The Part Above The Roof
Almost every chimney repair conversation in Toronto is about the section above the roof, and there are four reasons for that.
Short answer
Above the roof the stack has no heat from inside, takes weather on all four sides, has nothing above it, and goes through the most freeze-thaw cycles of any masonry on the house. That combination means it deteriorates years before the section inside, which is why a partial rebuild from the roof up is the standard Toronto repair.
The four reasons
No heat from inside. Below the roof the stack is in conditioned space and stays warmer and drier; above it, it is entirely exposed.
Weather on all four sides, unlike a wall which only faces one direction.
Nothing above it, so rain and snow land on the top and soak downward through the crown.
The most freeze-thaw cycles of any masonry on the house, because it gets no shelter and it both wets and dries fastest.
Add the fact that the crown is frequently a mortar wash rather than a proper crown, and the water is getting in from the top from early in the chimney life.
And add the flue gases on the inside on a working chimney, which is an attack from the other direction.
The termination height
While it is being rebuilt, check the height. A flue has to terminate a specified distance above the roof and above anything nearby, and a stack that was correct before an addition or a dormer was built may not be now. A short termination is both a draft problem and a code one, and the rebuild is the opportunity to correct it.
What the partial rebuild covers
Taking the stack down to sound masonry, which is frequently at or just below the roof line.
Rebuilding with matching brick and a compatible mortar.
New flue tile in the rebuilt section, or a liner through the whole flue.
A cast concrete crown, reinforced, oversailing the brick with a drip underneath.
A new stainless cap.
And new flashing at the roof line, with a cricket behind a wide stack, and the roof made good.
Why it is worth doing properly
A rebuilt stack with a proper crown, a cap and correct flashing lasts decades rather than years.
A rebuilt stack with a mortar wash on top instead of a crown is back in the same condition in a decade.
The flashing is the other half of the job, and doing the masonry without it means the water keeps coming in at the base.
The liner is the opportunity, because the flue is open during the work.
Access is the expensive part, so doing everything while the scaffold is up is far cheaper than returning.
And brick matching matters on a visible stack, which is worth the effort of sourcing rather than accepting whatever is available.
Common questions
Why does the chimney fail above the roof?
No heat from inside, weather on all four sides, nothing above it, and the most freeze-thaw cycles.
Is a partial rebuild normal?
It is the standard Toronto chimney repair.
What should be included?
Masonry, a proper crown, a cap, the liner and the flashing. All of it while the access is up.