How to Replace a Skylight During a Reroof
The roof is already open, so the flashing can be built into the new roof instead of patched into an old one.
- Advanced
- 4 to 8 hours added to a reroof
- Updated
The short answer
Replace the skylight during the reroof, after tear-off and before the new shingles go on. Remove the old unit and flashing, check and repair the framing and deck, size the new unit to the existing opening, install it with its matching flashing kit and a membrane collar, then shingle to the flashing. Do interior finishing and check the light shaft insulation last.
A skylight is the most leak-prone thing on most roofs, and a reroof is the best time to deal with it. With the old shingles off, the deck and framing are visible, the flashing can be woven into the new underlayment and shingles, and the skylight is installed in the order that makes it work: membrane first, then flashing, then shingles. Reflashing an old skylight into a new roof is possible, but the old frame, seals and glass are the same age as the roof you are replacing.
An old skylight that is fogged, stained, cracked or has been leaking does not get better with new shingles around it. Replace it now, while the cost of access is already covered by the reroof, and you avoid cutting into a brand new roof in five years.
This guide covers the swap. For the unit itself see how to install a deck-mounted skylight or how to install a curb-mounted skylight, and for the details of the metal work see how to flash a skylight.
Tools
- Flat bar
- Reciprocating saw
- Hook blade utility knife
- Level
- Tape measure
- Drill and driver
- Roofing hammer or coil nailer
Materials
- New skylight with matching flashing kit
- Self-adhered ice and water membrane
- Framing lumber for repairs
- Fasteners specified by the skylight instructions
- Shingles for the new roof
- Insulation and finish material for the light shaft
Safety first
- Work tied off. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury. A removed skylight leaves an open hole in the roof.
- Old skylight glass can shatter when it is removed. Wear eye protection and cut resistant gloves, and protect the room below with a drop cloth and tarp.
- Keep the weather in mind. The hole must be covered or dried in by the end of the day.
Step by Step
Step 1: Decide whether to replace or reflash
Signs that a skylight should be replaced: fog or moisture between the panes, cracks or yellowing in a plastic dome, corroded or bent frames, stains around the light shaft and a history of leaks.
An old unit that is still sound and is being reused needs new flashing, new seals and a careful check of the frame. Most reroofers replace it, because the unit's life is similar to the roof's and a skylight swap later means cutting into the new roof.
Step 2: Order the right replacement before tear-off
The model label is usually on the frame or the shaft. Give the model and size to a skylight supplier, who can often supply a direct replacement that uses the same rough opening. Matching the opening avoids any change to the framing.
Order early. Some units take weeks to arrive, and you do not want the roof open waiting for a skylight. Also order the flashing kit that matches the unit and the new roof covering.
Tip: If you are changing the size or type, such as going from a dome to a glass unit, check the new rough opening before anything is ordered.
Step 3: Protect the interior and remove the old unit after tear-off
Put down drop cloths and a tarp under the light shaft. Old glass and debris will fall. Remove the roofing around the skylight first, back far enough to see the deck and flashing flange.
Take out the fasteners holding the old frame, cut any sealant and lift the unit out. Remove the old flashing and all old membrane around the opening. Do not re-use old flashing with the new unit.
Step 4: Inspect and repair the framing and deck
Leaks around skylights often rot the framing and deck next to them. Probe the wood with a screwdriver. Replace soft sheathing and any framing that is damaged. If the original opening was cut without proper headers, this is the time to fix it.
Check that the opening is square and matches the new unit's rough opening size. Shim or re-frame as needed. A structural change needs a proper plan, and may need an engineer or a permit.
Watch out: Never cut or notch a truss member while repairing around a skylight.
Step 5: Install the underlayment and membrane collar
Run the main underlayment so it laps correctly into the opening, with upper sheets over lower ones. Then install ice and water membrane strips around the opening: sill, then sides, then head, each over the one below.
This is the advantage of doing it with a reroof. The new membrane and underlayment integrate with the skylight flashing from the start. See how to install ice and water shield.
Step 6: Set the new skylight and fasten it
Centre the unit in the opening, check for level across both directions and follow the roof plane. Fasten through the flange or brackets with the specified fasteners only, and avoid extra nails that are not in the instructions.
Step 7: Flash the skylight as the new shingles go on
Shingle up to the sill, install the sill flashing, then weave the side flashing with each course and finish with the head flashing tucked under the shingles above. Each piece laps over the one below.
If a cricket is recommended above a wide unit, build it now, before the shingles cover the area. See how to flash a skylight for the full sequence.
Step 8: Finish and insulate the light shaft
An old shaft is often uninsulated, poorly sealed or stained. Replace missing insulation, seal gaps and add a vapour barrier on the warm side before re-finishing. Condensation inside a cold shaft is the most common false alarm for skylight leaks.
Finish with drywall or trim, paint, and check the seal at the ceiling. See how to fix a leaking skylight if any staining appears later.
Why a Reroof Is the Right Time
Skylight flashing depends on how it is woven into the roof around it. On a new roof, the underlayment, membrane and shingles are all installed in the correct order from the start. On an existing roof, any skylight work means lifting shingles that are already nailed, which damages them and makes perfect lapping hard.
The cost of access, scaffolding, tie-offs and dry-in is already covered by the reroof. Adding a skylight swap is mostly the cost of the unit and kit.
Fraser Valley Notes
Skylights in this climate spend months wet. Old sealants age in the sun and cold cycles, and the moss and needle debris under trees collect at the upper edge. Replace old glazing with modern double-glazed low-e units, and keep the head of the skylight clear after the roof is done.
Common Mistakes
- Reusing old flashing with a new unit. The shapes do not match and the old metal is fatigued.
- Ordering the skylight after tear-off. The roof stays open or gets a temporary patch while the unit is delivered.
- Ignoring rotted framing. New flashing over rotten wood fails again quickly.
- Shingling before the flashing. The flashing then has to be forced under finished shingles and gets damaged.
- Leaving the light shaft uninsulated. Condensation stains continue and get blamed on the new install.
- Changing the opening size without a plan. A larger opening needs headers and possibly a permit.
When to Call a Professional
A skylight swap is part of a roof replacement, and the flashing is timed with the shingles, so it is usually best handled by the roofer doing the job. Get the skylight and kit ordered before tear-off starts.
If the opening needs to change size, the framing is damaged, or the roof is a truss roof, bring in a roofer and, where needed, an engineer before any cutting is done.
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 Replace a Skylight During a Reroof: Questions
Should I replace my skylight when I replace my roof?
Usually yes. Skylights are as old as the roof, their seals and flashing wear out at a similar rate, and the roof is already open. Replacing the skylight later means cutting into the new shingles. The extra cost during a reroof is mainly the unit and kit, since access and labour are already covered.
Can I reuse my old skylight with a new roof?
Sometimes, if the frame, glazing and seals are still in good shape. The flashing must be replaced with a new kit, and the seals are checked. Fogged glass, a cracked dome or a history of leaks means replacement is the better choice, because a leaking unit under a new roof is hard to fix later.
How do I know what size replacement skylight to order?
Find the model number on the label on the frame or light shaft, then give it to a skylight supplier. Many units have direct replacements that fit the same rough opening, so no framing changes are needed. If the label is missing, measure the rough opening in the framing carefully rather than the visible glass size.
Does the skylight go in before or after the shingles?
The skylight and its flashing go in during the shingle installation. Underlayment and membrane go on first, then the unit is set, then the flashing is added bottom up as the shingles go on around it. The flashing is woven into the shingle courses, so it cannot be added properly after the roof is done.
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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