Slate Roofing Ask the Builder

Slate Roofing Ask the Builder

Slate Roofing — Facts & Tips

Slate roofing is one of the most distinguished building products that I know of. It graces many of the most impressive buildings in the world. Cathedrals, palaces, residential homes, and castles have incorporated this long lasting building material as a weathershield. In fact, it was the king of roofing materials at the turn of the twentieth century. United States slate consumption was at its highest in 1902.

A Natural Stone Product

Slate is really nothing more than mud. Mud that has been squeezed and heated. Fine particles of clay and silt build up in thin layers at the bottom of shallow seas or at the mouths of large river deltas. These sediments then turn into a rock called shale. Mountain building processes can heat and pressurize the shale changing its chemistry.

The clay and silt crystallize into quartz, chlorite, and different micas, all of which are very durable materials. This is one of the reasons why slate lasts so long.

The chemistry change also imparts a negative quality. The metamorphosis that creates slate makes it brittle. This characteristic is responsible for many of the common slate roof failures.

Slate vs. Wood. Winner is.

Often people come home to a shattered slate shingle on their driveway. Or, a piece of slate has buried itself into the grass. These falling objects are dangerous.

They detach from the roof because of the brittle nature of slate. The slate may have fractured when it was installed or expansion and contraction forces at work on the roof deck creates stresses which crack slate.

Nailing slate shingles correctly is very tedious work. Drive the nail too far and you can easily crack a slate. Leave the nail proud of the surface and you risk causing a problem with the overlying shingle. This high nail head creates a pressure point below the overlying shingle.

Roof sheathing itself can cause problems. Wood is hygroscopic. It attracts and liberates moisture in an attempt to reach an equilibrium with outdoor humidity. This property cause the wood to shrink and swell. These movements can cause the nails that hold a slate shingle to pinch a corner. This stress causes the shingle to fracture and detach from the roof. Slate is not supposed to be nailed to a roof. It is supposed to hang from the nails much like a picture hangs on a wall.

Weight & Wind

A few years ago, a new home was built here in Cincinnati in a very exclusive neighborhood called Indian Hill. The design called for a slate roof. The house was built and slate began falling to the ground on a very regular basis. An autopsy of the situation revealed that the roof framing was inadequate. The steep roof slope was acting like a sail in the wind. Wind would hit the roof, flex and bend the trusses and break the slate. I believe this problem is still in our justice system, if you know what I mean. Once again, the brittle quality of slate rules supreme.

Older homes that have slate roofing have substantial roof framing members. The roof rafters are actual 2 x 6’s or 2 x 8’s 16 inches on center. the roof decks are 3/4 inch solid sheathing. There is very little, if any, flex.

In Europe, slate roofing is an art form and a science. New structures that use slate often install it on battens. Battens are strips of wood nailed perpendicular to the roof rafters. We call it spaced sheathing here in the USA. This method helps to extend the life of the slate if it is subjected to freezing temperatures or acid rain.

Fast Dry is Best

Slate Roofing Ask the Builder

Slate doesn’t absorb much water at all. However, the water it does absorb can and does destroy it over time. Rain water in industrial areas can turn into mild acid. This acid attacks the minerals in the slate and can dissolve them.

Water which turns to ice causes tiny micro fractures which blow the slate apart. Freezing and thawing cycles are like mini jack hammers that take layer after layer of slate off a shingle.

Slate that dries rapidly between periods of precipitation minimizes this damage. Spaced sheathing or battens allows air to circulate under the slate. This simple technique can add years to your slate roof if you are thinking of building a new home. The European roofers are real pros, don’t try to reinvent the wheel on your home!

Accessory Materials

Do you have a slate roof in good condition? Did you know that it is not uncommon for a slate roof to last 100 — 150 years? The other parts of your roof should be designed to last as long. For example, the metal flashings should be copper. Tin plated steel or galvanized steel have much shorter service lives. Besides, the copper costs just a little more! The labor to install these different metals is virtually the same. You should only have to pay for the difference in material costs.

The same is true for the nails. Consider using copper nails. A typical galvanized nail may only last 30-50 years before rusting. Special lead flashings for plumbing vent pipes are long lasting alternatives to the aluminum and rubber flashing you see at the home centers.

Look, No Nails!

If you are considering a new house or building and choose to use slate roofing, you now have a new option. There is a system by which the slate hangs from tracks. There are no nails in the slate that can pinch them. Furthermore the tracks hold the slate off the roof deck allowing adequate ventilation. It is an excellent way to attach slate to our modern plywood sheathed roofs. This system was developed by Slate International.


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