A window sill is located at the bottom of all wooden windows. It serves a twofold purpose. First, it is the bottom trim piece on a window. Without it you would see straight through to the framing. Second, it keeps the inner parts of the wall sealed from rain and pests. When it becomes rotten, water gets into the wall, which creates a breeding ground for termites and pests such as bugs and rodents. To replace it, you just need a few tools. It should take no more than two hours per window sill.
Remove the rotten window sill. The window sill is located on the bottom of the window. It will come apart into two pieces. The inside is separated from the outside and is usually a separate piece from the window. The outside window sill becomes rotted over time because of constant weather exposure. For the purpose of these instructions, we will be removing and replacing the outside window sill. It will be attached to the window jambs, so remove it in two pieces.
Raise the window and secure it into place. Using the wood chisel, gently pry the rotted sill from underneath the window jambs. If it will not move, cut the sill in half with the chisel. Slowly chisel away at the sill, without damaging the window sides or the interior sill. Once the sill is cut in half, you may continue to wiggle it until both pieces break free from the sides. (Remember that the window sides will rest on top of the sill.) Cut any nails that remain with wire cutters.
Clean out the opening and make sure there are no bugs or termites. If you find termites, you can purchase a special termite poison.
Measure the opening from one side to the other. Make sure your tape measure goes under the window sides when measuring all the way from frame to frame. Add an additional 2 inches for the amount that the sill will extend past both sides of the window. Measure 1 inch off the ends and make a square mark across the entire cut. Now, measure from where the inside sill and the outside sill meet and out to the outside of the window jamb or window sides. Make a mark from the back side of the cut and out toward the front using that measurement. You will have a section of both rear corners that will need to be removed. Cut that out with the jigsaw.
Replace the window sill underneath the window sides by gently tapping the sill into place. Nail the sill, on each side and in the middle, down into the framing beneath. Sink the nails with the nail set, fill with wood filler, caulk any exposed cracks and apply two coats of exterior primer.
Always be careful when using any power tools.