Of all the wooden parts in your home, windowsills are among the first to go bad. Once sills get wet, they become food for fungus, and fungus causes rot. If the rot isn’t so extensive that the entire sill is mushy, you can probably fix things up with a rot restorer. This is essentially an epoxy-based material you soak into the wood. Once it cures, it binds the punky fibers back together.
You can’t use ordinary epoxy resins or glues for this kind of work. You need a special water-thin formula, such as Git Rot or Cure Rot. These products are sold at marine supply houses, and some lumber yards, hardware stores and home centers. If your sill is too far gone for this kind of treatment, you’ll have to remove and replace it. Here’s how to remove and replace:
Because of the way window parts overlap and interlock, you’ll have to remove some window trim inside the house before you can remove the rotted sill. What trim parts need to come off? Normally, the two side casings (the vertical pieces that frame the window) and the stool (what most people call the windowsill). Carefully remove these parts with a thin pry bar and set them aside. Next, before you remove the sill, measure the distance between the two window jambs. Write this dimension down. If you pull the window out of alignment while pulling the old sill out you can use this dimension to get things back to normal.
Now take a hand saw and cut the sill in two, a few inches to one side of its center. Then cut through it again a few inches to the other side of the center. This will release a section out of the center of the sill that you can pry loose. Set it aside. With this center section removed, you can now pull the two ends of the sill free from their jambs. Work slowly and carefully. These sill pieces will be fastened to the jambs with nails, so it will take some judicious effort to work them loose.
Now, to install the new sill:
Take one of the scraps from the old sill to the lumber yard and pick up a piece of identical sill stock. Using the three pieces from the old sill as a pattern, cut the new sill, notching the ends of the new sill just like the ends of the old one. Measure the distance between the jambs to make sure it’s the same as it was before you started repairs. If not, knock the jambs back into place. Then slide the new sill into position. You’ll have to toe-nail it in place from both the inside and the outside of the house. Use eight penny finishing nails, and pre-drill for them to prevent splitting.
Inside the house, replace the old casings and stool. Then, back outside, give the whole sill a good soaking with wood preservative. Let this dry, then caulk all joints around the new sill, then paint. The best painting schedule is probably one coat of oil-base alkyd primer, followed by two coats of latex trim paint. Don’t cut corners on the caulking and finishing process. They are your assurance that the sill will stay dry so you’ll never have to repeat this time-consuming job in the future.