Skylights and Roof Curbs: The Penetrations That Outlive Their Flashing

A strata building has a leaking skylight over a top-floor corridor. The council approves a replacement, the unit is swapped out, and the leak stops. Eighteen months later it is back, in the same place, and the council is looking at an invoice for a repair that did not hold.

Nothing was wrong with either skylight. The problem was underneath both of them, in a curb built too low when the roof was last redone, and swapping the glass did not touch it.

Roof penetrations concentrate a disproportionate share of a building’s leak risk into a small area. Understanding which part of the assembly actually fails, and on what timeline, is what stops money going into the wrong component.

The glass is not the problem

A sealed glazing unit is a manufactured product with a long and reasonably predictable service life. It can fail, and when it does the usual symptom is a fogged cavity between the panes as the seal loses its gas fill and admits moisture. That is a visible, specific defect, and it does not cause water to run down a wall.

Water entering a building at a skylight comes from one of three places: the junction between the skylight frame and the curb it sits on, the flashing that bridges the curb to the roofing membrane, or the membrane termination at the top of the curb itself.

All three are site-built details. They depend on the sequence in which materials were installed, on the skill of whoever did it, and on decisions made possibly decades ago by people no longer available to ask. That is why they fail earlier and more often than the manufactured component sitting on top of them.

Curb height is the recurring defect

The curb is the raised frame built up from the roof structure that the unit sits on. Its purpose is straightforward: lift the opening above the water.

The roofing membrane turns up the sides of the curb and terminates near its top. That termination is a joint, and joints in a roofing system are the vulnerable part. Putting the joint high above the roof surface means water draining across the roof, ponding after a heavy rain, or backing up behind a blocked drain never reaches it.

When a curb is built low, the termination sits in the water path. It may hold for years because it is only occasionally wet. Then a drain blocks during an atmospheric river event, water builds on the roof, and it sits against a termination that was never designed to be submerged. The behaviour of roof drainage under that kind of load is set out in the guide on flat roof drains and overflows, and the storms that produce it are described in the post on atmospheric rivers and building envelopes.

Low curbs turn up most often on retrofits. A piece of equipment gets added to a roof years after it was built, somebody cuts the membrane to make the opening, and the curb and termination are improvised by whoever installed the equipment rather than by a roofer. The equipment works. The roof around it does not.

Flashing sequence: correct materials, wrong order

The second recurring failure is one that no material specification can prevent.

Flashing works by lapping, and every lap has to shed water onto the layer below it rather than into it. Upper pieces go over lower pieces, always, so that water running down the assembly stays on the outside of each layer in turn.

Install those pieces out of order and an upper element ends up tucked behind a lower one. What has been built at that point is a funnel. Water running down the flashing reaches the reversed lap and is directed into the assembly rather than shed off it.

Everything can be correct on paper. The right membrane, the right metal, the right sealant, and the detail still leaks because of the order in which things went on. This is the same failure mechanism described in the post on metal flashing detail failures in rainscreen assemblies, and it is why an inspection of a leaking penetration should look at how the layers overlap before it looks at whether anything is cracked.

A related version shows up where sealant has been used to compensate. When a lap is reversed, sealing the joint from outside will stop the leak for a while. The sealant is now the only thing holding water out of a detail that was built to fail, and sealant has a service life measured in years while flashing has one measured in decades. When it goes, the leak returns and looks like a new problem.

Condensation is not always a leak

Before commissioning a repair, establish which problem is actually being solved.

A skylight is a cold surface set into a warm ceiling. Warm humid air in a building rises, reaches that cold surface, and gives up its moisture as condensation. In a Metro Vancouver winter, with interior humidity from cooking, showers, and occupancy, this happens routinely.

The diagnostic is timing. Water appearing on a cold clear night with no rain is condensation. Water appearing during or shortly after rain is a leak. Water appearing in both circumstances means the unit has both problems, which is common on older single-glazed or thermally poor skylights.

Condensation is addressed by managing interior humidity, improving ventilation, or replacing a thermally poor unit with one that has a warmer interior surface. Reflashing the roof around it changes nothing. The distinction is covered in more detail in the guide on condensation versus a leak, and getting it wrong means paying for roofing work that was never going to help.

Hatches and operable units wear differently

A roof hatch or an operable skylight adds moving parts, and moving parts have their own failure mode.

The lid, the gasket, and the latching hardware all wear. A gasket compresses and hardens over years until it no longer seals when the lid closes. A latch that has been forced or has corroded may hold the lid nearly shut rather than fully shut. Water then enters at the lid, not at the curb, and no amount of work on the roofing will address it.

Roof hatches carry a second issue on strata buildings, because they are also an access point and part of the fall protection arrangement around the roof. Work on a hatch usually touches the access planning covered in the post on roof anchor and hatch access planning, and it makes sense to deal with both at the same visit.

Repair or replace

The decision comes down to which part has failed.

If the glazing and frame are sound and the failure is at the curb or the flashing, the repair is to strip the roofing back around the curb, rebuild the curb to an adequate height if it is deficient, install flashing in the correct sequence, and reterminate the membrane. The unit goes back on. This is normally the cheaper option and it addresses the actual defect.

If the unit itself has failed, through a broken seal, a cracked dome, a corroded frame, or thermal performance poor enough to cause chronic condensation, it gets replaced. Critically, the curb underneath still needs to be assessed and corrected during that work. Installing a new unit on an inadequate curb produces the eighteen-month repeat that started this article.

Where a roof replacement is already planned, skylights and curbs should be costed into it. The membrane around every penetration is being disturbed anyway, the access is already arranged, and the flashing is being rebuilt regardless. Doing the units at the same time avoids paying twice for the same disruption, and it avoids a new roof terminating against a unit with three years left in it. The same logic applies to the recover-versus-tear-off decision discussed in the guide on roof recover versus tear-off, where existing penetrations often drive the answer.

What to inspect and how often

Penetrations deserve closer attention than the open field of a roof, because that is where the risk sits.

At least annually, and again after major storms, someone should look at each skylight and curb specifically. The things worth checking are whether the membrane termination sits clearly above the surrounding roof surface, whether water ponds anywhere near the curb, whether the flashing laps run the correct way, whether sealant at the frame-to-curb junction is intact and still bonded, and whether the surrounding membrane shows any lifting, wrinkling, or open seams.

Photographs of each penetration, taken the same way each year, make deterioration visible that a single visit will not. A curb that is developing a ponding problem as the roof settles shows up clearly in a three-year photo sequence and not at all in one inspection.

Where the strata does not have a roof inspection programme, this is a sensible place to start one, since penetrations are a small number of locations carrying most of the exposure.

The practical summary

Skylights leak at the curb, the flashing, or the frame junction, not through the glass.

Curb height is the most common underlying defect, and it is often the legacy of a retrofit done without a roofer. A membrane termination sitting in the water path will eventually leak regardless of how well it was made.

Flashing sequence causes failures that no material can prevent. Check which way the laps run before concluding anything is worn out.

Separate condensation from leakage by when the water appears, because they need completely different fixes.

Reflash rather than replace when the unit is sound, and correct the curb whenever the unit is replaced, or the same leak returns on the same schedule.

Group the work with a planned roof replacement where one is coming, and inspect penetrations annually with photographs.

Sources and further reading

• Canadian Roofing Contractors Association, CRCA Roofing Specifications Manual, penetration and curb flashing details, referenced 2026
• National Research Council Canada, National Building Code of Canada, roofing and flashing provisions, referenced 2026
• BC Housing, Building Enclosure Design Guide, roof penetration detailing, referenced 2026
• RCI International, Manual of Practice, roof penetration inspection guidance, referenced 2026
• Allweathercoating roof inspection records, Metro Vancouver, 2026