Flat Roofs
Toronto has a great many flat roofs, and almost all of them fail at the same small number of places rather than in the middle of the field.
Short answer
Flat roofs fail at the details: drains and scuppers, parapet walls and cap flashing, penetrations, seams, and the transition to a wall. Ponding water is the accelerator, and freeze-thaw cycling is what opens the seam that was already weak. The repair depends on the membrane type, which is the first thing to establish.
Where they fail
Drains and scuppers, which block and leave water standing. On a Toronto roof that standing water then freezes.
Parapet walls and the cap flashing on top of them, which is the single most common entry point on a flat roof.
Penetrations: vents, stacks, mechanical curbs, conduit, anything through the membrane.
Seams and laps, which are the weak line in any membrane and which freeze-thaw cycling works open.
The wall transition and the base flashing, particularly where a lower roof meets a higher wall.
And mechanical units sitting directly on the membrane, which is both a wear point and a place nobody inspects.
Ponding is the accelerator
A flat roof is not meant to be flat — it is meant to drain. Water standing for more than a couple of days degrades the membrane, and in Toronto it also freezes and expands at every seam. If there is a ponding area, the slope or the drainage is the problem, and patching the membrane there is treating the symptom.
By membrane type
Modified bitumen, which is the most common residential flat roof here. Repaired by torch or cold-applied patching, and seam work is routine.
Built-up tar and gravel, older stock, repairable but increasingly at end of life and generally replaced rather than patched indefinitely.
Single-ply membranes, which are repaired with compatible material and the right welding method — and which are ruined by the wrong patch.
Liquid-applied systems, which can be recoated and which need the right primer and the right compatibility.
Rolled asphalt and older sheet goods, which are generally a replacement conversation.
And in every case, matching the repair material to the existing membrane, because an incompatible patch fails at its own edge within a season.
Repair or replace
Age and remaining life, which on most flat systems is a known number.
Whether the insulation underneath is wet, which a moisture survey establishes and which decides whether patching is worth anything.
How many patches are already there, because a roof with a dozen is telling you something.
Whether the drainage is right, because a roof that ponds needs the slope corrected whether it is patched or replaced.
Whether the deck is sound, which can only be assessed with the membrane open.
And the season, because in a Toronto winter an emergency repair is the realistic option and a proper replacement waits for conditions.
Common questions
Where do flat roofs usually leak?
At the details — drains, parapet cap flashing, penetrations and seams — rather than in the field.
Is standing water a problem?
Yes. It degrades the membrane and in Toronto it freezes and works the seams open.
Can a flat roof be repaired in winter?
An emergency repair, yes. Proper replacement waits for conditions.