A repaired leak comes back because the repair closed the membrane and left everything under it alone. Wet insulation stays wet, the cause of the original opening was never corrected, and the patch itself was often terminated in the wrong place. Three gaps, all invisible from the roof surface once the work is finished.
Gap one: the wet insulation nobody removed
This is the gap that produces most repeat calls. Water gets through a small opening in the membrane, spreads sideways through the insulation and along the top of the vapour barrier, and soaks a far larger area than the opening suggests. On a steel deck it runs along the flutes and can travel metres before it drops through a fastener hole or a deck lap.
A patch over the opening stops new water entering. It does nothing about the water already inside. Sealed under a new membrane, saturated insulation does not dry. It loses compressive strength, so the area settles and becomes a low spot that ponds. It stays in contact with the steel deck and the fasteners, which corrode. And it keeps releasing moisture into the space below, so the ceiling stain grows even though the roof is closed.
The owner sees a leak reappear near the repair and concludes the patch failed. Usually the patch is intact. The problem was never the patch.
A defensible repair cuts the wet material out. The cut is sized on the roof by probing or moisture scanning outward until readings come back dry, then insulation and cover board are replaced with matching thickness so the surface stays flush and the new membrane is tied into sound material at every edge.
Gap two: the cause that was never corrected
Membranes rarely open for no reason. Something made that spot fail, and unless the reason is corrected the new membrane will fail in the same place.
- Ponding. A patch in a low spot sits under water for weeks at a time. The repair holds until the freeze-thaw cycling works at its edges. Correcting it means addressing drainage, not membrane: clearing the drain, rebuilding a compressed sump, adding tapered insulation or a cricket to move water off the area.
- Movement. Splits over a deck joint, at a corner or beside an expansion joint are the assembly moving. A flat patch across a moving joint tears again. The detail needs relief, a proper expansion joint cover or a cant strip, not more membrane.
- Traffic. Punctures on the route between the hatch and the rooftop units are foot traffic. Patching them without adding walkway pads means patching them again.
- A bad detail. Base flashing terminated too low on the parapet, no cant strip at the wall junction, a pitch pocket where a proper curb belonged. The patch treats the symptom of a detail that was never right.
This is why we ask what else has been repaired on the roof and when. Three patches in the same twenty metres is not three failures. It is one unresolved cause. Our page on flat roof repairs sets out how the cause is established before anything is closed up.
Gap three: how the patch itself was terminated
Even a correctly located repair fails if it is finished badly, and the failure is usually at the edge.
A patch has to be bonded to a clean, dry, compatible surface across its full area, and it has to extend well past the damage in every direction so the perimeter of the patch lands on sound membrane. On a granulated SBS cap sheet that means embedding or removing granules so the patch has something to bond to. On TPO or PVC it means a clean weld with the seam probed afterwards. On EPDM it means the correct primer before tape or adhesive. On an aged, oxidised or previously coated membrane it means checking that the new material will bond to the old one at all.
The failures we open up most often are the shortcuts around that: mastic troweled over a crack with no reinforcement, a patch cut too small so its edge sits directly over the split, sealant applied over a wet or dusty surface, a patch laid on a gravel-surfaced built-up roof without the stone being cleared and the surface prepared, and unlike materials stuck together because that was what was on the truck.
How can you tell whether a repair went deeper than the surface?
You can often tell from the roof and from the paperwork.
- Walk the repair area. Does it feel soft or spongy underfoot compared with the surrounding field? Soft usually means wet or compressed insulation below.
- Look at the size. A patch the size of a dinner plate over a leak that stained several ceiling tiles almost certainly did not remove wet material.
- Look for a flush surface. A cut-and-replace repair sits level with the field. A patch straight over the old membrane sits proud, and its edge collects debris and water.
- Check whether anything changed besides the membrane. Was the drain cleared, was a cricket added, were walkway pads installed, was flashing rebuilt? If nothing changed, the cause did not.
- Ask for the photographs. A repair done properly produces images of the opened assembly, the wet material removed, the new insulation and cover board, and the finished detail.
An independent assessment is the reliable version of all that. A flat roof inspection with moisture readings shows the wet areas across the whole roof, which is the only way to see how much of the problem is still there.
Does this mean patch repairs are a waste of money?
No. Repair is the correct call on a large share of the commercial roofs we walk. A sound field with local failures is exactly what repair is for, and replacing a serviceable roof because of a few open laps would be the more expensive mistake.
What matters is that the repair is scoped honestly. That means the extent of wet material is established rather than assumed, the cause is corrected as part of the work, and the repair is detailed to the same standard as new construction. A properly built repair on a roof with life left in it is good value.
The point at which repair stops making sense is when the failures are no longer local: splits appearing in unrelated areas, surfacing lost across the field, fasteners backing out through the membrane, moisture spread through the insulation over a wide area. At that point each patch buys less time than the last, and the conversation moves to flat roof replacement. Our post on recurring leaks and when to replace works through where that line sits.
What stops the gaps opening in the first place?
Two things, and neither is complicated.
The first is a documented baseline. If someone has recorded the condition of the whole roof, with photographs and moisture data, then every repair afterwards is measured against a known state rather than against memory. It also means a new leak can be checked against what was already known to be wet.
The second is a maintenance routine that catches failures while they are small. Drains cleared before the autumn leaf fall and again before the spring melt, seams and flashing checked twice a year, debris removed, and damage from other trades on the roof reported and repaired rather than left. Most of the leaks that end up as emergency calls were visible as minor defects a season or two earlier. That is the entire argument for flat roof maintenance on a commercial building.
If your building has been patched repeatedly in the same area, call 416-456-0777 and we will start from what is under the patch rather than from the patch itself.