Cantilevered Deck-Beam Fix
Q: Can I repair deck beams that have rotted on the top?
The big 4x10 redwood beams that support the second floor of my house also support a cantilevered exterior balcony. The tops of the beams under the balcony have decayed. Can I repair them?
—Paul Maitland, Rowayton, Conn.
Norm Abram replies: It's possible, if the rot isn't too extensive. Remove the balcony decking so that you can inspect the beams. If the rot goes deeper than 1½ inches or is closer than 6 inches from the house, call in a professional. Otherwise, this is a job you can do yourself using 12-gauge, 1½-inch-wide, galvanized-steel straps and treated 2x lumber.
1. Set your circular saw to a cutting depth equal to the combined thickness of the strap and the treated 2x. Make repeated, closely spaced crosscuts through the rotted areas.
2. Hammer out the narrow pieces of wood left between the saw kerfs, then clean and flatten the bottom of the notch with a chisel.
3. Fill the notch with a piece of preservative-treated lumber bedded in a polyurethane construction adhesive. Secure this filler piece with ¼-by-3½-inch galvanized lag screws and washers, staggered and spaced no more than 12 inches apart. Drill and counterbore for each lag so that the heads sit flush with the top of the filler.
4. Chisel a shallow mortise into the center of the beam 6 inches beyond the ends of the patch so that the strap will sit flush with the top of the beam. Nail the strap into the mortise with hot-dipped galvanized 10d common nails. It will restore some of the beam's stiffness.
5. Before you install the balcony's decking, cover the tops of the beams with self-adhesive flashing, like the one made by Cofair Products, to prevent more decay.