There are two different types of skylights to install. One is a Curb Mounted Skylight and the other is a Self Flashing Skylight. The curb mounted skylights needs to have a curb pre-built on to the roof opening. The self flashing skylight already has the curb built on to the skylight.
Installing a Curb Mount Skylight:
Construct curb (normally using 2 x 4 lumber or larger). Your inside curb dimensions should be the same as your roof opening (ie Model # 2626 inside curb dimension should be 22 1/4 "x 22 1/4")
Attach the curb to the roof deck at your desired location (roofing felt should be in place first). Toe nail curb to roof deck and or roof rafters. Run your shingles up to the bottom edge of curb.
Now is the time to start flashing the curb. Preformed flashing is ideal, but step flashing is also sufficient. Be sure that your flashing overlaps as much as your shingles do. Flashing is placed down first over at least one rung of shingles, then at the bottom side of the curb, flashing overlaps each other up each side and across the top of the curb.
Now continue to complete the shingling of your roof, up the sides and across the top of the curb. Then place a strip of foam weather stripping or a small bead of silicone caulk on top of your wooden curb.
Finally, simply place skylight over your curb, making sure it slips all the way down on your foam or caulk to reduce air infiltration and ex-filtration. Secure the skylight to curb with either galvanized nails or stainless steel wood screws through the pre-drilled holes around the perimeter of the skylight frame.
Installing a Self Flashing Skylight:
Make sure roof opening and skylight inside dimensions are the same. Lay felt paper on roof and shingle up to the bottom edge of roof opening and not any further at this time.
Place ample amount (sufficient thickness to insure a complete seal of skylight frame to roof) of roofing cement, silicone or any other quality roofing compound, approximately three inches wide around the roof opening on all four sides.
Place skylight down on to the roof opening directly on top of the sealant and fasten with nails or screws on the top and on the two sloping sides on the outer edge of the built-in flashing. For extra assurance against leakage, caulk or compound over fastener heads.
Now you can resume roofing. Run your shingles all the way up against the curb wall of skylight on the two sloping sides and across top, leaving the bottom edge of the built-in flashing exposed and on top of shingles.
CAUTION: Some applications may require additional flashing (wood shingles, flat roof, etc.)