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Rotting Joists

Does sistering compensate for rot?

Save the Bay Tom Silva

I have a second-floor deck that cantilevers over the driveway. In a few places, the ends and tops of the joists are starting to rot. New joists would be very expensive to install because they connect with the house through a brick-veneer wall. Would "sistering" some new joists onto the rotting ones take care of the problem?
— Robb, Two Rivers, WI


Tom Silva replies: Sistering new joists to the old ones would just encourage rot by creating a place for water to collect. You could trim the rot off the ends of the joists back to good wood, although that would make the deck narrower. But it's the rot along the top of the joists that's a trickier problem.
If the tops of the joists aren't too badly damaged — if the rot is less than ¾ inch deep and occurs only in small sections — cut or scrape out all the rotted wood and fill it with epoxy. If the rot goes deeper or is more widespread, the affected joists should be replaced.
It might be a good idea to have a structural engineer design the connections and determine the size, species, and grade of joists you'll need, and determine whether the joists inside the house are solid. And when you install the new joists, protect them by brushing all surfaces with a couple of coats of wood preservative. Let it dry and brush on some more. Then before you reinstall new decking, cut some strips of 15- or 30-lb. building felt 1½ inches wide, and lay them on top of the joists before nailing down the decking on top.
One more thing: I notice from the photos that your decking boards are butted up tight to each other. This may be the source of your problem. When water can't drain freely through the deck, the tops of the joists stay wet longer. So be sure to leave a 1/8-inch gap between the boards to give the wood a chance to dry out.


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