Chimney Repointing: New Mortar for an Old Stack

• Sand or grit in the gutter under the chimney, or on the ground below it.
• Dark lines between the bricks where the mortar has weathered back.
• Plants or moss rooted in the stack.
• Damp on the chimney breast that comes a day or two after rain, not during it.
• A draught of daylight through a joint, seen from the attic.

1. Scaffold or a tower up to the stack.
2. Every joint raked out by hand to solid mortar, deep enough for the new mortar to key.
3. Dust brushed out and the joints damped.
4. New mortar pressed in and finished so water runs off the face, not into it.
5. The flaunching and the lead checked while the access is there.

It matters which. 268,078 Irish homes were built before 1946, and their chimneys were built in lime mortar with soft brick or stone. Lime lets a damp stack dry out through its joints. Hard cement mortar on that kind of stack traps the water in the brick, and the faces of the bricks blow off in frost. On a modern stack of hard brick or block, a cement mortar is right.

Ask what mortar is going in, and why. On an old stack the answer should be lime.

Many Irish stacks are plastered or dashed, not bare brick. When the render is cracked or hollow, it is hacked off and redone in two coats, with a drip at the top so water falls clear. Rendering over bad joints without repointing first only hides the problem: the render cracks along the same lines within a few winters.