If you have a rotting windowsill, chances are good that you are worried about its effect on the aesthetics of your home and on the safety of your window itself. If the windowsill is only rotted in a small area, you can usually contain it by applying a layer of epoxy mixture around the rot so that the rot does not spread and the area that's rotted out is strengthened. If your windowsill is totally rotted, you need to replace it. Here's how you can replace a windowsill.
1. Remove the Window Casing
In order to get at your rotted windowsill, you are going to need to remove the part of your house that surrounds the window and frames it. This part is known as the window casing. Take a knife and slide it through the caulk that is holding the casing to the building. Then, carefully pry off the casing using a crowbar to loosen it more if need be. This will allow you to get the casing off of your house but still leave it intact so that you can reinstall it over the new windowsill.
2. Get Rid of the Windowsill
Next, use a circular saw to get rid of the rotted windowsill. Make a clean cut as close to being flush with the house as you can. Use a hand plane to smooth the space that you just cut.
3. Drill Pilot Holes
Take a drill and drill pilot holes every two or three inches along the area where you are going to be applying the windowsill. Make sure that you can find these holes again if you cover them up by measuring exactly two or three inches from one of the sides and making a note as to how far up the hole is.
4. Apply the Adhesive
Next, apply a layer of adhesive that is totally waterproof. You are going to want to make sure that you get marine-grade adhesive either online or at your local hardware store. It is critical that you get strong adhesive in order to make sure that your replacement windowsill holds firm.
5. Attach the Windowsill
Finally, attach the windowsill by taking it and pushing it into place. Hammer nails into the approximate areas where you drilled the pilot holes. If there is a gap between your house and the new windowsill, fill that gap in with more of the adhesive you used earlier.
For more information, talk to a company that specializes in window and windowsill replacement (such as Ken Caryl Glass, Inc.).