How to Repair a Rotted Windowsill

Start by measuring your windowsill for the width, length and thickness of it. In order to get to the sill part of the window you must first remove the sash channel and the sidepieces of casing. Use a small pry bar supported on a small block of lumber to keep it from damaging the walls while working loose any trim. Remove all nails from the window jamb, walls and trim by using a pair of pliers. Once the old windowsill is removed, you can use it as a pattern for your replacement sill. The old windowsill can then be taken to your local home improvement center to purchase your replacement windowsill, do not forget to pick up some nails, preferably size 6d or 8d make sure they are finished.

On your lumber that was picked up, use the old windowsill to draw a pattern right onto the new lumber. When cutting out the lumber you want to make as smooth of a cut as possible, use a saber saw with a fine-toothed blade. Make your cut then place the new sill in place, use a few of your 6d or 8d nails to hold it in place. Add the trim with a few nails and once the fit is as you want it then securely nail all other pieces back in place.

There will probably be a few nail holes visible, just use some wood putty to fill them in. Use some paint or stain which ever matches your particular décor to finish the project.
However, if your plans are to paint the windowsill you may want to go with an inexpensive wood such as pine. If you plan to stain your new windowsill a wood such as oak can be used.

Congratulations, you have just repaired that damaged or rotted windowsill.