The Crown

What a proper crown has

Concrete rather than mortar. A crown is a structural slab exposed to weather, and mortar is not the right material.

Reinforcement, so a crack does not become two pieces.

An overhang past the face of the brick, with a drip edge underneath, so water drips clear rather than running down the masonry.

A slope away from the flue toward the edge.

A bond break or sealant joint where it meets the flue tile, so the tile and the crown can move independently.

And adequate thickness, which a mortar wash is not.

What is usually there instead

A mortar wash: a thin layer of mortar sloped off the top brick course, flush with the face, no reinforcement, no overhang, bonded hard to the flue tile. It cracks within a few years, the crack lets water into the top of the masonry, and that is where most Toronto chimney deterioration starts.

Why they crack

Freeze-thaw cycling in a thin unreinforced layer.

The flue tile expanding and contracting with heat and pulling on a crown bonded to it.

Differential movement between the crown and the masonry.

Shrinkage during the original cure, which in a mortar wash is substantial.

No overhang, so water runs down the face and the crown edge erodes.

And in many cases the crown was never a crown, only a wash.

Repair or replace

Hairline cracking on a sound crown: a crown sealer or a flexible coating, which is a legitimate maintenance measure.

Open cracks, missing sections, or a crown that is a mortar wash: replace it properly.

Replacement means removing the old material and casting a new reinforced slab with an overhang and a bond break.

Do it at the same time as any repointing, because the access is the same and the crown is what caused the pointing to fail.

Fit a cap at the same time, which keeps water out of the flue itself.

And do it in reasonable weather, because a concrete crown needs to cure rather than freeze.