Skylights & Roof Windows - Types of Roof Windows
A roof window (skylight) is a neat feature that adds a dramatic element of light to a room. The actual window/glass part of a roof window is typically bubble-shaped or flat glass with a rectangular frame. Roof windows are also classified by the shape of their light shaft framing.
Shaft: A straight corridor that funnels light into a narrow area. A shaft is needed when the roof line is more than a foot or so away from the ceiling.
Flared shaft: A bell-shaped, obtuse shaft that widens, allowing more area to be lighted.
Cathedral: Most common type. Ceiling is close to the roof line, thereby eliminating the need for a shaft.
The process of installing a roof window is much like installing a wall window, with a few added steps. Like most windows, the majority of the work involves framing the opening—two openings when framing a shaft.
It's usually less hassle to start installation by creating the interior opening first. This is done by marking the roof window opening on the ceiling or unfinished framing.
If you have to cut through into the attic, check for wiring that might be in the way and remove any insulation in that area. Cut the opening and remove the drywall or ceiling material.
Reinforce the outer ceiling joists with same-sized lumber pieces screwed on each side extending to the next header. In some cases this may be the entire ceiling length.
NOTE: Screws are suggested here because hammering may crack existing drywall seams.
Remove the ceiling joists that occupy the interior opening, cutting back to accommodate the headers (minimum double-headers = at least 3") on each side. Install the appropriate-sized headers. If you're not sure, contact a building inspector. But there should be at least two pieces of lumber on each side.
Recess header the thickness of the ceiling drywall. Install trimmer studs between the headers on each side of the opening at the proper width.
Later, you'll need to frame between the ceiling opening and the roof window frame in order to support the window and hang the drywall. For that reason, we'll frame the window on the roof next instead of finishing out the shaft.
NOTE: Some roof windows set in the frame and attach with brackets, while better designed windows have a flange that allows them to set on the roof over the frame.
Using a carbide-tip blade in a circular saw, carefully cut the opening and finish the corner cuts with a hand or reciprocating saw.
WARNING: Because the blade will gum up with tar and may kick back, some people prefer to cut through the shingles and roofing with a reciprocating saw. However, with this method it's more difficult to cut a straight line.
Pry off the cut piece and, returning to the attic, cut the exposed rafters in the opening, again, allowing for the headers on each side (usually 3"). Now, install the headers and trimmer studs for the correct height and width of the roof opening.
Back on the roof, remove the shingles surrounding the opening to allow for waterproofing measures (i.e. flashing, tar paper). Don't skimp here, roof windows are vulnerable to leaking if not installed properly.
Before fastening the window to the roof framing, consider putting down ice and water protection, in the roll form, or more messy but less expensive, roofing tar. Anchor the window according to the manufacturer's directions.
NOTE: Nailing/screwing the flashing in the incorrect place may cause water to leak in.
As an added measure, put down ice and water protection again, this time over the window flange. Add roofing felt over the protection and caulk where it butts up against the window.
Start at the bottom of the window with the step flashing, install a shingle, then flashing, etc. Overlap the flashing as you work toward the top. The manufacturer may include additional waterproofing channel flashing or framing.
With the window installed, the shaft is all that's left to frame. It helps support the window and provides framing for the drywall.
Reinforce under the four corners and support the side framing every 16" by beveling or notching 2x4s or 2x6s, nailing them to the headers and studs.
After toenailing the studs in, insulate the cavity and install a vapor barrier. That completes the roof window installation. Now the shaft is ready for drywall.