What is sarking, and why does it matter?
Sarking is the timber boarding fixed across a roof’s rafters, onto which the slates (and, on many roofs, a secondary waterproof or breathable membrane) are fixed. It is the layer that turns a rafter structure into a continuous surface, and it plays a major role in a slate roof’s overall performance, including how well the roof copes with the odd slipped or missing slate.
Traditional Scottish slate roofs were generally built with timber sarking boards, unlike some other regional roofing traditions that fixed slates directly to battens with open gaps behind. That solid boarded sarking is part of why a slipped slate on a Scottish roof does not always mean an immediate leak, as explained in our guide to whether a slipped slate will leak, though it is never a substitute for prompt repair.
Why ventilation matters on a slate roof
Roof spaces need a route for moisture and warm air to escape, otherwise condensation can form on the underside of sarking boards and any underlay, leading to timber decay over time that is separate from, and can be mistaken for, a leak from outside. Ventilation is usually provided through gaps at the eaves, ridge, or via dedicated ventilation tiles or fittings, allowing air to move through the roof space rather than becoming trapped.
How sarking and ventilation relate to slate repairs
When a slate is lifted for repair, it is also a chance to check the condition of the sarking boards immediately underneath. Sound sarking with a slipped slate above it is usually a straightforward repair. Sarking that has begun to rot from long standing damp or poor ventilation is a more significant issue, and one honest assessment is worth more than a quick slate swap that leaves the underlying problem untouched.
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Concerned about what's happening under your slates?
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