TYPES OF SKYLIGHTS

• Many people worry about cutting holes in the roof, but adding a skylight is actually easier than installing a window. Most skylights come with flashing systems that will seal the roof effectively. This brochure describes how to install a skylight and build a light well to carry the light into the room.
  
• There are two basic types of skylights: curb-mounted or frame in place. A curb-mounted skylight is raised above the plane of the roof; it either sets on a wood frame curb or the curb is an integral part of the unit (Fig. 1). A framed-in-place skylight is installed flush with the plane of the structure, much like a vertical window. It is held in place with L-shaped brackets (Fig. 2).
  
• Curb-mounted skylights may be glazed with a clear acrylic dome or with glass. Framed-in-place skylights are glazed with glass. Either may use insulating glass, and the glazing in top-of-the-line skylights may have low emissivity coatings and argon gas fill for added energy efficiency.
  
• The flashing system on a curb-mounted skylight typically consists of a head flashing, a sill flashing and two pieces of side flashing that run the length of the skylight (Fig. 3). The head flashing is slipped under the shingles above the opening. The side flashings are slipped under both the head flashing and the shingles on the side of the unit. The sill flashing goes under the side flashings but is set over the shingles below the skylight. The configuration allows water to run around and off the skylight.
  
• A framed-in-place skylight also has a solid head and sill flashing, but the side flashing consists of a series of step shingles that match the 5" exposure of each row of roofing (Fig. 3). Like a curb-mounted skylight, the head flashing goes under the shingles and the sill flashing goes under them. The step shingles are woven into the roofing, slipped under each shingle.
  
• Flashings may be made of galvanized steel or aluminum; most are aluminum, often finished to match the color of the skylight frame.

FRAMING THE ROUGH OPENING

• A skylight framing assembly consists of three parts (Fig. 4):
  
• The roof opening is framed with headers–framing lumber run horizontally across the opening, securely nailed to the rafters to support the structure. Headers are cut from lumber the same size as the rafters.
  
• A curb-mounted skylight also requires a framed curb, typically 2x6s but usually specified by the skylight manufacturer (the frame has to project far enough to accommodate the flashing). The curb is set on the roof sheathing around the perimeter of the opening, and the skylight is set on the curb.
  
• The ceiling opening is framed in the same way as the roof opening–headers are nailed between the ceiling joists to support the existing framing. Ceiling headers should be the same size as the ceiling joists.
  
• The light shaft is the framing that connects the roof opening with the ceiling opening. It is typically framed with 2x4s, insulated like any interior wall and finished on the inside with drywall or other material. The light shaft may be vertical or flared–wider at the bottom than the top. A flared light shaft is a little more difficult to frame but will admit more light.
  
• To frame the opening for a skylight, first decide on the approximate location of your skylight. It should be roughly centered in the room.
  
• Next, locate the ceiling joists, then realign the opening so it fits between the joists. Depending on the size of your skylight, you may or may not have to cut the ceiling joists and rafters; many skylights are made to fit between 16" or 24" on center joists and rafters without removing any framing.
  
• Mark the two corners of the ceiling opening closest to the outside wall. Take care to make sure the points are parallel to the wall. The width of the ceiling opening should match the rough opening width of the skylight (specified by the manufacturer). The length will vary, depending on the type of light shaft you want.
  
• Drill small holes through the ceiling at your marks and push two pieces of stiff wire up into the attic so you can easily spot the location from above.
  
• Go up to the attic; if possible, bring a 2' by 4' piece of plywood with you to lay across the ceiling joists so you won't accidentally put a foot through the ceiling. Locate your corners and clear the insulation away from the area. If you have to reroute electrical wiring, always turn the power off first and double check by testing a fixture on that circuit with an electrical tester.
  
• Use a plumb bob to locate the two points on the underside of the roof that are directly above the lower corners of the ceiling opening. Mark those points on the roof (Fig. 5). Double check to make sure they match the correct rough opening width, then measure up the underside of the roof the specified rough opening length. Drive four nails up through the roof to mark the corners of your rough opening.
  
• Go up to the roof and remove the shingles from around the opening, far enough to allow room to install the flashing. Snap chalk lines between the nails, then cut away the roofing felt with a utility knife.
  
• Set your circular saw so the blade depth is slightly more than the thickness of the roof sheathing, then cut away the sheathing and remove it (Fig. 6).
  
• How you frame the opening depends on the size and position of your skylight. If the skylight will fit between two existing rafters without cutting, simply cut two headers to fit between the rafters (they should be the same material as the rafters, typically 2x6s) and nail them in place with 16d common nails, flush with the cut edges of the sheathing (Fig. 7).
  
• If you have to cut a rafter, framing is a little more complex. First, nail two 2x4s across the rafters to provide temporary support (Fig. 8). The 2x4s should be long enough to reach at least two rafters on each side of the one that will be cut.
  
• Then use a reciprocating saw to cut the rafter out of the opening 3" back from the edge of the sheathing on each side. Cut four headers to span the distance between the uncut rafters on each side of the opening. Nail the first pair flush against the two ends of the cut rafter. Use three 16d common nails through the existing rafters into the headers on each end and three more nails through the header into the end of the cut rafter.
  
• Then nail the second pair of headers in place. Use 16d nails to nail through the existing rafters into the ends of the header and a pair of 8d nails every 16" to nail the two headers together.

INSTALLING THE SKYLIGHT

• Curb-Mounted Skylight–Build the curb by nailing four 2x6s into a box (the size is specified by the manufacturer). Square it, then toenail it into place over the opening with 8d galvanized nails. Run a bead of caulking all around the top of the curb (unless the manufacturer says not to), then set the skylight in place and fasten it down. Replace the shingles, leaving enough working space between the edge of the shingles and the curb so you can slip the flashing into place.
  
• Install the sill flashing first, according to the manufacturer's instructions. As a rule, flashings are nailed into the curb and sometimes cemented to the roof with plastic roof cement, but not nailed to the roof.
  
• Install the side flashing and fasten it in place. If you're working with step shingles, slip them under each row of roofing shingles, working your way up from the bottom. Slip the base of the head flashing under the shingles and set it in place over the top of the side flashing. Fasten it in place. Go back to the attic and remove any temporary supports.
  
• Framed-in-Place Skylight–Mount the brackets on the side of the skylight, set it in the hole and fasten it in place. Make sure the skylight is square and not twisted in the hole.
  
• Install the flashing as described above, or as specified by the manufacturer. Go back to the attic and remove any temporary supports.

BUILDING THE CEILING OPENING AND LIGHT SHAFT

• From inside the room, find the other two corners of the ceiling opening. Use a carpenter's square to make sure the opening is square or rectangular. Use a keyhole saw to cut away the drywall.
  
• Go back up to the attic. If you'll need to cut a ceiling joist, place temporary supports across the opening as described above, then cut the joist 3" back from the edge of the drywall. If the light shaft will be flared, cut the joist at the angle of the flare.
  
• Install headers across the ceiling opening as you did for the roof opening–a single header at each end if no ceiling joists are cut or double headers if a ceiling joist has been removed.
  
• Use a T-bevel to determine the angle of the rafters and ceiling joists, then cut 2x4 studs for the corners of the light shaft, angled on each end to fit flush against the rafters and ceiling joists. Studs should be placed 16" on center around the opening; you can use the corner studs as templates for the field studs across the top and bottom of the opening.
  
• Nail 2x2 cleats to the inside edges of the corner studs to act as backing for the drywall. From the attic, nail rigid foam insulation over the outside of the light shaft; then finish the inside of the shaft with drywall (Fig. 10).

SAFETY PRECAUTIONS

• Always follow standard safety procedures for working on the roof and using power equipment. When working on the roof, always wear loose, comfortable clothing and rubber-soled shoes. Set your ladder so the base is away from the building 1/4 the height of the ladder plus the width of the soffit. Always wear eye protection when working in the attic and when using power equipment.