Rotted Skylight Curb: What It Means and How It Is Fixed
| # | Table of Contents |
|---|---|
| 1 | What a Skylight Curb Is |
| 2 | Why Skylight Curbs Rot |
| 3 | Signs the Curb Is Rotting |
| 4 | How a Rotted Curb Is Repaired |
| 5 | Rebuild the Curb or Replace the Skylight? |
| 6 | FAQ |
What a Skylight Curb Is
Skylights mount one of two ways. A deck-mounted skylight attaches directly to the roof deck. A curb-mounted skylight sits on top of a raised frame, the curb, which is built from lumber around the roof opening and flashed into the roofing. Older homes often have site-built curbs with a skylight or plastic dome fastened on top.
The curb lifts the skylight above the water running down the roof. The building code requires one on flatter roofs: under the 2024 International Residential Code, unit skylights on a roof with a pitch of less than 3 in 12 "shall be mounted on a curb extending not less than 4 inches (102 mm) above the plane of the roof, unless otherwise specified in the manufacturer's installation instructions" (R324.6.8). VELUX's instructions for its FCM curb-mounted skylight call for curb lumber 1 1/2 inches thick and at least 3 1/2 inches high.
Why Skylight Curbs Rot
A curb is bare framing lumber sitting in the path of roof water. It only stays dry if everything around it is detailed correctly:
- No underlayment wrap. VELUX's instructions say: "To ensure waterproofing, skylight curbs shall be completely wrapped in VELUX adhesive skylight underlayment as indicated. Underlayment must be applied before installing the flashing." Many older site-built curbs were only flashed, with nothing behind the flashing to catch water that got past it.
- Failed or improvised flashing. The code requires flashing around roof openings (IRC R903.2.1). Curbs flashed with roofing cement and scraps of metal instead of a matched flashing kit tend to open up as the roof heats and cools.
- A low curb on a low slope. On a shallow roof, a curb shorter than the code or manufacturer minimum lets water and debris build up against the uphill side.
- Debris on the uphill side. Leaves and granules collect against the top of the curb and hold moisture against the flashing.
- Condensation from inside. Warm, humid indoor air can condense on a cold skylight frame and run down onto the curb. VELUX warns that on low-slope installs, "any condensation that forms on the glass due to high humidity may drip," and its instructions require any vapor barrier in the roof to be "extended and connected to the skylight curb."
Signs the Curb Is Rotting
| Where you look | What you might see |
|---|---|
| Inside the skylight shaft | Water stains, peeling paint or bubbling drywall near the top of the shaft |
| The frame or trim around the skylight | Dark or soft wood, mold spots, rust on fasteners |
| On the roof | Shingles sagging or curling next to the curb, cracked sealant, loose or bent flashing |
| The skylight itself | A unit that moves when pushed, or gaps between the skylight and curb |
| In the attic | Stained or soft framing around the opening |
A screwdriver that sinks easily into the curb wood is a reliable sign the rot is more than surface deep.
How a Rotted Curb Is Repaired
- Remove the skylight and flashing and the shingles around the curb.
- Cut out rotted curb lumber and check the roof deck and framing around the opening; soft decking around a curb is common and should be replaced too.
- Rebuild the curb to the skylight manufacturer's dimensions and at least the code height on low-slope roofs.
- Wrap the curb in self-adhered underlayment before any flashing goes on, tied into the roof underlayment.
- Install the flashing kit made for that skylight and the roofing material, then reinstall shingles around it.
- Connect any interior vapor barrier to the curb, and finish the shaft so humid air is not reaching the cold frame.
Rebuild the Curb or Replace the Skylight?
Once the skylight and flashing are off, the cost of putting an old unit back is close to the cost of a new one. Flashing kits are made for specific skylight models, and VELUX states that failure to install the skylight and flashing "in strict accordance with these VELUX instructions shall void the VELUX warranty." Its "No Leak" installation warranty applies only with its matching flashing on roofs within the stated pitch range. If the existing skylight is old, cloudy or an obsolete size, replacing it while the curb is open is usually the better value.
If your roof is due for replacement soon, plan the curb rebuild with the new roof so the skylight is flashed into fresh underlayment. Our guide to what to know before installing a skylight covers placement and sizing, and our roof repair team can open up a soft curb, check the deck and rebuild it.
Sources: International Residential Code (2024) R324.6.8 and R903.2.1; VELUX FCM Curb Mounted Skylight Installation Instructions (VAS 452846-2021-10).
Frequently Asked Questions
What is a skylight curb?
It is the raised wooden frame built on the roof around the opening that a curb-mounted skylight sits on. It lifts the skylight above roof water and is flashed into the roofing.
Why is the wood around my skylight rotting?
Usually because water is reaching the curb: the curb was not wrapped in underlayment, the flashing has failed, debris holds water on the uphill side, or indoor humidity condenses on the frame and runs onto the wood.
How tall does a skylight curb need to be?
The 2024 IRC requires at least 4 inches above the roof on roofs flatter than 3 in 12, unless the manufacturer specifies otherwise. VELUX’s FCM instructions call for at least 3 1/2 inches of curb height.
Can a rotted skylight curb be repaired without replacing the skylight?
Sometimes, if the skylight is in good condition and its flashing kit is still available. Often the old unit is replaced because the flashing must be redone anyway and kits are model-specific.
Is skylight curb rot covered by insurance?
Gradual rot from long-term moisture is often treated differently from sudden storm damage. Coverage depends on your policy, so check with your insurer and keep photos of what is found when the curb is opened.
