Common Failures in Slate Roofs

Elements of Slate Roofs

• The four basic elements of slate roofs are: The slate, which should be a hard variety with a reputation for durability; the roof deck, which should be made of board (not laminated) at least 3/4 inch thick; fasteners, which can be copper, stainless steel, cut steel or hot-dipped, galvanized steel nails; and flashing or sheet metal joints in valleys and around vents, pipes,dormers, and chimneys. Problems with any of these components can result in a leaking or failing slate roof.

Flashing-Related Failure

• The weakest component of a slate roof is often the flashing. Flashing on historic slate roofs is typically made of copper (sometimes lead coated), galvanized steel, lead sheet, or terne coated steel. These materials often deteriorate before the slate. If the flashing leaks, the roof leaks. Repairing flashing with tar or other sealants is not effective. Leaking flashing should be replaced, not repaired.

Slate-Related Failure

• The durability of a slate roof depends on the type of slate used. Some slates are harder than others. Though most slates should last well over 100 years, softer slates may begin to flake, crack and crumble after 50-60 years. Slate roofs cannot be repaired or restored after the slate has reached the end of its useful life. The entire roof should be replaced if more than 25 percent of the slate tiles are damaged or deteriorated.

  Hard slate roofs can develop cracks and breaks over time. Slate is stone and and some individual slate pieces may have tiny imperfections or fractures that are not evident at the time of installation. Environmental factors such as freeze-thaw cycles may eventually cause these slates to crack, break or fall off.

Fastener-Related Failure

• Properly installed nails should last as long as the slate. Fastener failure is usually caused by slate deterioration or poor installation. Moisture penetrating a deteriorating or soft slate roof can also penetrate and weaken the nails. Slates begin to fall off the roof when the nails can no longer anchor them to the base.

  Improper installation can also cause fastener-related failure on slate roofs. Nails that are too tight can crack the slate, and nails not hammered in adequately allow moisture to penetrate through the countersink drilled into the slate. Either situation can cause rapid deterioration and leaking.