Curb Mounted skylights
The curb, in the case of curb-mounted skylights, is a raised site-built frame to which skylights are attached to. The curb raises the skylight above the roof line.
Curb-mounted skylights require you to build a curb (typically a wood curb) and to attach it to the roof deck around the roof opening.
The skylight only includes a frame with the insulated glass panes and, in many cases, rubber gaskets and a flashing system.
You have to install the curb around the rough opening, and to water-proof it by using self-adhesive, mechanically fastened underlayment or other type of waterproofing system. The skylight is then mounted on the curb, and flashed.
The curb should be thoroughly wrapped with mechanically fastened or self-adhesive underlayment, before installing the flashing, to protect it from rainwater and elements.
A deck-mounted skylight is directly attached to the roof deck.
It comes with a built-in frame/curb, and you need to seal i, using waterproof materials (underlayments). A flashing kit is also used to cover the seam and the underlayment, to make the installation water-tight.
The flashing system varies with brands.
Curb-mounted vs. deck-mounted skylights
Deck-mounted skylights with their engineered curbs and their matching cladding and flashing kits are easier to install. You do not need to build and watertight the curb.
But they come in a limited number of standard sizes, and may not fit to existing roof holes.
Installing curb-mounted and deck-mounted skylights
Before even considering the type of skylight, you should assess your roof system and truss spacing.
Since you will have to build a light shaft, you should consider the possible need of reinforcement or modifications in the roof system, and to select the exact location for the skylight, by taking into account the room you want to light.
You must select the most advantageous spot between the ceiling joists and roof rafters; and to the effect, you need to consider electrical wiring, ducts or other obstructions…
Be also aware to the roof slope. Installing a skylight in a flat roof requires special care and especially designed skylights, to avoid water problems and to get adequate illumination.
A low-slope means less solar heat (and natural light) in winter, and more in summer, which is the opposite of the goals of any homeowner.
Metal flashing systems
Whatever the type of skylight - deck or curb-mounted - flashing and underlayment are critical for water-tightness and air sealing.
The metal flashing system offered by Velux is a reference for quality skylight flashing for both deck and curb-mounted skylights (its basics haven't changed much since the middle of the last century). Manufacturers like Wasco, Roto, or Andersen… do not use metal flashing systems.
It consists of a metal head flashing for the top of the skylight, sill flashing for the lower end of it and step flashing for the two sides. The flashing is supplemented by underlayment and cladding on the top, to seal the sash.
This design doesn't require mastic, caulks, cement or tar, though it does require a waterproof membrane (underlayment) - attached to the roof and flowing right up the side of the curb (of curb-mounted skylights), protected from the UV radiation by the flashing.
Mechanically fastened underlayments and self-adhesive underlayments should be properly installed around the skylights (or the site-build curb). They are crucial for water-tightness and also for air-tightness.
Though the basic design is the same, Velux has a wide variety of choices for different types of roof coverings (shingles, asphalt, Spanish tile…) and different skylights (curb-mounted, deck-mounted, sun tunnels...).
See here, for details on the Velux flashing systems: Skylight and flashing installation videos.
Flashing, Underlayments and insulation
Skylight flashing is critical for water-tightness. But the wrapping of the curb of the skylight (or the wrapping of the skylight in deck-mounted skylights) with underlayment is also critical for water protection and air sealing. And the same can be said of insulation.
Insulation is often forgotten or minimized in their importance. But it shouldn't.
Whatever the skylight - curb-mounted, deck-mounted or even solar tube skylighg - the rough opening should be carefully sealed and the light shaft properly and thoroughly insulated to high levels, to avoid energy loss in winter and unwanted heat gains during the summer (see image at left).
Consider very high levels of insulation all over the walls of the light shaft. That's important for energy-efficiency and to minimize thermal bridging (heat loss and unwanted heat gains through framing). There should be no gaps in the insulation layers covering the shaft; consider a continuous insulation layer all over its walls up to the curb/frame of the skylight.
Problems with skylights
Velux skylight glass options
Indoor and outdoor skylight covers
Leading skylight manufacturers and brands
Plastic aerogel acrylic skylights
Solar tubes vs. glass skylights
Skylights for cold and hot climates