Blog
26 October 2026
A leak on a flat roof can’t be left alone for long. What starts as a simple damp stain on the ceiling can lead to wet insulation, mould and structural damage if the repair is delayed. In Terrebonne, as everywhere in Quebec, alternating freezes and thaws, snow buildup and spring rains put stress on roofs year after year.
A successful leak repair comes down to two things: finding the real source of the water, then restoring the seal with materials that are compatible with the existing membrane.
A flat roof doesn’t shed water by gravity alone the way a sloped roof does. Water has to be directed to the roof drains by slight slopes. When it pools, it seeks out the slightest weakness in the membrane. These weaknesses come from aging materials, insufficient maintenance, a poorly executed detail during previous work or movement of the building.
Frost makes things worse: water that gets into a small crack freezes, expands and widens the opening. Season after season, this cycle ends up compromising the seal.
Inside, watch for yellowish or brown stains on the ceiling, blistering paint and a persistent musty smell. On the roof, the clues are a blistered membrane, open seams around penetrations or puddles of water that remain long after the rain. If you notice any of these signs on your building in Terrebonne, have the roof inspected by a roofer who specializes in flat roofs.
Locating a leak on a flat roof isn’t as simple as it seems. The spot where water appears inside rarely matches the spot where it gets in. Water can travel under the membrane and through the insulation for several metres before showing up.
This diagnosis makes it possible to work in the right spot and avoid repairs that don’t solve the real problem.
The most vulnerable points of a flat roof are rarely in the middle of the surface. They are found around penetrations: plumbing vents, ventilation ducts, air conditioning unit bases, roof drains and skylights. These junctions between the membrane and another element move with temperature swings, and their collars and flashings eventually come loose.
Roof drains deserve special attention. A partially clogged drain lets water pool around it, and the connection between the membrane and the drain can deteriorate. Resealing around roof drains and penetrations is one of the most common repairs on a flat roof.
For a localized leak, the damaged area is cut out and cleaned, wet insulation is replaced if needed, then a patch of elastomeric membrane made of SBS modified bitumen is installed, either torch-applied or self-adhered, depending on the existing system. The quality of the bond and the overlaps determines how long the repair will last. Sloppy work can hold for a few months before a new leak appears.
When the leak comes from a detail, such as an upstand or a vent collar, the entire detail is redone, not just the visible spot where the opening is.
A repair is appropriate when the membrane is in good overall condition. When damaged areas multiply, leaks come back from one season to the next or the insulation has been soaked repeatedly, it is better to consider a roof replacement. An assessment of the roof’s actual condition makes it possible to decide without unnecessary work.
An inspection in spring, to see the effects of winter, and another in fall, before the first snow, help spot risk areas before they leak. Cleaning the roof drains, inspecting seams and penetrations and checking the overall condition of the membrane help extend the life of the roof.
Toitures OBM specializes in elastomeric membrane flat roofing for commercial, industrial and multi-unit buildings, as well as residential flat roofs. Based at 1215 rue Tellier in Laval, the company works in Terrebonne and elsewhere on the North Shore, as well as in Laval and Montreal. Flat roof repair is one of its services: localized leaks, resealing around roof drains and penetrations, cracks and loose sections.
Before recommending a repair, the Toitures OBM team assesses the condition of the roof and the needs of the building to avoid unnecessary work, then clearly explains its recommendations. The company is Soprema PAQ+S certified, CAA-Québec Recommended and holds RBQ licence 5778-2823-01.
Most often, the real source wasn’t found, the detail at fault wasn’t completely redone or the membrane has reached the end of its life. A complete diagnosis of the area makes it possible to target the right cause.
In spring, a large amount of meltwater has to drain toward roof drains that are sometimes still blocked by ice or debris. Water then stands longer and can seep in through an upstand, a seam or a crack that holds up to an ordinary rain.
Only if it is wet or damaged. Waterlogged insulation loses its thermal value and can promote mould, so it is replaced in the affected area during the repair.
A leak that is properly located and properly repaired protects your building and keeps the damage from spreading. To have your flat roof inspected in Terrebonne, contact the Toitures OBM team at 514-449-5469, Monday to Friday from 8 a.m. to 5 p.m., or fill out the form to get a free quote.
OBM Roofing
Elastomeric membrane specialists for commercial, industrial and multi-unit buildings in Laval, Montreal and the North Shore.
