Wood that has dry rot is not really dry. It's infested with a fungus that thrives when wood is repeatedly in contact with water.
Check the condition of the wood on your home's exterior. Are the windowsills free of splits? Is the siding free of cracks? Are the casing and the sill joints still solid? Are the thresholds sound and firm?
If you've answered no to any of those questions, you probably have some fungus-infected, rotted wood. If left untreated, the problem can lead to major, expensive repairs.
What to Look For
Sometimes the fungus itself is visible. More often, the wood surface is brown and crumbly or whitish and spongy. Rotted wood may yield easily to the probe of an ice pick, or it may sound hollow when tapped with a hammer.
Look for dry rot in wet areas, near plumbing and in any wood that touches the soil or is exposed to weather or water. The fungus usually enters at the end grain. The base of a foundation post, the joint of a beam and the place where a window frame butts against the sill are all likely spots.
Pay particular attention to any side of your house where wind can force water deep into a joint or crack.
Do-It-Yourself or Pro?
You may be able to fix windowsills, door frames, porch rails, siding and other nonstructural wood. House structural members require the services of a professional.
At a minimum, cut away the affected area, plus 6 inches on each side, and splice in new wood. It's generally better, however, to replace the whole piece. If the repair is done incorrectly, it may just promote further decay and damage. Carefully saw or pry the old part away, keeping it intact if you can, for use as a pattern. Check that the area beneath is not affected. Then buy a new piece and cut it to fit.
If dry rot has infested only a small area, you can often repair it with two-part epoxy resin system, available at hardware and boating supply stores.
First, a resin-and-hardener liquid consolidant is used to penetrate and reinforce the wood fibers. Then a second resin-and-hardener mix, a paste, is used to build up and fill in the decayed area. Once the resin dries, it can be cut, planed or sanded to match the area surrounding the repair.
* Remove all the loose, decayed wood.
* Drill a honeycomb pattern of six-sixteenths-inch holes into (but not through) the decayed wood. This allows the consolidant to penetrate and bond well with the sound wood.
* Mix the two parts of the resin in a plastic squeeze bottle. Pour the mixture slowly into the holes. Add more resin as the first dose soaks in. Over the course of several days, resin will seep into the pores of the wood, replacing the decayed wood with plastic.
* Prepare the paste resin-and-hardener mix. Wear close-fitting vinyl gloves that protect your hands but let you feel what you're doing.
* Apply the paste filler with a putty knife. Smooth the area flush with the surrounding wood to minimize sanding.
* After the filler dries, sand the area with fine sandpaper to prep it for painting. Wipe off all dust and residue.
* Apply an alkyd primer, making sure it covers the joint between the repair and the surrounding solid wood. Finish with two coats of latex or alkyd paint.
Caution: When working with epoxy resins and hardeners, follow the manufacturer's directions. Wear safety goggles, vinyl gloves and a mask. And work in a well-ventilated area.