Instructions for Roof Curbs

Consult your skylight installation manual for the recommended outside curb dimensions for your particular model. For example, the precise curb dimensions for a nominal 2-by-4-foot skylight may be 25 1/2 inches by 49 1/2 inches.

Cut with a circular saw two pieces of pressure-treated dimensional lumber (2-by-4-inch lumber or larger) to the width called for by your instructions -- in this example, 25 1/2 inches. Cut two additional pieces for the height called for in the instructions, minus 3 inches to account for the depth of the first two pieces you cut. In this example, you would cut two pieces of lumber 46 1/2 inches long.

Clamp the long pieces between the short pieces atop the rough opening in the roof, and check that the resulting box closely matches your rough opening. Unclamp the pieces and move them elsewhere on the roof or to a flat landing or appropriate work and storage area.

Apply construction adhesive to the ends of the long pieces of lumber in a zig-zag pattern. Clamp them again between the short pieces. Drill pilot holes with a countersink bit. Screw two 3-inch wood screws into each butt joint from the short piece into the long piece.

Allow the roof curb’s adhesive to dry overnight. Place a thick bead of polyurethane caulk around the rough opening on the roof surface to create a waterproof bond between the curb base and the roof. Move the curb back in place over the rough opening on top of the caulk.

Kneel on the top of the curb, one side at a time, to press it into the caulk as you screw it in place. Toe-screw the curb, in two places on each side of the curb, by starting a 3-inch wood screw into the curb above its lower edge at an angle so it will pierce the center of the rough opening’s framing directly below.

Wrap the exterior of the curb with self-adhesive membrane, and press the membrane into the crease where the curb meets the roof and onto the roof itself. Wrap the curb and surrounding roof with roofing underlayment. Complete the seal between curb and roof with shingles, metal flashing or EPDM rubber, depending on which one best matches the current roof surface.