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Q.I often encounter rot on joists and sills in my remodeling work, and have to sister on new wood members to support the structure. Is it okay to leave some rotten wood in place, or must I remove it all?

A.Paul Fisette, director of the Building Materials and Wood Technology program at the University of Massachusetts in Amherst, responds: Wood rots when it is attacked by rot fungi — living organisms that need four things to survive: food (wood), favorable temperatures (32°F to 100°F), water, and oxygen. If wood is rotting, you can stop the process by eliminating one of these four things.

Generally, the most effective approach is to keep the wood dry — below a moisture content of 22% (which corresponds to around 90% relative humidity). Once the wood is dry, the rot will go dormant, but it can reactivate when the moisture content again rises above 22%.

Before you sister joists onto wood with dormant rot, I would spray the existing wood with a product like Bora-Care. It is sold by Nisus Corp of Knoxville, Tenn. (800/264-0870; www.nisuscorp.com.) It’s fairly inexpensive and will kill any fungi that are still on the wood. After you spray the wood (using a garden sprayer), be sure to dry the existing wood and take care of the conditions that led to wetting in the first place before adding the new wood.