A commercial flat roof repair runs in four stages. The entry point is traced from the interior evidence back to the roof. The assembly is opened and probed until the wet material ends. Everything saturated comes out, down to a sound deck. The insulation, cover board and membrane are then rebuilt and tied into the field.
Why does the repair start inside the building?
Because the ceiling stain is the only hard evidence anyone has, and it is almost never under the hole. Water entering a low-slope assembly travels. On a steel deck it runs along the flutes and can drop many metres from where it got in. On a concrete deck it spreads sideways and shows as a broad damp area with no source at all.
So the first work is interior. What sits directly above the stain, what direction do the deck flutes run, where are the nearest penetrations, curbs, drains and wall transitions, and what was happening outside when the water appeared. A leak that only shows in driving rain from one direction points at a wall or parapet detail. One that only shows during a thaw points at ice backing up behind a drain or a parapet. One that runs after any rain at all points at the field or at a drain.
That interior picture gives the roof crew a short list of places to check first, which is the difference between a targeted repair and an afternoon of patching likely-looking spots.
How is the entry point confirmed on the roof?
On the roof, the crew works the shortlist. The usual findings sit at the transitions: base flashing pulled away from a parapet behind its termination bar, an open lap at a curb corner, a pitch pocket whose filler has shrunk away from the conduit inside it, a drain where the membrane clamp has loosened, a split at a cant strip, cap flashing with open joints or missing fasteners.
Where the source is not obvious, the assembly gets probed. Wet insulation feels soft and different underfoot, and moisture readings across the area map how far it has spread. A test cut, a small square opened through membrane, cover board and insulation down to the deck, is the only way to see every layer at once: what system is on the roof, how many layers it already carries, whether the insulation is saturated, and what condition the deck is in.
The cut is not damage. It is closed watertight the same day, and it is what stops a scope being written on assumptions. Our flat roof repair quotes come after that check rather than before it.
How much of the roof has to be opened?
The size of the cut is set by the water, not by the hole. Wet insulation inside a sealed assembly does not dry out. It stays saturated, loses compressive strength, corrodes fasteners and the deck below, and keeps the ceiling damp long after the membrane opening has been closed. A patch installed over wet insulation is a repair that fails again, usually in the same place.
So the crew probes outward from the entry point until readings come back dry, and that boundary becomes the cut line. On a small flashing failure caught early, that may be a square metre. On a leak that has been quietly running for two winters, it can be a large section, because water spreads laterally between the membrane and the vapour barrier.
This is the single most common surprise in commercial roof repair, and it is why any price issued before the roof is opened is provisional. It is also the argument for finding leaks early, which is what a scheduled roof condition report is for.
How is the assembly rebuilt?
Once the wet material is out and the deck is exposed and confirmed sound, the layers go back in the order they came out.
- Deck. Cleaned and inspected. Corroded steel deck or deteriorated concrete is a structural question and goes to an engineer rather than being covered over.
- Vapour barrier. Tied back into the existing vapour control layer so the repair does not become the cold spot where condensation collects.
- Insulation. Replaced to match the existing thickness so the new area sits flush. Where the low spot was part of the problem, tapered insulation or a small cricket is used to move water toward the drain instead of rebuilding the pond.
- Cover board. Reinstated. It protects the membrane from foot traffic, hail and fastener back-out, and skipping it shortens the life of the patch.
- Membrane. Installed in the same system as the surrounding roof, lapped into it by the amount that system requires.
Terminology on any of these layers is set out on our roofing glossary page if you are reading a scope and want the definitions beside you.
Does the repair method change with the roof system?
Substantially. The membrane on the roof dictates how the new work is joined to the old, and mixing systems at a lap is how patches fail.
SBS modified bitumen
The existing cap sheet is cut back in a stepped pattern so base sheet and cap sheet laps are staggered rather than stacked. New plies are torched, mopped or self-adhered depending on what the surrounding roof is and what is safe on that building. Torch work carries a hot work procedure and a fire watch after the torches are shut down.
TPO and PVC
Thermoplastic membranes are heat welded. The area is cleaned, the weld surface prepared, and a patch welded with a hot air welder at settings tested on a scrap of the same sheet that day, because temperature, wind and the age of the membrane all change the weld. Every weld is probed with a seam probe before the crew leaves.
EPDM
EPDM is seamed with primer and cover tape or with adhesive. The surface has to be cleaned and primed properly, which on a weathered black sheet takes more preparation than owners expect.
Built-up roofing
The aggregate is spudded back, the flood coat opened, new plies set into hot or cold adhesive, and the surfacing reinstated over the repair. More detail on the systems themselves is on our flat roof types guide.
What about the flashings and details around the repair?
Most repairs are flashing repairs, so the detail work is the job rather than an afterthought. Base flashing runs up the wall or curb to a proper height and terminates behind a bar or into a reglet, not stopped short and sealed with a bead. A cant strip or a formed corner absorbs the movement at the ninety degree change of plane. Cap flashing or counterflashing sheds water over the top of the base flashing rather than relying on sealant to do the waterproofing.
Sealant is a maintenance item, not a repair. Any detail whose only defence against water is a bead of caulking will be back on the deficiency list within a few seasons. That is the difference between a repair and a delay, and it is covered further on our roof flashing and siding page.
What if the weather does not allow a permanent repair?
Then the roof is dried in temporarily and the permanent work is scheduled. Cold, wet or windy conditions genuinely limit what can be installed: adhesives and self-adhered membranes have application temperature limits, welded seams behave differently in the cold, and no insulation should be installed into an assembly that cannot be kept dry through the day.
A temporary dry-in stops interior damage now. That might be a mechanically fastened patch over the opening, a peel and stick membrane in an area that can be dried and warmed, or a covering held down properly against wind. It is a holding measure with a limited life, and it is recorded as such so it does not quietly become the permanent repair. Active water entry outside working hours goes through our emergency roof repair line at 416-456-0777.
What should you get at the end of the job?
A repair that leaves no record is hard to defend later. What should come back to you:
- Photographs of the opened assembly, showing the wet material and the deck condition, before anything was closed in.
- The location and dimensions of the repair, marked on a roof plan or described by area.
- What was found, in plain terms: which detail failed, how far the water had travelled, what came out.
- What went back in, layer by layer, including the insulation type and thickness.
- Any deficiencies noticed elsewhere on the roof while the crew was up there, so they go on the list rather than into the next leak call.
That record is what makes the next decision easier, whether that is another repair, a maintenance program or a replacement year.