Several different things can affect a window sill and cause a need for replacement, from condensation to sun damage to warping and more. As long as you have the right tools, a new piece of wood for your new sill, and some patience, you can replace it yourself. Here, our window replacement experts tell you how to replace an interior window sill:

1. The first step for how to replace an interior window sill is using your utility knife to cut the caulk between the window sill and the trim.
2. Next, cut underneath the window sill to free the sill from the trim board under it.
3. After that, place your putty knife between the wall and the trim and pry the trim away from the wall. If needed, use your pry bar or chisel to complete the task.
4. Once all of your trim is lifted away, use your pliers to remove all of the finishing nails in it.
5. Open the window, then use your putty knife to remove the side window trim.
6. Next, wedge your pry bar or chisel underneath the window sill and loosen it as much as possible.
7. Repeat step 6 on all sides of the sill until it comes loose and you can easily remove it.
8. Scrape away any old glue, caulk, or debris from the framing to prepare it for your new window sill.
9. Place your old window sill on top of your new piece of wood and use a pencil to trace the exact shape onto the wood.
10. Cut out your new window sill with your table saw.
11. Next, place your new window sill where your old window sill was to make sure it’s the right fit. If not, make adjustments as needed.
12. When you have the exact shape you want, use sandpaper to sand down the edges.
13. Next, paint or stain the window sill to match the surrounding woodwork.
14. Once it’s dry, tap the finished window sill into place using your rubber mallet.
15. After that, use your hammer to drive finishing nails into the back of each corner.
16. Continue to secure with nails every 8-10 inches along the back.
17. Re-install your side trim and the trim from underneath the sill.
18. Set your nails using a nailset, then fill in any holes using wood putty.
19. Finally, re-caulk your seams, corners, and cracks.