Skylights bring daylight into the middle of a deep commercial floor plate, which no window can do. In exchange, every unit adds a curb, a flashing detail, a fall hazard and an ongoing maintenance item to a low-slope roof. Whether that trade is worth it depends on the building, not on the product.
What do skylights actually give a commercial building?
Three things, in our experience with warehouses, plants and older industrial conversions across the GTA.
Daylight in the middle of the plate. A warehouse or manufacturing floor sixty metres deep gets nothing useful from perimeter glazing past the first few bays. Skylights and roof monitors are the only way to put natural light in the centre of that space. In converted brick-and-beam buildings, the original roof monitors were installed for exactly this reason and are often worth keeping.
Smoke and heat venting where it is required. Some industrial occupancies require venting provisions, and openable skylights or vents form part of that. This is an Ontario Building Code question for the designer and the fire consultant, not a roofing preference, but the roof has to accommodate the result.
Access and function. Roof hatches, detailed in almost exactly the same way, provide controlled roof access. Buildings without a proper hatch end up with staff on ladders at an unguarded roof edge.
What a skylight does to your utility bills depends on glazing, area, orientation and controls, and it is a modelling exercise for a designer rather than a claim a roofer should make. What we can tell you is what the roof has to do to carry it, which is what our skylight installation work covers.
What has to be right in the detailing?
A skylight on a low-slope roof is not a window. It is a hole in the waterproofing with a manufactured cover, and everything depends on how the transition is built.
- The curb. The unit sits on a raised curb, not on the deck. The curb has to stand high enough above the finished roof surface that snow, ponding and drifted meltwater cannot reach the frame or top of the base flashing. On an Ontario roof this matters far more than it does further south, because a skylight sitting in a snowdrift is under water for weeks each spring.
- Base flashing up the curb. Membrane run up the curb over a cant, terminated properly, and reinforced at all four corners with separate cut or moulded pieces. Curb corners split for the same reason parapet corners split.
- Counterflashing. The skylight frame or a separate cap must lap over the top of the base flashing so water is shed outward, not caught behind it.
- Drainage around the unit. Water has to get past the skylight, not stop at it. On a wide unit or a run of units sitting across the flow, that means a cricket or saddle on the upslope side to divert water around the obstruction.
- Structural framing. Cutting a deck opening removes structure. Headers and framing go to an engineer.
- Vapour control and thermal bridging. The curb is a break in the insulation. It has to be insulated and the vapour barrier tied into it, or the curb becomes a cold surface that condenses interior moisture and drips.
Why do skylights get blamed for leaks they did not cause?
Because condensation looks exactly like a leak from underneath.
A skylight is the coldest surface on the ceiling of a heated building in January. Interior humidity, from occupancy, from process moisture in a plant, from a humidification system, condenses on that cold surface and runs down the frame and onto the ceiling below. It appears in cold, dry weather with no rain, which is the giveaway. Adding sealant to the outside of the unit does nothing for it.
The fixes are different from leak fixes: glazing with a thermal break in the frame, an insulated curb, working condensation gutters, and above all dealing with the interior humidity load and the ventilation that should be removing it.
Genuine skylight leaks are almost always at the curb rather than at the unit: a split corner in the base flashing, counterflashing that has been reversed so it sits behind rather than over the base flashing, or a curb that is too low and is sitting in ponded water. Diagnosing which of the two you have is the first job on a flat roof repair call to a skylight.
What maintenance does a skylight add?
Real, ongoing, and worth budgeting for.
- Keep the drainage path around the unit clear. Debris collects on the upslope side of any obstruction on a roof. Left there it dams water against the curb.
- Check the curb flashing annually, particularly the corners, and particularly after any winter where snow drifted against the unit.
- Inspect the glazing. Older acrylic and polycarbonate domes become brittle and craze with age. Sealed units can lose their seal and fog.
- Check the frame seals and condensation gutters.
- Watch for impact damage after hail, which crazes a dome without opening it.
And a safety item that outranks all of the above. An aged skylight dome is not a walking surface and it is not a guardrail. Falls through skylights are a known cause of serious injury in roofing and in building maintenance work, which is why fall protection at skylights is treated as a hazard in its own right under Ontario working-at-heights requirements. Older units on industrial roofs are often brittle enough to fail under a person's weight without warning, and on a snow-covered roof they are invisible.
Screens or guards over skylights, or a fall protection plan that identifies each unit, is not optional on a roof that anyone accesses. If your building has skylights and no plan for them, that belongs in the maintenance programme before anything else does.
Should skylights be replaced at the same time as the roof?
Almost always yes, and this is one of the clearer decisions in commercial roofing.
The reason is the curb. During a re-roof, the curb flashing is stripped and rebuilt anyway, the roof surface height may change if insulation thickness changes, and the crew is already on the roof with everything open. Replacing a unit at that moment costs the incremental price of the unit and its installation. Replacing it two years later means opening a brand new membrane, cutting into completed flashing and creating a tie-in that did not need to exist.
The same logic applies to roof hatches, vents and anything else penetrating the assembly. If a component has less remaining life than the new roof, it goes now. On a flat roof replacement scope, we list every penetration and its condition so that decision is made deliberately rather than discovered halfway through.
The exception is a heritage or converted industrial building where the original roof monitors are a defining feature. There the question becomes whether the existing units can be restored and reglazed rather than replaced, which is a heritage and design conversation before it is a roofing one.
Which buildings are good candidates and which are not?
Good candidates:
- Deep-plan warehouses and distribution centres where the centre of the floor gets no daylight at all.
- Manufacturing floors where task visibility matters and the roof is otherwise uninterrupted.
- Converted industrial buildings that already have monitors or existing openings, where the framing exists and the character is worth keeping.
- Atriums and circulation cores in institutional and office buildings.
Poor candidates:
- Roofs with chronic ponding that has not been corrected. Adding curbs to a roof that already holds water adds dams to a roof that already holds water.
- Buildings with high interior humidity and no plan for the condensation that will follow. Food processing, laundries, indoor pools and plants with process moisture need the vapour and ventilation question answered first.
- Roofs crowded with mechanical equipment where there is no clean drainage path left to give up.
- Buildings with process exhaust that will craze and etch glazing.
- Roofs at the end of their service life. Put the skylight in with the new roof.
If you are weighing it up, the question to settle first is the condition of the roof it is going into. A walk and a written roof condition report answers that before anyone specifies a unit.
How does a skylight project run on an occupied building?
The disruption is mostly indoors, which surprises owners who assume roof work stays up there.
Cutting a deck opening drops debris, dust and noise directly into the space below. That space has to be cleared, sheeted off and kept out of use while the opening is made, framed and closed in. On a plant, that means coordinating with production. On a multi-tenant building, that means notice to the tenant underneath and usually work outside their hours.
The roof side follows the ordinary sequence: framing and curb, then the assembly built up around it, base flashing, cant, corners, then the unit set and counterflashed. Nothing is opened that cannot be made watertight the same day, so an opening is framed and dried in before the crew leaves.
Access, crane or hoist placement, lane occupancy and permit requirements are confirmed with the municipality before mobilisation, and structural work on the deck goes to an engineer. If you want to talk through whether your building suits skylights, call 416-456-0777 or book a walk through the contact page.