How to Flash a Skylight: Kits, Step Flashing, Head and Sill Pieces
Bottom up, each piece over the one below, with a membrane collar underneath as the backup.
- Advanced
- 3 to 6 hours
- Updated
The short answer
Flash a skylight with the manufacturer's kit, installed from the bottom up: a self-adhered membrane collar first, then the sill flashing, the side flashing and finally the head flashing, each lapped over the one below it. Sides are either woven step flashing or continuous sidewall pieces depending on the kit and roof. Shingles lap over the flashing, and a cricket or diverter protects the head on wide units.
Skylight flashing is the part of the job that decides whether the skylight leaks. The glass is sealed at the factory. The flashing is built on the roof, by hand, to a detail that has to shed water from every direction, and it shares the roof with shingles that also have to lap correctly. Most skylight leaks start at a missing piece, a reversed lap or a nail in the wrong place, not at the glass.
The best approach is always the manufacturer's flashing kit for that exact skylight and that roofing type. Kits are tested with the unit, come with instructions, and are often required to keep the warranty. Custom flashing is possible, but it should follow the same principles: water sheds over every joint, and the upper layer always laps the lower.
This guide covers the flashing itself on a pitched shingle roof. For the whole installation see how to install a deck-mounted skylight, and for the curb-mounted equivalent see how to install a curb-mounted skylight.
Tools
- Hook blade utility knife
- Tin snips
- Hand seamer
- Roofing hammer or coil nailer
- Tape measure
- Chalk line
- Caulking gun
Materials
- Manufacturer's flashing kit matched to the exact skylight model and roofing type
- Self-adhered ice and water membrane
- Roofing nails, 1-1/4 inch
- Shingles to match
- Sealant specified by the kit, used only where the instructions call for it
- Plywood and lumber for a cricket, if the unit needs one
Safety first
- Work tied off. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and where a fall could cause serious injury. The area around a skylight is slippery with membrane and loose shingle scrap.
- Metal flashing edges cut. Wear cut resistant gloves when handling and trimming pieces.
- Stay off the skylight glass and dome and keep tools from falling onto it.
Step by Step
Step 1: Read the kit instructions and check the parts
Kits differ for shingle, tile, metal and flat roofing. Check the parts: sill piece, side pieces, head piece, corner pieces or boots, sealing strips and fasteners. A missing piece found on the roof wastes a trip.
Read the instructions through once before starting. Note the required shingle clearances, the nail locations and any sealant called for, because these differ between manufacturers.
Step 2: Install a self-adhered membrane collar around the frame
Clean and dry the deck and frame. Cut membrane strips long enough to extend several inches beyond each corner. Install the sill strip first, onto the deck and up the frame, then the side strips lapping over the sill strip, then the head strip over the top.
This collar is your backup layer. If any flashing piece is ever breached, the water meets membrane and runs out over it. See how to install ice and water shield. Press it firmly into the corners with a hand roller or a dry cloth so there are no bridged air pockets.
Step 3: Install the sill flashing
The sill piece at the bottom carries all the water that runs down across the skylight glass and frame. It goes over the shingles that run up to the skylight from below, never under them, so water is delivered onto the roof surface.
Fasten it only where the kit shows, usually along its upper edge or the roof leg, and never through the part that water runs across. Shingle courses below are cut back to clear the flashing as the instructions direct.
Step 4: Install the side flashing
Two systems are common. Step-style side flashing uses individual pieces woven into the shingle courses exactly like step flashing: one piece per course, one nail each, each covered by the next shingle. This is the most widely used on shingle roofs because every course sheds its own water.
Continuous sidewall pieces are a long single strip per side with a sealed, shaped design, often used on metal or tile roofs or in kits made for that profile. Do not substitute one for the other unless the kit supports it. Whichever system the kit uses, each side piece laps over the sill flashing at the bottom.
Step 5: Install the head flashing
The head piece is last on because it lies on top of the side pieces at the corners but tucks under the shingles above, so water coming down the roof from above flows over it and onto the side flashing. Getting this lap right is the whole purpose of the bottom-up sequence.
The shingles above the skylight are lifted or cut back so the head flashing slides under them. Nail it only along its upper edge, in the pattern shown, where the shingle above will cover the nails.
Step 6: Add a cricket above wide units
A wide skylight interrupts a large slice of the roof and dams the water running down toward it. A small peaked cricket above it splits that flow to either side, as with a chimney cricket. Many skylight manufacturers state a width over which a cricket is recommended, so check the instructions.
Under big fir and cedar trees in the Fraser Valley, needles and debris pile up at the head of skylights and dam water. A cricket helps, and so does keeping the area clear.
Step 7: Shingle over the flashing and leave correct gaps
Shingles go on top of the flashing legs, never under them on the lower and side sections. Cut them to clear the frame, not to touch it, because a tight fit wicks water. Keep nails out of the flashing channel and at least the distance shown from the frame.
Follow any sealant instruction exactly. Some kits call for a small bead under specific shingle tabs, but sealant is a supplement to laps, not a substitute. See roofing sealants.
Step 8: Inspect every lap and test with water
Look at each corner, where water collects and pieces overlap. Check that no nail sits in the open, that no piece is reversed and that shingles are not pressing on the frame.
Run a hose on the roof above the skylight, working from the lower edge up, for several minutes per section. If you see water inside, stop and find the source before continuing. If unsure, see how to fix a leaking skylight.
Kit Versus Custom Flashing
A factory kit is designed and tested with the skylight, and using it is usually a condition of the skylight warranty. Custom flashing made from sheet metal is sometimes needed for odd locations, old skylights or roofs with unusual profiles. It must follow the same lapping logic, be made from compatible metal and use a proper membrane collar underneath.
Never mix metals in a way that causes corrosion, and never use sealant to replace a lap.
Step Flashing Versus Continuous Sidewall Flashing
- Step-style pieces: one per shingle course, woven in. Best for shingles. Every course drains its own water.
- Continuous sidewall pieces: long single strips, used with kits made for metal, tile or other profiles. Depend on their sealed edges and laps.
- Corner pieces and boots: moulded or folded corners at the bottom corners of the skylight, where most leaks start.
Common Mistakes
- Installing the head flashing first. Water then runs behind the side pieces instead of over them.
- Leaving out the membrane collar. Flashing alone has no backup if it is breached.
- Nailing through the flashing channel. Each nail is a hole where water runs.
- Replacing lapped metal with caulking. Sealant cracks and fails within a few seasons.
- Reusing the old flashing on a new skylight. It will not match the new frame and is often corroded or bent.
- Shingles jammed hard against the frame. They wick water and trap debris.
When to Call a Professional
If the skylight is on a steep or high roof, is a large or multi-unit installation, or the flashing has to be fabricated because no kit fits, a roofer should do it. Flashing is the detail that depends most on practice and on seeing how the existing shingles lie.
Also call a roofer if previous flashing work has failed more than once. The fault is often in the roof around the skylight, not just the skylight.
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 Flash a Skylight: Questions
Do skylights come with flashing kits?
Most skylight manufacturers supply or sell a matched flashing kit for each model and roofing type, such as shingle, tile or metal. Some units include it, others need it ordered separately. Using the matched kit is normally required to keep the warranty, so confirm what is included before you buy, and do not substitute generic flashing.
What goes first when flashing a skylight?
Work from the bottom up. First a self-adhered membrane collar, then the sill flashing at the bottom, then the side flashing, and last the head flashing at the top. Each upper piece laps over the lower one so water sheds across every joint. Reversing the order drives water behind the metal.
Do you use step flashing around a skylight?
Often yes. Many shingle roof kits use individual side pieces woven into the shingle courses, exactly like step flashing along a wall. Other kits use continuous sidewall pieces, particularly for metal and tile roofs. Use whichever the skylight's flashing kit specifies, and do not substitute one style for the other.
Can I use roofing cement to flash a skylight?
Only as a supplement where the kit says so. Roofing cement and caulking crack, shrink and peel after a few seasons of sun, rain and cold. Lapped metal and membrane do the real work of shedding water. A skylight sealed mainly with sealant will leak sooner or later.
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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