Keeping Water Out
Waterproofing a chimney is a legitimate measure with one absolute rule: the product has to let the masonry breathe.
Short answer
Masonry has to be able to dry outward. A breathable vapour-permeable water repellent — typically a siloxane or silane — reduces absorption while still letting moisture out. A film-forming sealer traps water inside the brick, which in a Toronto winter means accelerated spalling behind the sealed face.
The breathability rule
Masonry absorbs water from rain, from a failed crown, from the joints and from the back face, and it has to be able to release it.
A vapour-permeable repellent reduces liquid absorption while still allowing water vapour out.
A film-forming sealer — an acrylic, a urethane, a paint-like coating — stops both directions.
Trapped water then freezes inside the brick, and the face spalls off behind the coating.
That damage shows up a few years later and is frequently blamed on the weather rather than the product.
And it is worse on a chimney than anywhere else, because the chimney has the most water coming in from above and the most freeze-thaw cycles.
Fix the water sources first
A repellent on the face does nothing about a cracked crown, open joints, a missing cap or failed flashing — and those are where most of the water is getting in. Sealing a chimney with a failed crown is treating the one surface that was not the problem. Crown, cap, pointing and flashing first.
When a repellent is worth it
After the crown, the cap, the pointing and the flashing have been dealt with, as the final step.
On a chimney in an exposed position taking a lot of driving rain.
On relatively absorbent brick, where reducing the uptake meaningfully reduces the freeze-thaw damage.
On a stack that has been repointed, to protect the new work.
With a product rated for masonry and stated to be vapour permeable, applied according to its instructions.
And understanding that it is a maintenance item with a service life rather than a permanent treatment.
What else keeps water out
A proper crown with an overhang and a drip edge, which is the most important single item.
A cap, which keeps water out of the flue entirely.
Sound pointing with a compatible mortar.
Correct flashing at the roof line — step flashing, counter flashing cut into the joint, and a cricket behind a wide stack.
A sound flue liner, because water inside the flue attacks the masonry from the inside.
And drainage at the base, because a chimney in a valley or behind a parapet can sit in standing water and ice.
Common questions
Should I seal my chimney?
Only with a breathable vapour-permeable repellent, and only after the crown, cap, pointing and flashing are sound.
What does a film-forming sealer do?
Traps water inside the brick, which then spalls the face off behind the coating.
Is it permanent?
No. It is a maintenance item with a service life.