How to Seal a Flat Skylight Window

What You'll Need
Roofing cement

By sealing a flat skylight window, you can eliminate the possibility of it leaking water. You won’t need much in the way of materials or specialized skill but you will need a good head for heights. If you’re uncomfortable with heights, you are best advised to hire someone to do the work for you. The more thorough you are, the less the chance of a leak developing. As long as you seal the area well, you should have good service from the skylight for many years.

Step 1 - Inspect the Window

Before sealing the window, you need to inspect the window itself as well as the flashing and the roof. With a skylight window that opens, proper installation should mean a good seal on the window itself. Open it and inspect the seals then close to make sure the window is completely square in the frame. If it isn't, you will experience leaks from the window and you will need to replace the entire skylight in time.

There should be no holes in the flashing, and the corners should be secure and without gaps. If the flashing has been improperly installed, you will need to replace it.

If the skylight window has been placed on a roof with a low pitch, it will be more susceptible to water pooling and leaks which can cause ongoing problems in prolonged wet weather. Make sure that the roof is clear of any obstructions that could cause an ice dam in winter as this could also lead to leaks. Make sure that there are no loose or missing shingles. If there are, secure and replace them.

Step 2 - Sealing Flashing

To avoid leaks in the flashing, start by raising the shingles all around the window. Where the flashing joins the roof, use the trowel to spread roofing cement in all directions around the window to a depth of ¼ inch. This will form a good seal on the flashing. If you discover holes in the flashing, spread roofing cement on them and also over any seams in the flashing.

Step 3 - Shingles

To seal more of the area around the skylight window, spread roofing cement on the underside of the shingles you’ve raised around the skylight window. Push the shingles back down, especially where the flashing and shingles meet. Now, spread more of the roofing cement over the surrounding shingles. The wider you spread the area of roofing cement, the less chance of leakage later from the skylight window and the better your seal.

You should pay special attention to the area of the roof above the skylight window. This is where water will flow down from the peak of the roof. Be very generous in applying roofing cement all around the skylight. Remember that even tiny holes can lead to leaks that can cause major damage in your house so be very thorough. It will definitely pay off later.