Blog
10 August 2026
Flat roofs are common on commercial buildings, apartment buildings and many homes. They provide useful space for mechanical equipment, but they require well-maintained waterproofing, since water drains slowly and can seep into the smallest opening.
If you own or manage a flat-roof building in Saint-Jérôme, here is how to recognize a leak, understand its cause and choose the right repair.
On a flat roof, water follows a gentle slope to the roof drains. It can therefore sit longer and take advantage of the slightest defect to seep in. Freeze-thaw cycles, snow, the spring melt and heavy rains in Quebec’s climate weaken waterproofing membranes over the years. Leaks often show up in spring and fall, when these conditions are at their toughest.
The first signs are usually seen inside: moisture stains on the ceiling, yellowish rings, paint that blisters or peels. You may also notice mould in the corners of rooms or a musty smell in a closed space.
These signs should never be ignored. An untreated leak damages the insulation, then the wooden components of the roof structure.
On the roof, watch for blisters in the membrane, separation at the seams, cracks and areas where water sits long after a rain. Flashings and upstands around roof drains, chimneys, vents and walls can also come loose over time.
Understanding where a leak comes from is essential before undertaking a repair. Causes can combine, which sometimes makes the diagnosis more complicated.
An elastomeric membrane made of SBS modified bitumen stands up well to temperature swings, but it ages. UV rays wear down its surface, and repeated building movement puts stress on the seams. At some point, cracks appear and let water through. Age alone is not enough to judge a membrane: what counts is its actual condition, as found during the inspection.
Poorly welded seams, insufficient overlaps or badly secured upstands can cause early leaks, even on a fairly recent roof. These defects are often concentrated in the most complex spots: corners, parapets, roof drains and penetrations.
Clogged roof drains prevent water from draining. The accumulated water adds weight to the roof and eventually finds the nearest weak point. Regular roof drain cleaning is one of the most effective preventive measures.
Before any work, the roofer inspects the entire roof to locate the source of the water infiltration. It is not always where you think: water can travel some distance before appearing inside. A thorough diagnosis avoids ineffective repairs and unnecessary work.
When the damage is limited, the damaged section is removed or prepared, the area is dried, then a patch of compatible membrane is torch-applied. Around a roof drain or penetration, the waterproofing work covers the entire detail, not just the visible crack.
If the damage is extensive or the membrane is too deteriorated, a repair does not solve the underlying problem. A partial or complete replacement of the elastomeric roofing will then be recommended, with a clear explanation of what is behind that recommendation.
It can be tempting to cover a leak with caulking or a makeshift patch. These fixes sometimes help in the short term, but they don't address the cause and often complicate the permanent repair that follows.
Repairing a flat roof yourself may seem economical, but working at heights carries real risks, and a poorly done repair can make the water infiltration worse. Torch-applying a modified bitumen membrane also requires specific skills and precautions. A roofer specializing in flat roofs assesses the overall condition of the roof and recommends the right solution for your situation.
Based at 1215 rue Tellier in Laval, Toitures OBM serves Saint-Jérôme, Laval, Montreal and the North Shore. The company specializes in elastomeric membrane flat roofs for commercial, industrial and multi-unit buildings, as well as for residential flat roofs.
Its services include flat roof repair, roof replacement, resurfacing, maintenance and inspection, as well as new construction. Toitures OBM is Soprema PAQ+S certified, CAA-Québec Recommended and holds RBQ licence 5778-2823-01. Financing is available through Financeit.
A stain that grows after rain or during snowmelt often points to the roof. Other causes, such as condensation or plumbing, are possible. A roof inspection makes it possible to tell the difference.
A recent roof should not leak. When it does, the cause is often a waterproofing detail, such as an upstand, a seam or the area around a roof drain. A precise diagnosis makes it possible to fix the right spot.
Not necessarily. If the membrane is in good overall condition, a localized repair is often enough. Roof replacement is recommended only when the condition of the roof justifies it.
A leak on your flat roof in Saint-Jérôme? 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.
