Slate Delamination

Delamination is the separation of a roofing slate into layers along its natural cleavage planes. As slate weathers, the surface can flake, spall, soften and split, and eventually individual slates break or crumble. It is one of the main ways slate wears out, alongside fixing failure (see nail sickness).

Why it happens

Slate is a metamorphic rock that splits into thin sheets along cleavage planes, which is what makes it useful for roofing. Its durability varies greatly by geological source. Some slates contain minerals, such as certain carbonates or iron sulfides, that react with water and air, expand and break the slate apart. Repeated wetting and drying, freeze-thaw cycles and acidic pollution speed up the process. Softer slates may last far less time than hard, dense slates; heritage guidance in the UK and Ireland stresses that the service life depends heavily on the quarry and on matching like with like. Testing standards such as EN 12326 in Europe and ASTM C406 in North America are used to classify slate durability.

Signs

• Flaking or powdery surfaces, often worse at the exposed lower part of the slate.
• Slates that sound dull rather than ringing when tapped.
• Edges that crumble, and slates that break when handled.
• Softening around nail holes, causing slates to slip.
• Rust-coloured staining from oxidising pyrite in some slates.

Assessment and repair

Conservation bodies such as Historic Environment Scotland, Cadw and national parks authorities recommend regular inspection and timely replacement of individual defective slates with matching slate of similar size, colour, thickness and durability. When a large proportion of slates are delaminating, the roof is usually stripped and reslated, salvaging sound slates for reuse where possible and making up the shortfall with compatible new or reclaimed slate. Mixing poor-quality substitutes into a historic roof can shorten its life and change its appearance. Walking on slate roofs damages them and should be avoided; access is normally by ladders, hooks and roof ladders.