Making a Skylight Leakproof
Updated: Feb. 16, 2023
Use special flashing and these five steps
For a leakproof skylight use a special flashing kit and installation techniques we show here.
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Leakproof skylight flashing
Photo 1: Apply self-sticking underlayment
Install 6-in.-wide strips of a self-sticking waterproof underlayment. This underlayment will direct any water that may get through the metal flashing onto the roofing felt. Install from the bottom up, lapping each piece on top of the lower one.
Photo 2: Set the lower flashing
Set the bottom one-piece flashing on top of the shingle course. Nail it only at the upper corners; don’t nail it into the roof.
Photo 3: Add step flashing
Weave the step flashing up the side of the skylight as you install each shingle course. Nail at the upper corner.
Photo 4: Set the top flashing
The top (head) flashing is one piece that wraps around the sides. This fabricated piece has soldered and sealed seams and corners. Fit and install the next row of shingles, leaving about 1/2 in. of flashing exposed.
If you have to replace shingles around a skylight, examine the old flashing. Most residential skylights sit on a raised curb made watertight with metal flashing. If it’s carefully removed when the old roof is torn off, it should be reusable. If not, you’ll have to buy new flashing ($50 to $100) either from the skylight manufacturer or from a roofing company that has a metal fabrication shop (look under “Roofing” in the Yellow Pages or online).
First remove the skylight’s counterflashing. On some units, you have to remove the whole glass frame (lead photo below). On other models, you simply unscrew a metal channel from the curb. Next pry off the old shingles, carefully pulling nails to save the flashing. To reassemble, follow the steps as outlined by the photos, and you’ll have a leakproof installation. Note that the metal flashing pieces don’t need to be sealed to each other. Since water runs downhill, they only need to be installed in the right order. So put away that roofing cement.
If you’re hiring a roofing crew, ask them to carefully remove the old flashing and reuse it if it’s in good shape.
Prevent Ice Dams
Correctly Flashed Skylight
This skylight shows the correct flashing technique to make it waterproof. Numbers 1 – 4 correspond to the 4 photos above. Number 5 shows how the final step, counterflashing, is integral to the skylight and covers the upper edges of the roof flashing.
Required Tools for this Project
Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.
Required Materials for this Project
Avoid last-minute shopping trips by having all your materials ready ahead of time. Here’s a list.
Originally Published: November 29, 2019