What is a skylight curb?

A skylight curb is the raised box or frame that sits on the roof deck and supports a skylight. The skylight fits onto the top of the curb, and the roofing material and flashing are worked up around its sides. Most curbs are built from framed lumber sheathed in plywood or OSB, though many installers now use prefabricated curbs made from insulated foam, plastic, or metal that arrive ready to set over the rough opening.

The curb does three jobs. It lifts the skylight above the surrounding shingles or tile so water runs off instead of pooling against the glass or frame. It gives the unit a flat, level seat so the glazing stays square and the seals stay tight. And it creates a clean vertical surface for step flashing, counterflashing, or a manufacturer's flashing kit to tie into. Without a properly built curb, the joint between the skylight and the roof is the most likely place for leaks to start.

On a flat or low-slope roof, the curb height matters even more because water has less natural fall to carry it away. On a pitched roof in a humid, storm-prone area like Citrus County, the curb also has to handle wind-driven rain and the expansion and contraction of roofing materials under Florida sun. Installers often check whether an existing curb is solid, correctly sized, and insulated before reusing it with a new skylight.

Curbs are sometimes called skylight wells or light wells when the term refers to the shaft below the opening, but the curb itself is the part at roof level. For a homeowner, the curb is one of the first things to look at when a skylight is being replaced or repaired, since a worn or poorly flashed curb can cause damage to the ceiling and framing below. Local skylight installation providers can tell you whether an existing curb is worth keeping or needs to be rebuilt.

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