Years of exposure to the elements will eventually rot wooden window sills. You need to know how to replace a window sill to prevent the damage from spreading farther into your home’s structure. You can expect this project to take about half a day.

How to Replace a Window Sill Steps

1. Cut through the caulk around the window frame and trim with a utility knife.
2. Carefully lift away trim pieces with a pry bar. Take care not to break the pieces because you can reuse them.
3. Have a partner hold the window.
4. Using the reciprocating saw with a fine tooth blade carefully cut around the window. The blade will sever the nails holding the window in place.
5. Take out the window and the bottom framing member.
6. Cut a central section out of the old sill with the reciprocating saw.
7. Pry out remaining pieces of sill and make sure to pull or pry out any remaining nails.
   1. Driving a screw into the old sill to give the pry bar something to grab can be helpful.
8. Measure the old framing member and cut a new one from a cedar 2 x 4.
9. Secure it into place with 16-gauge 2-1/2-inch nails.
10. Measure the old sill and cut a new one with notched corners from 2 x 6 cedar board.
11. Replace or add some insulation into the cavity beneath sill.
12. Use a table saw to make 1/4-inch deep kerf cut into the bottom of the sill parallel with its outer edge.
13. Fit new sill into place and nail it.
14. Add wood filler to any holes and cracks in wood around the window.
15. Cover exposed wood around the window with insulation paper.
16. Re-install the window and put the trim back on.