Each week we’re bringing you an in-depth look at one of the standard conditions we encounter and document during inspections of buildings and civil structures. 

Part 19: Slate Conditions

Slate is a metamorphic rock that is easily split along its layers and dressed, yielding an attractive, durable roof tile. In the U.S. slate is quarried in Pennsylvania, Virginia, New York, Maine and Vermont. Natural color variations in slate – including gray, green, purple, and red – are often combined for architectural effect.

Cracked and missing slate

Slate conditions include failed fasteners; cracks; various types of soiling; and unsecured slate.

Loose slate resulting from a failed fastener

Slate roof tiles are generally very durable, with a well-designed slate roof having a service life of at least 60 years, and often 100 years or longer. Slates with greater amounts of impurities are subject to destructive weathering with wetting and thermal cycles. Weathered slates lose strength, becoming more prone to cracks. Steeply-pitched roofs shed water more efficiently, reducing weathering and resulting in substantially longer service life. Metal fasteners are prone to corrosion and eventual failure, leaving loose or unsecured slates. When only a few roofing slates are deteriorated, a roof that is in otherwise good condition can be easily repaired by a skilled roofer.

Failed fasteners and missing slates

Next in this series: Stucco Conditions

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