At times, window sills must be replaced due to damage. The causes for this type of damage vary from a badly-executed original installation to a building structure that is poorly designed. Replacing the sill is straightforward, but dealing with the underlying reason for the procedure may be complicated.
Cut the sill in half. This is done to simplify the removal process. First, adjust the circular saw, so that the depth of the saw blade is the same as the thickness of the sill. Begin the cut at the outside edge and proceed towards the window. Take the cut as far as space will allow then turn the saw off and remove the saw. You will need a different tool to finish the cut. That tool can be a reciprocating saw, a hand saw or wood chisel. Choose which ever instrument best sits your skill level and finish the cut.
Pull one half of the sill out at a time. Use a flat pry bar, if you are not strong enough to free each piece of board with your hands. Wear a good heavy-duty pair of work gloves, while you attempt this. Take your time and you should be able to work each piece free. Set the wood aside.
Pull all nails out with a pair of vice grips. Set the jaws of the vice grips, so they just barely bite into the nail. Next close the jaws around the nail and then slowly work each nail loose. Sometimes the nose of the locking pliers can be used to apply leverage.
Determine the exact size of the original sill from your two removed pieces of old sill. Don't forget to figure in the thickness of the saw blade, when you calculate the overall length.
Cut a new sill from some softwood stock. Do not build-up the wood to create your new sill, but instead use one piece of stock if at all possible. If moisture appears to be a major problem use redwood, cypress or cedar.
Plane the outside edge so that it is rounded.
Sand the board thoroughly and paint every side and edge with a primer paint.
Slide the new sill into place and nail it tight with the galvanised casing nails. Set each nail just a little ways below the surface and fill with caulking. Also caulk around all edges of the new sill.
Paint the new sill with a finish exterior-grade paint.
If the old sill is rotten, it may come out without any cutting. Water damage is not the reason for replacing a sill, but is by far the major concern.
Look for water damage or rot in places beyond the sill, such as the wooden frame of the building. If you wait too long to replace a defective sill, you may have bigger problems. Always replace the sill as soon as you notice the damage.