Integrating Skylights Into a Standing Seam Roof
A skylight in a standing seam roof needs a curb, a cricket, and a panel layout planned around it before the first panel goes down. Flashing kits made for shingles do not translate.
A skylight in a standing seam roof is a small chimney with glass in it. It interrupts the panel run, it obstructs water and snow moving down the slope, it has to be flashed on four sides in a material that cannot be woven or lapped like shingles, and it has to allow the panels around it to keep moving. Done properly it is a durable detail. Done as an afterthought it is the leak everyone remembers.
Curb-Mounted, Not Deck-Mounted
Skylights come in two mounting styles. Deck-mounted units sit low, integral to the roof plane, with a factory flashing kit designed to weave into shingle courses. Curb-mounted units sit on a built-up curb that raises the frame above the roof surface. For standing seam, curb-mounted is the assembly that works.
The reason is geometry. A curb creates four vertical faces to flash against, which is exactly what metal roof detailing is good at — upturned legs, hems, aprons and counter-flashing all have something to work against. A deck-mounted unit's factory kit assumes overlapping courses of flexible material, and there is no way to make a rigid, seamed panel behave that way.
Curb Height
Curb height is a snow-country decision. The curb has to be tall enough that the flashing legs come up well clear of anything that will sit against them — which in Ontario means drifted snow, packed ice at the uphill side, and meltwater sitting under that snow through a January thaw. A curb sized for a rain climate is not adequate here. Taller costs nothing meaningful during construction and eliminates a whole category of winter problems.
The Four Sides
The flashing assembly follows the same logic as a chimney.
- ✓Downhill: an apron with a vertical leg up the curb and a horizontal leg lapping over the panel below, hemmed at its lower edge so water drips clear rather than running back underneath.
- ✓Sides: formed pans with an upturned leg against the curb and a hem interlocking with the adjoining panel, so the panel can move without dragging the flashing.
- ✓Uphill: a cricket on anything of width, for exactly the same reason as a chimney — a flat back pan collects snow, ice and debris.
- ✓Over the top of all of it: counter-flashing or head flashing coming down from the curb face over the base flashing legs, so no top edge relies on sealant.
- ✓Under all of it: self-adhered membrane up the curb faces and out onto the deck generously on all four sides, installed before any metal.
A skylight flashing kit sold for asphalt shingles is not a standing seam detail. Those kits are designed to be woven into shingle courses and rely on that overlapping to shed water. On a metal roof the flashing is fabricated to suit the panel profile and the curb, and it interlocks with the panels by hems and cleats. Fitting a shingle kit to a metal roof produces a detail that only works until the first driving rain.
Panel Layout Around the Opening
This is the part that has to be decided before any panel goes on the roof, and it is where retrofits get difficult. Ideally the curb sits between two seams, entirely within the flat of a panel run, so the side flashings meet ordinary panel pans. The next best outcome is that a seam lands right at the edge of the curb, where it can be terminated cleanly into the side flashing.
The outcome to avoid is a seam that runs into the middle of the curb's width and has to be cut off, because a seam terminated mid-run needs its rib closed and dressed into the flashing, and it is both the ugliest and the most leak-prone version of this detail. Planning the panel layout around existing or planned skylights is a five-minute conversation at setting-out that saves a great deal of grief.
Letting the Panels Move
The curb is fixed to the structure. The panels around it are not — they expand and contract along the slope. So the side flashings connect to the panels by hem and cleat, not by screwing through the panel into the curb. Screwing a panel to a fixed curb pins it at that point, and everything downhill and uphill of it then fights that anchor every time the temperature changes. It shows up as distorted pans around the skylight and, eventually, as a torn flashing.
Adding a Skylight to an Existing Metal Roof
It can be done, and it is more involved than cutting a hole. Panels have to be unseamed and removed back to a sensible boundary, the opening framed and headed off properly, the curb built, membrane installed, new flashings fabricated to the panel profile, and panels re-cut and re-seamed around it. On a mechanically seamed roof, unseaming and reseaming is specialist work. It is worth pricing honestly and worth deciding at the time of the re-roof if a skylight is anywhere in the plans.
Condensation Is Not a Leak
Water appearing around a skylight in winter is often condensation rather than infiltration. A skylight is the coldest surface in a room, and in a house with high indoor humidity, moist air contacts the glass and the frame and condenses. It runs down and collects at the bottom of the frame, and it looks exactly like a leak. The tells are that it happens on cold days rather than during rain, that it is worst in bathrooms and kitchens, and that the water is clean. The fix is humidity control and ventilation, not roofing.
Frequently Asked Questions
Can a skylight be added to an existing standing seam roof?
Yes, but it requires unseaming and removing panels, framing the opening, building a curb, and fabricating new flashings — not simply cutting an opening. Mechanically seamed roofs need the right tooling to reseam. If a skylight is a possibility, it is far cheaper to do it during the re-roof.
Does a skylight need a cricket above it?
Anything with meaningful width across the slope does, for the same reason a chimney does. Without one, the uphill side of the curb is a shelf that collects snow, ice and debris, and holds meltwater against the flashing all winter.
Why is water dripping from my skylight only in winter?
Most often condensation. The glass and frame are the coldest surfaces in the room, indoor humidity is highest in winter, and moisture condenses and runs. If it happens on cold dry days rather than during rain, and the water is clean, humidity control is the place to start.
Curb-mounted or deck-mounted for a metal roof?
Curb-mounted. The curb gives the vertical faces that metal flashing details need, keeps the frame clear of drifted snow, and lets the flashings interlock with the panels properly. Deck-mounted units and their shingle-oriented flashing kits do not adapt to a seamed panel.
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