Surfaces and decks
Hairline crack
A hairline crack is a very thin crack in the surface finish that is often cosmetic.
Hairline cracks are common in plaster and decks. They often come from drying shrinkage or normal temperature swings. They are narrow, and a fingernail does not catch on them.
Watch them. If the crack grows, gets wider, shows rust stains, or the pool drops water, call a pro. A bucket test can help sort out evaporation from a leak. Cosmetic cracks can often be patched by a finish contractor. Keep photos with dates so you can compare later.
Related terms
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