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 in this series of Step-By-Steps 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.
Dutchman Repair overview
Remove Loose, Rot-Damaged Wood
Fed by water trapped behind a storm window with plugged weep holes, the rot fungi disintegrated this end of the windowsill. Fish out the loose stuff with a Japanese nail puller.
Make Smooth, Straight Cuts for Tight Joints
A dutchman's joints must be tight, so cuts into the old wood have to be smooth and straight. Mark for the cross-grain cut using a square and a utility knife, then make the cut with a Japanese handsaw.
Chisel out a Straight Edge
A sharp chisel removes the remainder of the rotted wood and carves out a straight edge in the solid part of the sill to glue the dutchman to.
Trim and Test-Fit Patch
Here, Mike Vietri of Vietri Construction trims and test-fits a piece of primed western red cedar several times to ensure that the patch sits snug and flush with all mating surfaces. He chose cedar because of its rot resistance.
Apply Polyurethane Glue
Polyurethane glues cure by reacting with moisture, so first dampen the unpainted surfaces slightly, then spread the glue onto the raw wood on both the patch and the old sill.
Attach the Dutchman
After drilling pilot holes into the patch, hold it in place with a couple of stainless steel trim screws driven just below the wood surface.
Tip: Try not to touch the sticky, uncured glue. It's next to impossible to remove from skin and tools.
Level the Joint and Prep for Priming
Polyurethane glue foams as it cures, filling minor gaps as it oozes out of the joint. When the foam becomes dry and crusty, a chisel or 100-grit sandpaper levels the joint; 150-grit readies it for spot priming. For big ridges between the two pieces, a block plane feathers wood faster than sandpaper.
Finish the Job
The screw heads are covered with an oil-based glazing putty, then the patch is covered with a coat of oil-based primer. Two top-coats of 100-percent acrylic paint follow. The paint isn't there just to look pretty—it's the wood's first line of defense against water and sunlight.