How to Replace a Wood Windowsill
- 1). Cut the old windowsill into three pieces with a hand saw for easy removal. With a pry bar, remove the center section first and then jostle the two side pieces out from the frame. Use a hammer and chisel to dislodge any difficult wood pieces. If the sill rests under the window frame, it may be necessary to cut any nails with a hacksaw.
- 2). Cut a new windowsill by butting the three sections of the old sill together and use it as a template. Make sure to choose wood stock that is the same thickness as the old sill. Additionally, choose stock that has a drip groove notched into it. When installed on the outside edge, the drip groove will prevent water from following the underside of the windowsill.
- 3). Sand the new sill and round the edges. Treat the sill with two coats of wood preservative such as copper or zinc naphthenate, which will not only add years to the life of the sill, but accepts paint readily. Let it dry.
- 4). Slide the new windowsill into the old sill's position. Fasten it into place with galvanized finishing nails. Sink the nails with a nail set and fill the holes with wood putty. Allow the wood putty to dry and sand to a smooth surface.
- 5). Stuff fiberglass insulation into any gaps, then fill with exterior caulking. Leaving the window open a few inches, prime the new sill and let it dry 24 to 48 hours. For extra protection against the weather, give the sill two coats of paint.
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