Rot happens, even to the best of houses. All it takes is wood, water, and warmth, and before you know it solid lumber turns to mush. Exterior trim is the most vulnerable to attack by rot fungi, and it doesn't have to be very old; the trim shown on these pages was installed only 10 years ago.
Fortunately, rotted trim is generally easy to repair. (Rot-infested framing or mudsills pose a much bigger problem.) But before you can fix it, you have to find it. With screwdriver or awl in hand, scrutinize areas that are nearly horizontal and don't drain well, such as windowsills, drip caps, and water tables. Look for paint that is cracked, peeling, or blistering, or wood that's darker than the surrounding area or green with algae. Probe anywhere there's end grain, which wicks up water like a celery stalk in a grade-school science experiment. Pay particular attention to joints, which dry slowly, and to all wood that's close to dirt, concrete, or masonry. If you're able to push the tool's tip easily into a suspect board, then it's time to root out the rot.
When rot afflicts a relatively confined area, filling the damage with two-part epoxy resin is a smart option that yields a seamless repair and doesn't require a lot of experience. Here, John Stahl of Advanced Repair Technology, who restored the old windows for This Old House TV projects in Milton and Salem, Mass., takes us through a typical repair of a rotted window mullion.
