How to Install a Deck-Mounted Skylight in a Shingle Roof
Cut the opening, frame it square, set the unit on the deck and flash it with the manufacturer's kit, working from the bottom up.
- Advanced
- 1 to 2 days
- Updated
The short answer
A deck-mounted skylight is installed by marking the opening from inside, cutting the roof deck, framing headers and trimmers if a rafter is cut, fastening the unit to the deck, then flashing with the manufacturer's kit. The sill flashing goes on first, then the sides, then the head, each lapped over the one below. Self-adhered membrane goes under the flashing as a collar. Follow the unit's own instructions.
A deck-mounted skylight sits directly on the roof deck, with a factory flashing kit that laps into the shingles. On a pitched shingle roof it is the standard skylight, and on a well-built roof it is one of the most reliable penetrations. On a poorly framed one, it is a leak waiting for the first wet November.
Most of the work is not the glass. It is the rough opening, the framing that carries the load of the roof around it, the underlayment collar and the order in which the flashing pieces lap. This guide walks through that sequence for a typical unit. The manufacturer's installation instructions for your exact model always govern, including rough opening size, curb or frame details and how many fasteners each piece takes.
If you are choosing a unit, read skylight types first. For the flashing in detail, see how to flash a skylight. For a unit on a raised frame, see how to install a curb-mounted skylight.
Tools
- Reciprocating saw
- Circular saw
- Drill and long bit
- Plumb bob or long level
- Framing square
- Hammer or framing nailer
- Hook blade utility knife
- Tape measure
- Chalk line
Materials
- Skylight unit with the matching manufacturer flashing kit
- Framing lumber for headers and trimmers, same depth as the rafters
- Structural framing nails or screws
- Self-adhered ice and water membrane
- Roofing nails, 1-1/4 inch
- Shingles to match
- Insulation and vapour barrier for the light shaft
- Plywood or drywall for the shaft
Safety first
- Work tied off. WorkSafeBC requires fall protection where a fall could be 3 metres (about 10 feet) or more, and an open skylight hole is itself a fall hazard. Cover it the moment the deck is cut.
- Check the attic for wiring, ducts and plumbing before cutting anything from above. Turn off power to any circuit that runs through the area.
- Cutting a rafter or truss changes the structure. Do not cut a truss. A rafter cut needs proper double headers, and may need an engineer and a permit.
Step by Step
Step 1: Locate the opening from inside and confirm the framing
Start in the attic. Measure the rafter spacing and decide where the light shaft will come down into the room. The ideal is a unit that fits between two rafters with no cut, because then you only need short headers at the top and bottom of the opening.
Drive a nail up through the roof at each corner of the planned opening, then go outside and check that the location is at least a few feet clear of the ridge, a valley, a chimney or a vent. Skylights need room above them for water to divert and room below for the sill flashing.
Watch out: If the roof is trussed, do not cut any truss member. Ask the truss engineer or a structural engineer for a plan, because trusses carry load as a system.
Step 2: Cut the opening in the roof deck and remove the shingles
Remove shingles in a rectangle larger than the skylight so the flashing kit has a clean, flat deck to lie on. The kit's instructions give the area, often at least a foot to either side and more above and below.
Snap lines for the rough opening from the nails you drove through, cut the deck with a circular saw set to the sheathing depth and finish corners with a reciprocating saw. Cut from above so you can see the rafters, and keep the saw off them.
Tip: Tarp or plywood the hole immediately if rain is coming. Fraser Valley weather turns quickly.
Step 3: Frame headers and trimmers if a rafter is cut
Install headers across the rafters at the top and bottom of the opening, sized and nailed per a structural plan. A common approach is doubled lumber the same depth as the rafters, fastened with structural nails and joist hangers. The cut rafter ends bear on the headers, and the trimmers carry the loads down.
This is a structural change. Span, snow load and rafter size decide what the framing has to be, and Hope and the canyon carry much heavier snow loads than Chilliwack. A building permit and sometimes engineer-stamped framing are required in many places, so check with your municipality before cutting.
Watch out: Do not cut the rafter until the headers are designed and the lumber is on hand to support the roof on both sides.
Step 4: Check the opening is square and plumb
Diagonals should match within about 1/8 inch. Shim or re-fasten a header if they do not. The opening should also be the exact size the unit calls for, since most skylights are made for a tight fit with limited play.
Check that the top and bottom headers are level and that the deck around the opening is sound and flat. Patch soft or damaged sheathing now.
Step 5: Install the underlayment and self-adhered membrane collar
Run the underlayment so it is lapped over the lower edge and tucked under the upper edge of the opening, following the normal shingle-style lapping: the upper layer over the lower. Then install strips of ice and water membrane around the opening, the sill strip first, then the sides, then the head, each lapped over the one below.
This membrane collar is the backup if a flashing piece ever lets water through. See how to install ice and water shield. Turn it a few inches up the framing inside the opening where the kit instructions call for it.
Step 6: Set and fasten the skylight to the deck
Lift the unit into position, centre it in the opening and check the gap around it is even. Level it across the width and check it follows the roof plane.
Fasten through the brackets or nailing flange with the specified fasteners. Some units fasten to the top of the deck, others to the side of the framing. Do not add extra fasteners where the instructions do not show them, and do not drive nails through the flashing channel.
Step 7: Install the sill flashing first, then the sides, then the head
The sill piece, at the bottom edge, goes over the shingles that run up to the skylight. Then the side flashings go on, woven with the shingle courses like step flashing. The head piece goes last at the top, tucked under the shingles above, so water running down the roof from above is diverted around the unit.
Each piece follows the kit's instructions for nails and sealant. The details and the common variations are covered in how to flash a skylight, and the same lapping logic as step flashing applies on the sides.
Step 8: Shingle up to the flashing and finish the roof
Keep shingle nails out of the flashing channels and away from the unit's edges. Trim shingles so they stop short of the skylight frame and sit on top of the flashing pieces, not under them.
Finally, add the cap or counterflashing piece that the kit specifies over the top of the frame, and check that all seams and nail heads are covered.
Step 9: Build and finish the light shaft inside
Frame the four sides of the shaft from the roof opening down to the ceiling opening, plumb or splayed to spread light. Fill the walls with insulation, with no gaps, and tape or seal a vapour barrier on the warm side.
An uninsulated or poorly sealed shaft is cold, and condensation on it looks like a roof leak. See how to fix a leaking skylight. Finish the shaft with drywall or plywood and paint it a light colour.
Why the Order of Flashing Matters
Water always runs downhill and the flashing is lapped to follow that. The sill piece laps over the shingles below, the side pieces overlap the sill piece and each other, and the head piece is tucked under the shingles above. Reverse any of that and water gets behind the metal instead of off it.
Permits and Structure
A skylight that fits between rafters is usually a simple job for the structure. Cutting a rafter or enlarging the roof opening changes how the roof carries load. Check the requirements with your local building department, and follow an engineer's or designer's framing plan where one is needed.
Common Mistakes
- Cutting a rafter without proper headers. The roof sags around the opening and the unit distorts and leaks.
- Flashing the head before the sill. Reverse lapping drives water behind the flashing.
- Skipping the membrane collar. Without it, there is no backup when the flashing is breached.
- Changing the flashing kit. Mixing parts or cutting pieces out to fit breaks the tested system.
- Driving nails through the flashing channel. Every hole is a leak path.
- Leaving the light shaft uninsulated. The result is condensation and staining that gets blamed on the roof.
When to Call a Professional
If the opening needs a rafter cut, the roof is trussed, the pitch is steep, or the roof is high and you cannot tie off safely, hire a roofer and, where required, an engineer. Structural framing, permits and flashing together are not a first-time job.
Roofers also handle tying the skylight flashing into the surrounding shingles, which is where most do-it-yourself skylight leaks begin.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Skylights service or the towns we cover.
How to Install a Deck-Mounted Skylight: Questions
Do I need a permit to install a skylight?
It depends on the municipality and the work. Replacing a skylight of the same size usually does not need one. Cutting a new opening, cutting a rafter or changing the structure often does, and may need an engineer's framing plan. Check with your local building department before cutting anything, since permit rules differ between Chilliwack, Abbotsford and Hope.
Can you install a skylight without cutting a rafter?
Yes, if the unit is narrow enough to fit between two rafters. Many skylights are sized for 16 or 24 inch rafter spacing for that reason. You then only need short headers at the top and bottom of the opening. A wider unit requires cutting a rafter and framing around it with doubled headers and trimmers.
What goes under a skylight flashing kit?
Roofing underlayment lapped into the opening, then strips of self-adhered ice and water membrane around the frame. The membrane acts as a waterproof collar and backup if the flashing is ever breached. It is lapped bottom first, so each upper piece sits over the lower one.
In what order is skylight flashing installed?
Bottom up. The sill piece goes on first, then the side pieces, then the head piece at the top. Each piece laps over the one below it and is woven into the shingle courses. This makes water run over every joint instead of into it. Always follow the order in the manufacturer's instructions.
How long does it take to install a skylight?
A straightforward deck-mounted unit between rafters takes about a day for the roof work, plus extra time for framing and finishing the light shaft inside. Cutting a rafter, building headers, or finishing a long shaft adds time. Weather matters, since the roof opening should not be left open overnight.
Related Guides
All Skylights guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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