When you get a skylight estimate, one of the first things we look at is how the existing unit is mounted. It isn’t something most homeowners think about. But it affects how the skylight sheds water, which roofs it suits, and especially what happens years from now when it has to be replaced.
There are two basic ways to mount a skylight: on a curb, or directly on the roof deck. Here’s what each one means.
What a curb is
A curb is a short box frame built around the opening in your roof. It’s usually made from 2×4 or 2×6 lumber fastened to the roof deck, and it’s waterproofed with underlayment and flashing that tie into the surrounding roof. The skylight then sits on top of the curb, like a lid on a shoebox, and is fastened in place.
So a curb-mounted skylight is really two pieces: the curb, which is part of the roof, and the skylight, which sits on it.
What a deck-mounted skylight is
A deck-mounted skylight attaches directly to the roof deck without a separately built wood curb. Many deck-mounted units are self-flashing, meaning they have a built-in metal curb or flange that’s designed to be flashed straight into the roofing around it.
Deck-mounted skylights sit lower and have a sleeker look on a pitched roof. The tradeoff is that the skylight and the roofing around it are tied together much more closely.
If you’re looking at VELUX models, the product names tell you which is which. VELUX uses “curb mount” in the names of its curb-mounted units, like the FCM fixed curb-mount skylight, and “deck mount” for units that install directly to the deck.
Which roofs suit which
Flat and low-slope roofs: curbs
On a flat or low-slope roof, a curb isn’t optional. Water drains slowly on a low-slope roof, and a skylight sitting flush with the surface would be sitting in standing water. The curb raises the skylight above the roof so water runs around it instead of against it. California’s residential code requires skylights on roofs flatter than 3:12 to sit on a curb at least 4 inches above the roof, unless the manufacturer’s instructions say otherwise.
This is the reason nearly every skylight on a flat commercial roof is curb-mounted, and why curbs are central to our commercial skylight work. It also applies to plenty of LA homes. Flat-roofed mid-century and modern homes have curb-mounted skylights for the same reason.
Pitched roofs: either can work
On a sloped roof with shingle or tile, both methods work. Water runs off a pitched roof quickly, so a deck-mounted unit can sit lower without trouble. A curb-mounted unit is just as common, and it sits a little higher.
Why it matters most at replacement time
Here’s the part that most often surprises homeowners. The mounting type doesn’t affect day-to-day use much, but it makes a big difference when the skylight eventually needs replacing.
Curb-mounted skylights are usually the simpler replacement. The curb is part of the roof and is often still sound after the skylight itself has failed. We remove the old unit, check the curb, and set the new skylight on it. That’s the most common job we do. A standard-size replacement of a 2×2 dome usually runs $800 to $900, and replacing a 2×4 VELUX glass unit typically runs $900 to $1,100, or less with a Solar Industries unit, because it’s one trade and usually one visit. More on that in skylight replacement.
Deck-mounted and self-flashing skylights are more involved to replace. The unit is flashed directly into the roofing, so taking it out means working on the roofing around it: lifting shingles or tile, re-flashing, and resealing. That’s more labor, sometimes another trade, and higher cost than a curb-mounted swap.
Neither one is wrong. But if you’re installing a new skylight and expect to own the house for a long time, it’s worth knowing what the next replacement will involve.
Leaks: where each one tends to fail
Most skylight leaks aren’t the glass. They’re at the flashing where the skylight meets the roof. We go into the common causes in why skylights leak.
With a curb-mounted skylight, the usual weak spots are flashing around the base of the curb, and on flat roofs, the joint where the roofing membrane turns up the curb. A curb that’s too short for a low-slope roof can also let water reach the top edge.
With a deck-mounted skylight, a leak usually means the flashing between the unit and the roof has failed. Because that flashing is woven into the roofing, the repair often means opening the roofing around the unit.
If your skylight is leaking, the skylight repair page explains how we tell whether it’s the skylight, the flashing, or the roof itself. If the problem is the roof, we’ll tell you, and that’s a job for your roofer.
Can you switch from one to the other?
Sometimes. When an old acrylic dome on a curb gets replaced with flat glass, the curb is often reused, though a flat glass unit on a low-slope roof may need the curb pitched so water runs off instead of pooling. Going from a deck-mounted unit to a curb, or the other way around, is a bigger change that involves the roof around the opening. We’ll tell you on the visit whether it makes sense for your roof.
For old domes specifically, our dome skylight replacement page covers the options.
What about new installations?
For a brand-new skylight, we cut the opening, build the curb, set and flash the unit, and finish the light shaft inside. The roof type usually decides the mounting method. Flat and low-slope roofs get a curb. On pitched roofs, we’ll talk through the look you want and what future replacement will involve. More in new skylight installation.
Quick reference
- Curb-mounted: The skylight sits on a raised wood box built into the roof. Required on flat and low-slope roofs, and common on pitched roofs. Usually the simpler, cheaper replacement.
- Deck-mounted or self-flashing: The skylight attaches directly to the roof deck and is flashed into the roofing. Lower profile, for pitched roofs. Replacement involves roofing work and costs more.
More terms you might hear during an estimate are in our skylight glossary.
Not sure what you have?
Most people don’t, and that’s fine. We’ll identify it on the visit. Hollywood Skylights has been in business for 12 years, with more than 20 years of experience in skylight replacement, repair, and installation across Los Angeles, from the Hollywood Hills to the San Fernando Valley and Santa Monica.
Call us at (213) 334-3947 or request a free estimate.