Cracks in a Stucco Chimney: Which Ones Matter

Hairline Shrinkage Cracks

Stucco is a cement plaster, and like any cement it shrinks a little as it cures. ASTM C926, the standard for applying cement plaster, accepts that fine cracking can occur. Thin, random hairline cracks that do not stain and do not grow are usually cosmetic.

They still deserve a look every year, because a hairline crack on the top of a chimney or below a crown can let water wick in during long storms.

Cracks That Let Water In

Stucco chimney cracking that follows the block joints in a stair-step or grid pattern, cracks that open wider than a credit card, and cracks with stains, rust or white powder below them are letting water through. Water behind stucco soaks the block, and in a framed chase it reaches the wood.

These cracks are repaired by opening them into a clean groove, filling with stucco or a flexible patch, and fixing whatever is sending water to them, usually the crown, the cap or the flashing.

Movement Cracks

A long diagonal crack, a crack that reopens after patching, or a crack where one side sits proud of the other suggests the chimney or its base has moved. Patching it without finding the cause only buys a season. The chimney should be inspected for settling, a failing footing or a loose top section.

Block Chimneys and Framed Chases

Not every stucco chimney is masonry. Many homes have a framed chase finished in stucco around a metal chimney pipe. Stucco chimney chase cracks matter more there, because water behind the finish reaches plywood and studs, not block. Soft spots, bulges and rust from the chase cover above are warning signs.

On a block chimney, the stucco is stuck fast to the masonry and cracks usually show the block joints behind them. Either way, the top of the chimney is the place to start when cracks appear.