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Slate Roofing, Outlasting the Competition

By Jonathan Stevens, Assoc. AIA, LEED BD+C

Slate roofing has been around for hundreds of years and adorns many historic buildings.  Architects and designers choose slate because it is beautifully elegant and a highly durable roofing material.  Slate, as we see and use it today, was formed in one of two ways millions of years ago by heat and pressure within the earth.  The most common type was formed from marine deposits of clay and sand called aqueous sediments.  The second type, which is rarer, is called igneous sediment and was formed from volcanic ash.

As a stone, slate is very hard, will not saturate with water, and is completely fireproof.  With the right tools slate can be split into thin pieces that can be used as a roofing material.  It is highly resistant to weather, varying temperature changes, and mildew allowing it to have a long lifespan with a relatively low maintenance cost.

Slate tiles are not hung or nailed, they are held down by the shaft of the nail.  The large heads of a roofing nail are important because they help prevent uplift in windy conditions.  Slate tiles are punched for nails from the back side of the tile, allowing the nail to be counter sunk from the front.  This allows the head of the nail to be flush with the top of the slate.  Nails that are set too tight can break the slate tile and nails that do not sit flush with the top of the slate can break the next overlapping course of slate.

There are a number of reasons why slate roofs fail or deteriorate over time.  Even though slate itself has a long life expectancy, the metal flashing used to help prevent water from entering the building tends to wear out and leak long before the slate itself fails.  The most common locations of flashing failure are in roof valleys, along dormers, and around penetrations.  Slate roofs, however, are able to be dismantled and put back together, allowing maintenance of flashing to be done without replacing the entire slate roof.  Another issue with older slate is that they could break or slide out of place allowing leaks to occur.  Broken or missing slate should be removed and replaced with slate matching in size, shape, and color.  Slate hooks and bibs are used to secure replacement slate in existing slate locations.

The weight of a slate roof is a major consideration to take into account when choosing slate as a roofing material, especially if you are installing it on an existing structure that doesn’t currently have a slate roof.  Slate can weigh up to three times as much as a similar roof using asphalt shingles.  Because of this the structural integrity of a roof should be evaluated by a structural engineer before slate shingles are added.

In order to keep your slate roof operating and looking its best for many years to come regular maintenance should be performed:

  • From the attic, check the roof rafters and roof sheathing for water stains at all the critical areas of the roof, at valleys and hips.  
  • Clean the gutters periodically to prevent water backup during heavy rain.  
  • Replace damaged slates promptly.  Replacing damaged tiles early will prevent the broken slate from damaging other tiles.  
  • Keep foot traffic off the roof.  If it can’t be avoided, the use of soft soled shoes and stepping only on the lower middle of the exposed portion of slate will minimize any possible damage.

Slate roofs are an important design feature on thousands of historical buildings all around the world.  By keeping up with yearly maintenance procedures we can preserve them for many years to come.