Blog
27 July 2026
A leak on a flat roof often starts with a small damp stain on the ceiling. You tell yourself it’s nothing serious, then the next rain makes the stain bigger. Before long, the insulation is wet and interior damage is piling up. In Laval, whether on an apartment building, a plex, or a commercial or industrial building, a roof leak is best handled when it is dealt with early.
Here is how to spot a leak on a flat roof, how a well-done repair is carried out and how to keep the problem from coming back.
On a flat roof, water drainage depends on three things: the condition of the roof drains, the slight slope that directs water toward them and the integrity of the membrane. When one of these fails, water pools, looks for a weak point and eventually seeps in.
Quebec's climate makes things worse. Freeze-thaw cycles, accumulated snow, the spring melt and heavy rains constantly stress the membrane and seams.
The first clue is often visible inside: a brown stain on the ceiling, paint that bubbles, a drywall panel that bulges slightly. In more advanced cases, a mouldy smell appears or drops fall from the ceiling during heavy rains.
On the roof, a visual check can reveal blisters, separation, cracks around flashings or penetrations, or a roof drain clogged with debris. If you notice any of these signs, an inspection is needed.
Leaving a leak in place allows water to seep into the insulation under the membrane. Saturated insulation loses its properties and promotes mould. What could have been a localized repair may then turn into a replacement of part or all of the roof. In an apartment building, a leak also affects several occupants.
A good repair starts with a thorough diagnosis. Water can travel several metres under the membrane before entering the building, so the source is not always above the stain. The roofer examines the entire surface, paying particular attention to seams, upstands, roof drains and penetrations.
For a crack or a loose section, the area is cleaned and prepared, then a patch of elastomeric membrane compatible with the existing system is installed. On an SBS modified bitumen membrane, the patch is generally torch-applied, which promotes good adhesion to the existing membrane. Around a roof drain or penetration, the waterproofing work covers the entire detail, not just the visible crack.
If the membrane has reached the end of its service life or the insulation is extensively saturated, piling up repairs does not solve the underlying problem. In that case, replacing the elastomeric roofing may be recommended. This recommendation is always based on the actual condition of the roof, to avoid unnecessary work.
Once the repair is done, it is a good idea to watch the ceiling during the next few rains. If a trace of moisture reappears, a new check can confirm whether there was a second source of water infiltration.
Regular maintenance remains the simplest way to avoid leaks. An inspection in spring and fall, roof drain cleaning and a check of the seams, upstands and penetrations make it possible to fix small defects before they let water in. When the membrane is sound but its surface is worn, resurfacing can also extend its lifespan.
Toitures OBM is a roofer specializing in elastomeric membrane flat roofs based at 1215 rue Tellier in Laval. The team repairs flat roofs in Laval, Montreal and on the North Shore: localized leaks, resealing around roof drains and penetrations, cracks and loose sections. It also handles roof replacement, resurfacing, maintenance and inspection, as well as new flat roof construction.
Toitures OBM installs Soprema systems. The company is Soprema PAQ+S certified, CAA-Québec Recommended and holds RBQ licence 5778-2823-01. Its recommendations are clear and explained before work begins.
Yes, when the membrane is in good overall condition and the damage is localized. A compatible patch is then installed on the affected area. If the membrane is worn everywhere, an isolated repair may not hold for long.
Water can travel under the membrane or through the insulation before finding an opening into the building. That is why the diagnosis covers the entire roof and not just the area above the stain.
A sealant applied in a hurry may help for a few days, but it does not replace a repair made with a compatible membrane. On an elastomeric roof, a lasting repair requires resealing that is suited to the existing system.
Do you suspect a leak on your flat roof in Laval? Contact Toitures OBM at 514-449-5469, Monday to Friday from 8 a.m. to 5 p.m., or request a free quote.
OBM Roofing
Elastomeric membrane specialists for commercial, industrial and multi-unit buildings in Laval, Montreal and the North Shore.
