Over time, a windowsill may rot and need to be replaced. The following steps will help you to remove and replace an old rotted or otherwise damaged window sill. It also will increase the durability of your new window sill.
Remove the old window sill by prying it away with a chisel and hammer. Use a ply bar to separate it from the adjoining sides. Cut any nails with a bucksaw blade.
Choose a new piece of wood with the same thickness as the old window sill. Place the old window sill on top of the new wood and cut it to its same dimensions. You are using the old sill as your guide.
Sand and treat the wood.
Position the new sill into the old spot and verify that it is level.
Nail the window sill under the window sides with galvanized finishing nails, sinking the nails beneath the surface.
Fill the holes with wood putty. Allow it to dry before sanding. Caulk any gaps.
Prime and paint the new window sill.
Keep the window sill painted and caulked for a longer life. Pressure-treated wood is the best choice for a new window sill. It will last longer.