Leaking Skylight: Causes, Diagnosis, and Repair (Montreal)

A brownish ring spreading around the window frame. A few drops on the floor during a warm spell in February. Sometimes just a lingering musty smell in the attic room.

When a skylight leaks, the cause is almost always found somewhere other than the glass. It’s hidden between the frame, the flashing, and the roof. And in Montreal, with the freeze-thaw cycles that repeat throughout the winter, this type of problem doesn’t stay stable for very long: water soaks the insulation, causes the drywall to swell, and then creates the conditions that mold needs to thrive. A delay of just a few days can significantly increase the scope of the repairs.

The 5 Key Points to Remember

• Condensation is often mistaken for a leak. Clear water spread across the entire window due to very cold, dry weather: that’s moisture from your home. Discolored water that appears in a specific spot during rain or when snow melts: that’s a leak.
• The flashing gives way more often than the rest. This metal connector between the skylight and the roofing material tops the list of potential culprits, ahead of the glazing and the frame.
• Water doesn’t flow in a straight line. It follows the rafters and can emerge up to one meter from the entry point. Hence the inspection order: interior, attic, roof.
• Most of the damage can be repaired. Flashings, sealant, and membranes need to be replaced. Persistent condensation between the panes, a warped frame, or a twenty-year-old unit that leaks every spring—no, thank you.
• Applying sealant without checking the surface first makes the situation worse. Waterlogging the area around the well traps water instead of draining it away, accelerates the decay of the frame, and may void your warranties.

Is it really a leak? Leak or condensation?

That’s the trap of a Quebec winter. Before concluding that a skylight is leaking, rule out condensation, because the two look very similar at first glance.

Condensation forms on the inside of the glass when warm, humid air from inside the house comes into contact with a cold surface. The water is clear. It appears evenly across the entire window during very cold weather, and it disappears as soon as the indoor humidity drops. Rain has nothing to do with it.

A real leak, on the other hand, is weather-related. It occurs during or after a downpour, snowmelt, or freezing rain. The water is often discolored because it carries dust and debris along its path. And it collects at a specific spot: a corner of the frame, the edge of the tunnel.

A hygrometer quickly settles the matter. Health Canada recommends keeping relative humidity below 50% in the summer and between 30% and 35% in the winter. When the outdoor temperature drops below -10 °C, CMHC lowers the target to 30%, or even 25% during extreme cold spells, specifically to prevent condensation on windows and cold walls. If your hygrometer reads 55% in January, the problem lies in the air inside your home, not on the roof.

The 6 Common Causes of a Leaking Skylight

1. The flashing. This metal piece connects the skylight to the roof covering, and it is by far the leading source of water infiltration. Whether it’s cracked, lifted, corroded, or installed without the original kit—in all four cases, water gets in.
2. Sealant and caulking. They have a limited lifespan, and Montreal’s temperature fluctuations cause them to harden faster than elsewhere. The sealant cracks, and then water seeps in through the crack.
3. The frame or the glazing seal. Wood swells, aluminum warps, and the seal between the glass panels eventually fails. Condensation trapped between the panes is the classic sign.
4. The original installation. A frame installed without sufficient clearance, a poorly connected vapor barrier, a tunnel left uninsulated: the defect was there from day one and becomes apparent years later, often after a heavy rain.
5. The membrane around the well. On a flat roof, water pools. If the elastomeric membrane has cracked near a penetration, it allows water to seep through without the drain being at fault. The same applies to a flat roof with backed-up drains.
6. Ice dams. The National Research Council of Canada describes the mechanism: Meltwater trapped by an accumulation of ice seeps by gravity into the roof deck or exterior walls, which wets the insulation, causes the wood to rot, and, in severe cases, damages ceilings and wall coverings. A skylight set into a sloped roof creates exactly the kind of obstacle where snow accumulates.

How to Locate the Exact Source of the Leak

Water does not flow vertically. It follows the rafters, runs along the vapor barrier, and sometimes emerges more than a meter away from the actual point of entry. That’s why the diagnosis is carried out in a specific order.

We start inside, noting exactly where the water appears and when. The upper corner, the base of the tunnel, the middle of the window: each location points to a different cause. Next, we go up into the attic to trace the path of the water, because that’s where the water’s path becomes clear: rings on the wood, compressed and darkened insulation, rusty nails. Finally, we inspect the roof, checking the flashing, the condition of the sealant, the membrane or shingles around the frame, and then the water flow upstream.

If you’re still unsure, a water test will confirm it: spray water in sections, starting from the bottom, while a second person observes the inside.

Visible signs of a leaking skylight

• A yellowish or brown ring that widens toward the top, around the frame
• Peeling paint, swollen drywall, and loose trim
• Condensation or a whitish deposit trapped between the glass panes
• Signs of runoff on the tunnel walls
• A musty smell in the room or in the attic
• Rust on the metal parts of the frame
• Ice that forms around the well in winter

Repair or Replace? What Can Actually Be Repaired

A modern skylight typically lasts 15 to 25 years, depending on the model, orientation, and quality of installation. The manufacturer’s warranties provide a useful guide. VELUX skylights in the No Leak line are covered by a 10-year warranty on replacement parts (skylights, roof windows, and flashings) and a 20-year warranty against seal failure—that is, moisture trapped between the glass panels. In addition, there is a 10-year installation warranty, contingent on the use of VELUX flashing—which clearly demonstrates just how important proper installation is, just as much as the product itself.

When it comes to repairs, the list is longer than you might think: replacing flashing, reapplying sealant, reworking trim joints, installing a section of membrane around a penetration, and replacing a screen or an opening mechanism.

Replacement is necessary when: there is constant condensation between the panes, when the frame is rotten or warped, when the glass is cracked, or when a unit that is more than twenty years old leaks repeatedly. In that case, you might as well take advantage of it: today’s ENERGY STAR-certified double- or triple-pane low-emissivity models significantly improve a room’s energy efficiency compared to a unit from the 1990s. Since 2020, Canadian requirements have included only a single climate zone—the coldest one. In other words, a model certified today is designed for our winters.

Not all types of skylights behave the same way, either. A tubular skylight, with its sealed dome and reflective tube, has fewer weak points than a large opening roof window, and the Canadian standard does indeed classify it as a skylight. A flat-roof unit, on the other hand, requires adequate slope; otherwise, standing water will eventually seep in.

What to Do Immediately to Minimize Water Damage

1. Place a bucket and protect the floor. If water is running down a wall, move furniture and other objects out of the way.
2. Disconnect the power from the mains as soon as water gets near a light fixture or an outlet.
3. If you’re comfortable doing so, drill a small hole at the lowest point of a plaster blister to drain the trapped water. It may seem counterintuitive, but it prevents the ceiling from collapsing. Otherwise, report it to the repair person as soon as they arrive.
4. Take pictures of everything—the ceiling, walls, and floor—before you clean anything. Your insurance company will need them.
5. Do not climb onto a snow-covered or icy roof. No repair is worth a fall.
6. Call an emergency roofing service to have the roof temporarily secured.

Should you fix it yourself or call a professional?

Touching up an interior trim joint or cleaning away debris that has accumulated around the frame from a stable ladder in July: that’s reasonable.

The rest, not so much. As soon as you touch the flashing, the membrane, or the roofing material, a sloppy repair almost always makes things worse. The most common instinct is to seal the perimeter of the skylight with caulk, and that’s exactly what you shouldn’t do: water gets trapped, the frame rots faster, and both the manufacturer’s warranty and your roof warranty could be voided.

A roofer with a valid RBQ license will identify the root cause, work safely at heights, and document the repair for your warranty purposes. If your skylight shows any of the signs described above, our Velux skylight repair and installation team serves the entire Greater Montreal area.

Prevention and Maintenance: Maintaining Watertightness Over Time

In most cases, two inspections a year are sufficient—one in the spring and one in the fall—focusing on the frame, flashing, and sealant. Take this opportunity to clear away any debris that has accumulated upstream of the drain: leaves, shingle granules, and small twigs. All of this traps water against the frame.

Also keep an eye on indoor humidity with a hygrometer, especially if the skylight is above a bathroom or kitchen. As for snow, a roof heating cable helps reduce ice dams in areas where the problem recurs every year.

And reapply the sealant around the tenth year, before it starts to crack. It’s the most cost-effective expense of the bunch.