How to Build Wooden Roof Curbs

Consult the manufacturer’s sizing instructions for the model of skylight you are installing. For example, a 3-by-3-foot skylight may call for a roof opening of 36 1/2 by 36 1/2 framed by 2-by-4s nailed between the roof rafters. The inner dimension of your wooden roof curb needs to match the roof opening precisely.

Cut pressure-treated lumber, typically 2-by-6s, with a chop saw to your dimensions. For example, for the skylight described in Step 1, the top and bottom of the curb need to measure 36 1/2 inches. The sides need to measure 33 1/2 inches, which when screwed to the top and bottom pieces, each 1 1/2 inches thick, makes the sides the desired 36 1/2 inches long.

Apply a tube of construction adhesive from a caulking gun in a squiggly pattern to each end of side pieces of lumber. Clamp the curb together with two long bar clamps pressing the top pieces against the side pieces.

Drill pilot holes and countersinks for 3-inch galvanized wood screws. Insert three screws into each corner of the 2-by-6, through the top and bottom pieces and into the side pieces. Allow the curb to sit for several hours or overnight, keeping the clamps in place, per the adhesive maker’s instructions for drying time.

Lay the completed curb over the roof opening to verify accurate placement. Remove the curb and caulk the juncture of the roof where it will meet the bottom of the curb heavily with polyurethane caulk in a caulking gun. Place the curb back in place and fasten it on the inside to the roof framing with two No. 10 galvanized nails or two No. 8 deck screws per side driven in at an angle, similarly to that used for toenailing boards.