Rebuilding
There is a point where repointing is money spent on a structure that has gone, and the usual line is at the roof.
Short answer
Rebuilding is needed where the brick itself has deteriorated rather than just the joints: widespread spalling, delaminating brick, a leaning or bulging stack, or a crack through the structure. In Toronto the damage concentrates above the roof line, which is why a partial rebuild from the roof up is the most common version.
When repointing is not enough
Widespread spalling, where the brick faces have gone and the units are now absorbent.
Delaminating or crumbling brick that cannot hold mortar.
A stack leaning out of plumb, which is a structural matter rather than a surface one.
Bulging or a visible crack running through the structure.
Open joints so deep that there is no mortar left to point into.
And a stack where enough individual bricks need replacing that rebuilding the section is cheaper and better than cutting them out one at a time.
Above the roof line
The section above the roof gets no heat from inside, takes weather on all four sides and has nothing above it. It deteriorates first and fastest, and a partial rebuild from the roof line up — with a new crown and a new cap — is the most common chimney rebuild in Toronto. It is a bounded job with a predictable scope.
What a rebuild involves
Taking the affected section down, carefully, with protection for the roof below.
Salvaging brick where it is sound and matching new brick where it is not, which on an older Toronto house takes finding.
Rebuilding with a compatible mortar — softer than the brick on older work.
New flue tile or a liner as part of the work, because the flue is open and this is the time.
A new reinforced concrete crown with an overhang and a drip edge.
And a new cap, new flashing at the roof line, and the roof made good.
The practical side
Season matters: masonry work needs temperatures above freezing for the mortar to cure, which in Toronto means a working window.
Access: scaffold or a lift, which on a narrow lot with a rear laneway is a real cost and a real constraint.
Permits may be needed depending on the scope and whether it is structural.
In a semi-detached or row house the chimney may be shared, which makes it a party wall matter and a neighbour conversation.
Heritage designation restricts material and appearance in several Toronto neighbourhoods.
And brick matching, which on a visible stack is the difference between a repair that looks right and one that does not.
Common questions
When does a chimney need rebuilding?
When the brick itself has gone rather than just the joints, or when the stack is leaning or cracked.
Is a partial rebuild an option?
It is the most common one — from the roof line up, with a new crown and cap.
Is winter a possibility for this work?
No. The mortar has to cure above freezing.