This Irving chimney had been patched before. A wash coat had been spread over the crown at some point to seal it, and it had cracked straight through again, taking surface material with it. The flue was left open to the weather.
A wash coat is a short term fix. It sits on top of the crown rather than replacing it, so it moves differently to the masonry underneath and fails along the same lines the crown already had.
What the Crown Was Doing
The crown is the concrete surface on top of the chimney, and its job is to shed water away from the brick and away from the flue. This one had cracked through, the patch coat was lifting, and the mortar joints in the brick below showed the deterioration you would expect from years of runoff tracking down the face.
With the flue open on top of that, rain was going directly into the chimney rather than around it.
The Cap We Fitted
We fitted a full width cap that covers the entire crown rather than just the flue opening. It carries a drip edge so water is thrown clear of the brickwork instead of running down it, and mesh screening on all four sides that keeps birds, squirrels and debris out while leaving the flue free to draw.
Covering the whole crown also protects the crown underneath from further weathering, which buys the masonry years it would not otherwise get.
