Rotted wood occurs as the result of fungal growth caused by the presence of moisture. Dry rot repair is ideal for areas of wood that have limited structural importance such as a window or door frames. Structural components such as joists and beams should be replaced if signs of rot are present. There are two ways to repair rotted wood, depending on the size of the damaged area and the condition of the surrounding wood -- fill it with wood filler or cut the rotted wood out and replace it with a patch.
Scrape away crumbling and rotted wood with a wood chisel. If you cannot scrape away all of the decayed wood to a sound, unaffected wood surface, you'll need to use a consolidant, which is an epoxy-like liquid hardener that penetrates and reinforces damaged wood fibers by working through the infected wood and bonding to the solid wood.
Drill a honeycomb pattern of shallow holes into the wood with a 3/16-inch twist drill bit. Do not drill completely through the wood.
Brush on consolidant, or wood hardener with a paint brush,, and leave it for about two hours, or the time recommended on the label.
Construct a form around the damaged section of wood using thin hardboard or scrap lumber. Structure the form so that it encases the sides of the damaged section, leaving one side open, and tape it in place using duct tape.
Spray lubricant on the forms before installing them around the damaged area. This ensures the form will come away from the wood filler when you're ready to remove them.
Fill the gaps between the damaged wood and the forms with wood filler. Leave this to dry for about 10 minutes, or until the filler is solid and rubbery.
Remove the forms and scrape away excess filler with a utility knife. Sand the filled wood with fine-grit sandpaper to shape and smooth it.
Apply a finish coat of paint over the wood to conceal the patch.
Cut a block of wood that is the same thickness, but slightly larger than the rotted section of wood.
Place the patch piece over the rotted area and trace the outline of the patch piece onto the rotted wood with a pencil.
Cut out the rotted section along your traced lines. Chisel out corners so that all edges of the patch fit snugly into the damaged area.
Apply polyurethane glue to the patch piece and reinsert it into the damaged area. Clamp the wood with a single-handed face frame clamp to hold it in place until the glue dries. This type of clamp applies pressure to the top and the sides of the wood to ensure the patch piece is pushed tightly into the gap.
Sand the patched section smooth with fine-grit sandpaper and apply a finish coat of paint or stain to conceal the patch.