- 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.