How To Replace An Interior Window Sill

The most common reason for replacing a window sill is the usual tear and wear surfaces suffer due to use and weather. Moulded and rotten wood may not come as a surprise after years and, as we said before, cosmetic repairs can increase the value on a property.

We encourage you to change your window sill yourself but keep in mind this is a delicate process. You won't be working from scratch, hence it's very important to know how to remove the existing sill without messing up your window frames!


What you'll need to replace a window sill

We've put together a full list of the tools that you'll need for this job. You may have most of them already, but make sure you are fully equipped beforehand. Some tools you may be able to replace with, for example, an oscillating multitool. 

Set aside: a utility knife, a putty knife, chisel, pry bar, pliers, a mallet and/or a claw hammer, a multitool, a reciprocating saw, a drill, finishing nails, strong adhesive, a crayon or pencil, and measuring tape. Finally, you'll also need a table saw to cut the new sill, but of course, you can get this done at your closest warehouse.


Stage 1: Removing the old window sill

Once you've decided to get this done and you have your brand new window sill all good to go, you'd have to first remove the old sill. Always remember to proceed with care.


Cut through the layers of the window sill trim

After opening the window, use the utility knife to free the old sill. The goal is to loosen up the wooden sill trims, so first, you must cut through the layers of caulk, paint, and glue. Then, use your putty knife to help the sill loosen up even more. You can also do this with the blade of your oscillating multitool.


Pry the window sill up and out of the window frame

Grab the reciprocating saw and cut a vertical line into the window sill. We'd recommend using a power tool only if you are already quite handy! You can also cut 2 lines -one vertical and one diagonal that will help you remove a small piece first and then keep going. Be careful! You don't want to cut the outer sill, plus, you shouldn't hit the nails that keep both holding to each other by mistake.

If you'll do it the old way, use the putty knife, a small saw, a pry bar, and the chisel to pry up the sill. You may want to have your hammer or mallet, just in case. Depending on the condition of the window sill it may be quite tight or loose. Take the time you need to do this.

Most likely, the jambs will be nailed at the bottom, so keep the diagonal pliers handy and carefully remove the nails.


Stage 2: Installing the new window sill

Once you've removed the old sill, make sure to get the right measures so you can get the right sizes from the get-go. Use the sill you just removed as a template if needed to ensure the thickness is accurate.

Finally, once you are all good to go, follow the instructions for fitting the sill on Step 6 of this guide: "Fitting your window sill." Be very gentle during the process as you don't want to split your beautiful brand new sill! Remember to give the sill a tiny tilt to prevent mould and rot.

One of the advantages of getting a finished interior window sill is that you can count on professional design, great materials and a polished finishing, making the task way easier!