Before you embark on a window sill repair of your old home windows, you will want to read this article and learn a few simple window sill repair tricks. While there is more than one way to replace a window sill, I came across the video below which offers a few tips that could save you a bunch of time. The end result will be a new window sill that is as good or better than the original.
Window Still Repair Steps
Inspection: The first thing you should do is inspect the entire window for water damage and wood rot. If you find that the only damage is the sill itself, then carry on with the steps below. If you discover extensive damage to the window frame itself and the window jamb, it may be time for new replacement windows. In this case check out our article on window replacement or contact the Window Repair Guy.
Cut the Nails: Use an oscillating saw to cut through the nails. Follow along the topside of the sill to separate it from the window stool. On the underside of the sill cut between the sill and the siding along the entire length of the sill.
Cut the Center Section of the Sill: Use a sawsall with a long reciprocating blade to cut out the center section of the sill. Your cut should be about 2 inches inside the side jamb. Insert a screw at either end of the front face of the sill so you can use a pry bar to remove the center section of the sill.
Remove the Sill End Sections: Use a hammer and chisel to carefully knock out the remaining end sections of the window sill. Take care to not damage the window jamb. With the two blocks removed you need to clear out any nails or other obstructions to make room for the new sill.
Cut the New Sill: Use the old window sill as a template to cut the new window sill. Carefully measure both the length and the width of the opening to ensure a good fit. Cedar is an excellent choice of wood for your new sill, it stands up well in harsh weather conditions. Refer to the old sill to set your saw to the proper angle so the new sill will fit properly against the siding. Use a circular saw to make a dado cut along the bottom edge of the sill which encourages the rain water to fall to the ground rather than wick its way back to the siding.
Paint the New Sill: Sand the entire sill with 150 grit sandpaper then prime all sides with a white, water based primer.
Install the Sill: Before installing the sill, apply a generous bead of caulking on the underside of the jamb and the casing. Slide the sill in place leaving a small gap along the backside momentarily so you can fill the void with expanding foam. The foam provides two benefits, it fills the space to insulate the window, while at the same time it helps to push the sill up into place for a tight fit. As soon as you apply the foam, tap the sill into position. Hold the sill up and nail it to the stool.
Caulk & Paint: The final steps of your window sill repair project are caulking and a finish coat of paint. Caulk the entire perimeter of the still both top and bottom. Allow the caulking to fully cure then apply a coat of high quality paint.