One of the most important elements of a brick or masonry chimney structure is the crown mortar, which acts as a sloped seal to drain rainfall off of the top surface of the chimney. Over time, this mortar cap can deteriorate and develop cracks that open the masonry to more extensive damage. Once the mortar crown has reached the point of decomposing, the best repair is replacing the crown to restore its function.
Access the chimney and chip away the old mortar crown using a hammer and cold chisel, then clear the chimney’s top surface of dust and debris with a stiff-bristled broom.
Mix the mortar mix in a wheelbarrow, using a hoe or shovel, following the manufacturer’s specifications. The mortar should be slightly stiff, or “dry,” and about the consistency of peanut butter.
Transport the mixed mortar to the roof using a 5-gallon plastic bucket, available at most home improvement centers, and position the bucket of mortar on the roof behind the chimney for easy access during application.
Spread mortar over chimney top surface using a trowel, sloped away from the clay chimney pipes to the outer edges. Maintain a minimum slope of 2 to 3 inches per linear foot, keeping the surface smooth with the trowel.
Allow the new mortar crown to set for a minimum of 24 hours before proceeding.
Apply a coat of elastomeric concrete sealer to the surface of the mortar crown using a paint brush, making certain the coating laps onto the clay chimney pipes at the top and onto the masonry sides of the chimney structure. Allow the sealer coat to dry following the manufacturer’s recommendation.