Ask the Authority: How to Prevent Wood Rot

Q: I enjoyed reading about your process for repairing a rot-damaged porch post but what can I do to prevent this type of damage?

A: The best defense against rot is a good coat of paint that keeps the wood dry. Rot spores are present in most untreated wood. The wood provides a food source for the organism, but it needs moisture and warm temperatures in order to feed and replicate. As long as the moisture content stays low (20 percent or less), the fungus will remain dormant.

If you?re the type of person who's meticulous about maintaining your exterior woodwork, you probably won't have any problems with rot. But for an extra measure of protection, I recommend you apply a borate-based wood preservative. Made from boric acid, "borates" are non-toxic to humans and other higher life forms, yet they kill wood rot and fungi on contact. They also exterminate any wood-destroying insects such as termites, carpenter ants and powder-post beetles that ingest the treated wood.

To treat large unpainted surfaces - or to wipe out active rot infestations - borates can be applied in liquid form.

To provide long-term insurance against rot, borates are available in time-release capsules. Impel Rods are crystalline tubes of boric acid that slowly dissolve when exposed to water. To protect a precious wood member, you simply drill a hole, insert the appropriate-sized Impel Rod and cap the hole with putty or a removable plug. If the wood stays dry, the rod remains intact; if it gets wet, the borates will diffuse and kill off any rot infestations before they can do any damage.

Simply drilling a hole and inserting an Impel Rod'a crystalline, water-soluble form of borate'provides long-term insurance against rot damage. Filling the hole with a removable plug makes it possible to add more protection in the future.


To protect a window's most vulnerable surface, small (1/3-in.W x 1-in.L) Impel Rods are inserted into holes drilled into the face of the sill. The holes are then plugged with epoxy wood filler.

Impel Rods are available in a range of sizes, the smallest are about the size of a prescription drug capsule ('-in. W x 1/2-in. L), the largest are as big as the bottle those pills come in (3/4-in.W x 3-in. L). I use the smaller sizes to protect valuable features such as porch railings, corner boards, door bottoms, windowsills, and casings. I use the larger ones for heftier objects such as fence posts, porch columns, and girder beams.

All of these products are available by mail order from Wood Care Systems (800-827-3480;, which provides first-rate customer service.

Tom O'Brien is a veteran restoration carpenter who writes frequently about construction practices and old houses.