Before you can begin cutting through the roof, you'll have to build a curb, or box frame, for your skylight if it's the curb-mounted type. You may find it convenient to assemble the curb on the roof. If your skylight boasts an integral curb, or is self-flashing and sits directly on the sheathing, you can avoid this step.
The curb should raise the skylight at least 4 inches above the roofing material. Usually 2 by 6 lumber works well for shingle, shake, and built-up roofs.
The inside dimensions of the curb should equal the dimensions specified by the manufacturer for the roof opening. Mark the lumber for cuts, taking care to keep knots away from the ends. Cut the pieces. As a further precaution against split ends, drill pilot holes for nails; then nail the pieces together.
Check the curb for squareness by measuring diagonally from corner to corner; the distances should be identical. You can also use a framing square.
It's a good idea to brace two opposite corners of the curb to keep it square until you're ready to nail it in place. Make the braces from lengths of wood or cut triangles from 1/2-inch plywood. Nail in the braces with 6 or 8-penny common nails; drive the nails halfway down so you can remove them easily later on.
Marking the Roof
In a home with finished ceilings, you mark the roof opening and the roofing material that needs to be removed in exactly the same manner for either a sloped or a flat roof. The amount of roofing material you remove to allow for the flashing and water runoff varies, depending on the type of roofing and the kind of skylight you use.
For a Curb-mounted Skylight
Set the curb over the four nails protruding through the roof. If the curb is the right size, there'll be a nail in each inside corner.
For a wood or asphalt shingle roof, use chalk, pencil, or a utility knife to mark lines on the roof along the outside edges of the sides of the curb. Extend the lines you just outlined 3 inches beyond the top and bottom corners of the curb. Mark a line between these points and parallel to the top and bottom of the curb. This is the area from which you'll remove the roofing material.
If you have a heavy shake roof, mark lines on the roof 1/2 inch away from the outside edges of the sides of the curb; extend those lines 3 inches to the top and bottom, and connect them in the manner described above.
After you've marked the lines, you can set the curb aside.
For a Self-flashing Skylight
Make your measurements from the sides of the proposed roof opening marked by the four nails. Mark lines approximately 10 inches beyond the nails on the top and the two side, and 2 inches below the nails on the bottom.
On a flat roof (hot-mopped asphalt or foam), mark lines 10 inches beyond the nails on all four sides. You'll have to remove the gravel first.