Cupped/cupping
Cupped or the cupping of wood, often found in cedar shingles, results from too much drying across the grain of the wood results in a cupped or curled appearance. Slight cupping is generally not an issue of weathertightness, but excessive cupping may warrant selective replacement. Cupping is generally the result of direct sunlight, excessive rainfall, or too much heat causing shrinkage across the exposed surface of the wood. Historic shingles benefit from under shingle ventilation by being on lath, open or skipped sheathing, so how the roof has been installed can be an issue for longevity and reduce cupping. It can also occur in wood that is not sufficiently fastened to a substrate such as a sidewall shingle in direct sunlight. Cupping is often seen in aging shingle roofing and is an indicator that the shingles may be reaching their limit of lifetime. Cupping can also be a resting place for moss or other algae in wet pockets which can accelerate deterioration of wood. See Algae – AL.