Commercial Skylight Installation and Repair
Curb-mounted skylights, hatches and translucent panels on low-slope commercial and industrial roofs.
Commercial skylights on flat roofs are curb-mounted: the unit sits on a raised curb that the roof membrane is flashed up and over, so the water line never reaches the glazing. Most skylight leaks come from the curb and flashing, not the unit. NorthCan Roofing installs, reflashes and replaces skylights across Toronto and the GTA. Call 416-456-0777.
How is a skylight installed on a flat roof?
The unit is the easy part. The curb and the flashing are the job.
- Opening and framing. The deck opening is cut and framed to suit the unit and the structure. Any framing that affects joists or long-span members is confirmed with a professional engineer.
- Curb. An insulated curb is set on the deck, high enough that the membrane terminates well above the finished roof surface, above ponding and above the snow that will drift against it in a Toronto winter. Curb height is the single most important decision on the installation.
- Vapour and thermal continuity. The vapour barrier and the insulation are carried into the curb so the detail is not a thermal weak point that condenses.
- Membrane. Base flashing is run up the curb over a cant or a properly formed corner, terminated mechanically at the head, and the corners are detailed as a system rather than improvised.
- Unit and cap. The skylight is set on the curb, fastened as specified, and counter-flashed over the base flashing so water is shed outward at every layer.
- Drainage around it. Crickets are built on the upslope side of a wide curb so water is diverted around it rather than pooling behind it.
The same layered logic applies to every curb on a roof, as described on roof flashing and siding.
Why is my skylight leaking?
Start by ruling out condensation, because a large share of reported skylight leaks are not leaks at all. Condensation appears on cold, clear days rather than during rain, shows as beads on the inner glazing and the frame, and points to interior humidity and a thermal break problem rather than water entry.
Actual leaks usually come from one of these:
- Curb height too low, so drifted snow and ponded water sit above the membrane termination.
- Base flashing failed at the corners, which is where curb flashings almost always split first.
- Counter flashing loose or missing, leaving the membrane head exposed.
- Failed gaskets and seals in an older unit, especially where the frame has been sealed rather than flashed.
- Cracked or crazed glazing, including acrylic domes that have weathered and impact damage from hail or dropped tools.
- Ponding behind the curb, where no cricket was built and the water has nowhere else to go.
We diagnose by inspecting the curb, the flashing and the unit separately, and we say clearly which of the three is at fault. Active water entry is handled through emergency roof repairs.
Repair, reflash or replace?
Repair suits a sound unit on a sound curb with a local flashing failure, a loose counter flashing or a failed seal. It is quick and it works when the curb geometry is right.
Reflash means stripping the membrane back around the curb, correcting the base flashing and, where needed, building the curb up to a proper height, then rebuilding the detail. This is the answer when the leak keeps returning because the curb was always too low.
Replace the unit when the glazing is crazed, cracked or has lost clarity, when the frame is deformed or corroded, when the unit is single-glazed and condensation is a constant complaint, or when the unit is simply beyond serviceable parts. Replacement is also the moment to raise the curb and correct the drainage in one visit.
Where the surrounding roof is being replaced anyway, all three questions get answered at once. See flat roof replacement.
What about skylights during a re-roof?
A re-roof raises the roof surface, because rebuilding to current code requirements normally adds insulation. If the skylight curbs are not raised with it, every curb on the roof loses flashing height at once, which is how a brand new roof develops old leaks. We assess every curb during the roof replacement and build up or replace the ones that need it.
It is also the practical time to decide the future of each unit. Old acrylic domes near the end of their life, units that are no longer wanted, and openings that would be better closed off entirely all cost far less to deal with while the membrane is open and the crew is on site than as a separate mobilisation two seasons later.
Are skylights a safety hazard on a commercial roof?
Yes, and it is the hazard most often overlooked on an industrial roof. An unguarded skylight or translucent panel is an opening in the deck that looks like a surface. Falls through skylights are a recognised risk in roof work, and weathered acrylic and fibreglass panels give no warning before they fail under a person's weight.
Under the Occupational Health and Safety Act and the Ministry of Labour, Immigration, Training and Skills Development working-at-heights requirements, our crews guard or cover openings and use travel restraint or fall arrest around them. For your building beyond our visit, the practical options are screens or guards over the units, permanent guardrails around them, or replacing weathered panels with units rated to resist a fall. We note every unguarded skylight in our roof condition report, and the wider access and fall protection picture is on industrial roofing.
To book an assessment, call 416-456-0777 or use the contact page.
Frequently asked questions
Can you add a skylight to an existing flat roof?
Yes, subject to the structure. Cutting a deck opening affects joists or purlins, so the framing is confirmed with a professional engineer before anything is cut. We then frame the opening, build an insulated curb of adequate height, flash it into the existing membrane and set the unit, with a cricket upslope if the curb is wide.
Is the water on the inside of our skylight a leak?
Check when it appears. Water during or shortly after rain is a leak, and the cause is usually the curb flashing. Beads and drips on cold, clear days are condensation, driven by interior humidity meeting a cold frame or glazing. Condensation is solved with thermally broken units, better ventilation and humidity control, not with sealant.
How high should a skylight curb be?
High enough that the membrane termination sits above ponding water and above snow drifted against the curb. Low curbs are the most common design fault we find on flat roof skylights in Ontario, and they are the reason a leak returns after every repair. When we reflash a low curb we build it up rather than repeat the failure.
Do you replace roof hatches as well?
Yes. Roof hatches are detailed in the same way as a skylight, with an insulated curb, base flashing and counter flashing, and they suffer the same failures at the corners and at low curb heights. We replace hatches, rebuild their curbs and fit safety railings around the opening where required.
Will a new skylight match the existing openings?
Where the opening size is retained, a replacement unit is ordered to suit the existing curb or to a new curb built to the same footprint, so the interior finish is undisturbed. Where the old unit is a discontinued size, we build the curb to suit a current unit and finish the interior reveal accordingly.
Ready for your free roof assessment?
Tell us the address, the roof size and what you are seeing. We will book a time to get up there and put it in writing.
Call 416-456-0777 for anything urgent. Our emergency line is answered around the clock.
Call 416-456-0777