Repointing
A chimney is the most exposed masonry on a Toronto house — wet on all four sides, with nothing above it, going through a hundred freeze-thaw cycles a year.
Short answer
Mortar joints fail from freeze-thaw cycling and from water saturating the masonry. Repointing means cutting out the failed mortar to a proper depth and replacing it with a compatible mix. On an older Toronto chimney the mix matters enormously: a hard modern mortar on soft old brick destroys the brick.
Why the chimney goes first
It is exposed on all four sides, unlike a wall which only faces one way.
There is nothing above it, so rain lands on the top and soaks downward.
It goes through the most freeze-thaw cycles of any masonry on the house, because it gets no shelter and dries fastest after a thaw.
The upper section above the roof has no heat from inside, so it stays colder and wetter.
Flue gases on the inside and weather on the outside, which from a hard-working appliance means the masonry is attacked from both sides.
And a failed crown or a missing cap puts water into the top of the masonry, which is where most of the deterioration starts.
The mortar mix
Older Toronto brick is softer than modern brick, and it was laid with a lime-rich mortar that was deliberately weaker than the brick. If the mortar is weaker, it takes the movement and the salt damage and can be replaced. A hard modern Portland-rich mortar is stronger than the old brick, so the brick face spalls off instead. On a pre-war chimney, the mix is the decision.
What proper repointing involves
Cutting out the failed mortar to a proper depth — generally at least twice the joint width — rather than smearing new mortar over the old.
Cutting by hand or with care, because an angle grinder run along a joint widens it and cuts the brick arrises.
Matching the mix to the existing mortar strength, colour and sand, which on a heritage or older chimney may mean analysis.
Matching the joint profile, which is both an appearance and a water-shedding detail.
Damping the joint and curing the new mortar properly, which in a Toronto summer means keeping it from drying too fast.
And not doing it in freezing weather, because mortar that freezes before it cures is worthless.
Repoint or rebuild
Sound brick with failed joints: repoint.
Spalled, delaminating or crumbling brick: the brick has to be replaced, and if enough of it is gone, the section is rebuilt.
A chimney leaning, bulging or with a visible crack through the structure: rebuild, and get it looked at properly rather than patched.
Deterioration concentrated in the section above the roof, which is common, and which is a partial rebuild from the roof line up.
A failed crown and no cap, which needs fixing whichever route is taken.
And an unlined chimney serving an appliance, which is a liner question running alongside the masonry one.
Common questions
Why does the chimney deteriorate before the walls?
It is exposed on all four sides with nothing above it, and it takes the most freeze-thaw cycles.
Does the mortar mix matter?
Enormously on older brick. A hard modern mortar spalls soft old brick.
Will a mason repoint in the cold months?
No. Mortar that freezes before it cures is worthless.