What does a skylight curb do?

It raises the skylight above the roof surface and creates the mounting base the skylight attaches to. That height helps manage drainage and allows proper flashing around the opening so water flows past without pooling against the skylight frame.

Does every skylight have a curb?

No. Deck-mounted skylights attach more directly to the roof sheathing and framing without a raised frame, while curb-mounted skylights sit on a raised curb. The installation type depends on the skylight design, roof slope, and roofing material.

How do I know if a skylight curb is bad?

Common signs are chronic leaks around the skylight, soft or rotted wood visible from the roof or attic side, condensation stains on the shaft interior, and poor or deteriorated flashing around the skylight base. A new skylight will not solve those problems if the curb itself is failing.

Can a roofer rebuild a skylight curb without replacing the skylight?

Sometimes, but it depends on the age and dimensions of the existing unit. The skylight must be carefully removed, the curb rebuilt, and the unit reinstalled. If the skylight is already near the end of its life, rebuilding the curb and replacing the unit together is usually the cleaner and more cost-effective repair.

Do you need a permit to replace a skylight curb?

Often yes, especially when the work changes roof framing, flashing, or the skylight opening dimensions. Permit rules vary locally, but curb replacement is usually treated as roof work rather than a minor interior repair, and an inspection may be required to verify flashing and structural support.

Skylight Curb - Raised Base for Curb-Mounted Skylights

A skylight curb is a raised framed box that projects above the roof surface and provides a mounting base for a curb-mounted skylight.

Skylight Curb diagram — labeled parts, dimensions, and installation context

What It Is

A skylight curb lifts the skylight above the roof plane so water can shed around the opening and the skylight can be fastened to a stable, level perimeter. On low-slope roofs, the curb is a critical part of keeping the installation weather-tight because the shallow pitch does not move water away from the opening as quickly as a steep roof would.

The curb itself is separate from the skylight unit. It is typically constructed from 2x6 or 2x8 lumber, built to the dimensions specified by the skylight manufacturer, and fastened to the roof structure through the sheathing into the rafters or blocking around the opening. Once installed, the curb is flashed into the roofing membrane or shingle system and insulated on the interior side to prevent condensation and heat loss.

Curb height matters for drainage. Most manufacturers and roofing standards call for a minimum curb height of 8 inches above the finished roof surface, though some specify more on very low slopes. A taller curb gives water and debris more clearance to move past the skylight without pooling against the frame.

Types

Site-built curbs are the most common type, framed from lumber on the job and flashed by the roofer as part of the roof installation. These allow flexibility in sizing and height, and they can be customized for unusual roof conditions or non-standard skylight dimensions.

Prefabricated metal curbs are available from skylight and roofing manufacturers. These are typically galvanized steel or aluminum frames with pre-formed flanges for easier flashing integration. Metal curbs are common on commercial flat roofs where standardized curb sizes match catalog skylight units. Some include built-in insulation or condensation gutters.

Insulated curb adapters are aftermarket frames designed to retrofit a new skylight onto an existing curb or to upgrade thermal performance. These adapters reduce heat loss through the curb wall and minimize condensation in cold climates.

Where It Is Used

Skylight curbs are used on curb-mounted skylights, especially on low-slope and commercial roofs. They are also found on some residential roofs where the skylight design or roof condition favors a raised mount over a deck-mounted unit. Curb-mounted installations are standard on roofs with a slope of 3:12 or less, where the additional height of the curb is essential for drainage.

On steeper residential roofs, deck-mounted skylights are more common because the steep pitch naturally sheds water and the lower profile looks cleaner from the ground. However, curb-mounted units are sometimes preferred even on steeper roofs when the skylight is very large or when the building owner wants the ability to swap the skylight without reworking the flashing.

How to Identify One

From the roof, look for a raised box-like frame under the skylight. The curb will project several inches above the surrounding roof surface and the skylight will sit on top of it rather than being recessed into the roof plane. Flashing will wrap from the roof surface up the sides of the curb.

From inside, the curb may correspond to the perimeter of the skylight shaft opening, but the curb itself is part of the roof-side assembly and is not always visible from the interior once the shaft is finished. Look for signs of moisture staining, soft wood, or condensation drips around the interior perimeter as indicators that the curb may be compromised.

Replacement

Replacement is needed when the curb is rotted, out of square, undersized, improperly flashed, or incompatible with a new skylight. Replacing a skylight without addressing a failed curb often leads to repeat leaks, so this is roofing work rather than a simple glazing swap. The roofer will need to strip the flashing, remove the old curb, inspect the roof sheathing and framing, build a new curb to the correct dimensions, and reflash the installation.

If the skylight is also being replaced, coordinate the curb dimensions with the new unit before construction begins. A mismatch between curb size and skylight size is one of the most common causes of callbacks on skylight installations.