repairs that could cause more harm than good.

Smith advised property owners not to accept exposed nails or slates with too little overlap – ideally four inches on the sides and at least three inches on the tops. If the headlap is not long enough, there is no other option but to tear out the tiles and relay the roof, Smith said.

Homeowners should also be wary of roofers who attempt to use tar, roof cement or paint to fill in slate roofs to make them waterproof. Smith said roof cement and tar should never be used on a slate roof and silver paint is only suggested for soft roofs on their last legs.

“People will tell you they have a clue when they don’t,” Smith warned.

There are also some problems that can’t be fixed.

Namely, bad slates. When slates get old, they start to absorb water and flake and need to be replaced, Smith said.

Smith concluded with the classic rule, “if it’s not broke, don’t fix it.”

No matter how old the roof is, if slates look smooth, solid and clean, they’re probably OK, Smith said.


  1. Thanks for the great post. Can I ask, what is your opinion on synthetic slate tiles? Are they any good in comparison to natural slate or is it just price that makes them desirable. Thanks, Jane

    • Hi Jane, I have seen and heard about curling and faded tiles. Also, like any new roofing product, the jury is still out on the actual service life of these. I am not a fan.

  2. I must agree with you – I know a house from early 30′ with original roof! Which is, i think – incredible.


  3. Your point about the importance of correct installation of slate roofs to make them last longer was very well explained. It’s pretty amazing that slate roofs have the potential to last for more than a century. One of my friends told me about the felt underlayment that’s used during slate roof construction to protect buildings from water during construction. Is that the secret behind a roof that lasts 120 years? I’ve heard about how water can be very harmful to roofing materials, so having something that’s used to protect it would definitely make it last longer.

    • Hi Deanna,

      Underlayment is only needed to keep the roof weather-tight until the slate is installed. Many of the roofs we work on have extremely deteriorated underlayment, if it was used at all.


  4. There is definately a lot to find out about this subject.

    I really like all the points you’ve made.

  5. If you really love natural slate, then you will admire even the classic workmanship of American slaters. Am I jealous – you bet I am. American slate outclasses ours with its subtle range of colours and textures and outstanding kerb appeal. In England we do not board – sheath our roofs before tiling them – we should but we don’t- we use wood laths, and I’d like to ask Barry a question: (Q) when you have boarded a roof with OSB board do you next cover it with breathable membrane, then lath it vertically, then lath it horizontally like the Brits do, then lay the slates on top, or do you don’t bother with the laths at all and nail the slates directly onto the membrane covered OSB board. I wish thee well cousins, and do admire your work and safety procedures.

    • I don’t recommend OSB for sheathing under slate or tile. It would be the weak link in the chain. Traditionally solid wood boards (one inch thick) were used. These were usually installed with minimal space between them, though not always. Tar paper underlayment (which is not breathable) was sometimes installed. and then the slate was nailed directly to the sheathing. No lathe was used. This is the system I recommend for a new slate roof, except for the underlayment. We use the newer synthetic underlayments sometimes or 30# felt paper.

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