Roof flashing: the hidden cause of 80% of infiltrations in Montreal

When water shows up on the ceiling, the first instinct is to blame the shingles. Yet in the vast majority of residential infiltrations, the shingles aren’t at fault. The culprit, almost always, is a small metal element placed at the roof’s junctions: the flashing. These strips that no one notices are nonetheless the first line of defence against water — and the first to give way as they age.

Understanding how flashing works, where it hides and which wear signs it sends is how you avoid 80% of roof leaks. And it also means recognizing that a targeted $600 repair today can prevent a full $4,000 ceiling restoration three years from now.

The essentials in 30 seconds

Flashing ensures watertightness at the roof’s junctions: chimney, vents, valleys, dormers, walls.
According to industry statistics, it is responsible for about 80% of residential infiltrations.
Its lifespan is generally shorter than that of the shingles: 12 to 18 years versus 20 to 25 years.
Inspecting the flashing every 2 to 3 years in Montreal makes it possible to act before the infiltration.

The Technical Role of Flashing: Understanding the Vulnerability

A shingled roof is designed to channel water away through overlap: each shingle partially covers the one below. This system works perfectly on a flat, continuous surface. The problem starts as soon as an obstacle interrupts that continuity: chimney, plumbing pipe, vent outlet, dormer wall, junction between two roof planes. At each of these points, the break in watertightness must be filled manually. That’s exactly the role of the flashing.

A classic flashing is a strip of aluminum, galvanized steel or lead bent to fit the junction. It slides under the shingles on one side and rises against the element to be sealed on the other. A second piece, called counter-flashing, sometimes seals the whole thing on top. The system relies on several sealants, mortars and elastomer joints which, themselves, age faster than the metal.

According to Les Couvreurs Duro Toit, across 89 infiltration cases handled in Montreal in spring 2025, 71 traced their origin directly to a defective flashing — nearly 8 cases out of 10. That’s a statistic consistent with the observations of all roofers specialized in the region.

Flashing location	Common cause of leak	Impact if neglected
Around the chimney	Detached sealant joint, cracked mortar	Infiltration into interior wall, ceiling stains
Base of vents and pipes	Rubber cracked by UV	Direct drip onto the top-floor ceiling
Valleys (roof valleys)	Metal flashing too short or bent	Major infiltration during heavy rain
Wall-roof junction (dormer)	Poor overlap with the cladding	Chronic moisture in the partition wall

Recognizing a Weakening Flashing Before the Leak

Flashing sends visible signals well before the first drip on the ceiling. The challenge is that it’s located up high and no one thinks to look at it. Here’s how to carry out a targeted visual inspection from the ground or a window.

Cracked or missing mortar at the base of a chimney — easily visible with binoculars or a phone’s telephoto lens.
Metal flashing that is dull, rusted or detached from the cladding — a sign of oxidation or excessive expansion.
Cracked rubber at the base of a vent or plumbing pipe coming out of the roof — classic UV ageing.
Trace of runoff or rust on the shingles directly beneath a flashing — indicates water has already begun its path.
A ceiling stain aligned exactly under a roof junction — a near-certain correlation with a defective flashing.

If you spot even a single one of these signs, don’t wait to call in a roofer. A flashing repair generally takes a few hours and costs a fraction of the price of a full restoration, whereas a flashing left alone can destroy the insulation, the structure and the interior finish in a single season.

Why Flashing Suffers Especially in Montreal

The Montreal climate hits flashing far harder than shingles. Three specific mechanisms accelerate its wear and explain why a biennial inspection is more relevant here than elsewhere.

First factor: freeze-thaw cycles. The flashing’s metal expands in summer and contracts in winter. With each cycle, it exerts a pull on the sealants and mortar holding it in place. Over 15 years in Montreal, that’s nearly 1,500 cumulative cycles — well beyond what most sealants can handle without cracking. Second factor: ice dams. When water creeps back behind an ice barrier at the roof’s edge, it exerts reverse hydraulic pressure on valley and eave flashing, stressing zones normally protected by gravity.

Third factor, specific to Montreal: urban density. Many residential roofs have chimneys, vents and dormers very close together, multiplying the junction points per home. A typical Plateau duplex can have 8 to 12 distinct flashings, versus 3 to 5 on a suburban bungalow. It’s simple math: more flashing means more potential failure points, so more importance to regular maintenance.

Frequently asked questions

What exactly does roof flashing do?

Flashing is a strip of metal or membrane that ensures watertightness at the junctions between the roof and other elements: chimney, wall, vent, dormer. It’s the most vulnerable point of any roof.

How much does redoing chimney flashing cost in Montreal?

Between $400 and $1,200 depending on accessibility, mortar condition and the type of flashing required. A quick intervention avoids far higher costs in interior damage.

Can you repair flashing yourself?

A temporary patch with roofing sealant is possible, but a durable repair requires removing the adjacent shingle and replacing the flashing. Best left to a roofer.

Take action with Les Couvreurs Duro Toit

An ageing chimney, a suspicious dormer or a faint stain on the ceiling? Les Couvreurs Duro Toit inspect and replace flashing throughout Montreal and the North Shore. Avoid the infiltration with a quick, targeted intervention.