How To Replace A Window Sill Inside: Easy DIY Guide

An interior window sill is a horizontal surface installed inside a window frame. It can be made from wood, metal, or stone and used for decoration.

The window sill is a very important part of the window. It provides support for the window, and it also provides a surface that can be used to display items such as plants or flowers. The sill also helps seal any gaps between the wall and the windowsill.

The interior window sill replacement is a project that many homeowners often overlook. The window sill is the border that separates the inside of a home from the outside world. It provides an aesthetic and functional purpose for being there. If you are experiencing water damage or other problems with your window sills, it may be time to replace them.

I will show you how to replace a window sill inside in this article. So, stick with me until the end.

5 Easy Steps To Replace A Window Sill Inside

StepsEstimated Required Time
Remove The Window Sill Moulding30 – 60 minutes
Pry Out The Sill60 – 120 minutes
Cut The New Sill60 – 80 minutes
Install The New Sill30 – 60 minutes
Final Touches60 – 120 minutes

Step 1: Remove The Window Sill Moulding

Window sill moulding is the case or trim that surrounds windows and doors on the outside. Since it rests on top of the sill and prevents it from elevating, you’ll probably need to remove it.

Required Tools

  • Utility knife.
  • Crowbar.
  • Cleaning brush.


  • Cut the caulk line on both sides that conceals the drywall and window framing seal to remove the casing.
  • Remove all of the caulk by dragging a utility knife from top to bottom.
  • Remove all caulk from the sill area as well.
  • Pull the moulding from the drywall with a crowbar.
  • Use bracing against the wall if prying to avoid denting the drywall.
  • Remove the nails and staples and clean up any excess caulk.
  • If you are cautious, the case can often be saved and reapplied, but usually, it needs to be changed.

Step 2: Pry Out The Sill

Required Tools

  • Hammer.
  • Pry bar.
  • Screwdriver.
  • Utility knife.
  • Rubber mallet.
  • Diagonal pliers.
  • Spray foam.
  • Cleaning brush.


  • Use a hammer or a pry bar to lift up the sill.
  • You can also push it towards your body by positioning the pry bar’s tip under the sill on top.
  • If the pry bar does not fit, try using a screwdriver.
  • Continue lifting at the sill from all sides until the nails begin to pull free.
  • To remove the nails, pound the sill in all directions using a rubber mallet.
  • To remove the sill from the window sash and framing, rock it up and down with your fingers.
  • Remove any dirt, caulk, or old glue from the structure where the sill was moved.
  • Use the diagonal pliers to remove any potential remaining nails.
  • Take the window spray foam, apply it to the gaps, and let it dry.
  • Before installing the new sill, use a utility knife to remove any extra foam.

Step 3: Cut The New Sill

For your new window sill to fit properly and not look out of place, it is crucial to have the proper measurements.

Required Tools

  • Pencil.
  • Table saw.


  • Make a template out of the sill you just removed from the window.
  • Cut the new sill to the proper size using a table saw, but do not cut off any notching.
  • This enables you to check whether the new sill’s thickness will be exact or equivalent to the previous one.

Step 4: Install The New Sill

Required Tools

  • Nails.
  • Hammer.
  • Pencil.
  • Rubber mallet.
  • Measuring tape.
  • Jigsaw.


  • Install the wood board into place using nails or brad nails, making sure to nail into the structure below.
  • Tap it back as far as it will go using the rubber mallet.
  • Draw the necessary notches using a pencil and a tape measure before cutting them with a jigsaw.
  • Reinstall the sill and cut the notches to fit it tightly on the frame.

Step 5: Final Touches

Required Tools

  • Hand block.
  • 100-grit sandpaper.
  • Hand drill.
  • Hammer.
  • 1-inch finish nails.
  • Caulk gun.
  • Paint or primer.


  • Use a hand block and 100-grit sandpaper for sanding the sill and the corners.
  • You can paint the sill or give it a finish to match the surrounding woodwork.
  • Drill holes through the sill, then use a hammer to put 2-inch nails into the holes.
  • Nail the trim pieces using 1-inch finish nails in the order they were removed.
  • Use a putty crayon with a matched color to fill in every hole.
  • Caulk the edges of the trim and moulding.
  • Match them with the existing trim of the wall using paint or prime.

Congratulations, you have successfully managed to replace your window sill. But you should remember that the window sill lasts only 10 years with proper maintenance. So if you live in a pre-purchased house, it is necessary to check for its rotting symptoms.

How To Tell If Your Window Sill Wood Is Rotten?

Wood is the most vulnerable part of a window sill that is exposed to rot. Look for the following signs if you ever feel like your window sill wood isn’t like it used to be.

  • The exterior of the wooden window frame feels fragile, spongy, crumbly, or mushy to the touch.
  • Mould can be seen on the external window frame. Initially, this might appear as black flecks.
  • Rotten pieces will be apparent.
  • The paint or sealer on the hardwood frame has cracks in it.
  • The corners of the frame are either missing pieces or don’t fit together properly.
  • The surrounding shingles or trim is discolored.
  • On the outside wall near the window, there are rotten bricks.

Bottom Line

Interior window sill replacement is a cost-effective and smart solution for homeowners looking to enhance their house’s appearance and value.

Also, keep in mind that when removing a sill, it’s crucial to use extreme caution to avoid breaking any obstructions. When working with wood, be sure to carefully remove any obstructions to the sill and the trim, and wear safety glasses at all times.

However, when replacing an inside window sill, you can always hire a professional to be on the safe side.


What is an internal window sill called?

The area of the window that is frequently referred to as the “sill” within the house is actually the stool.

However, even window experts frequently refer to the stool as the sill.

Can a damaged sill be repaired?

While the homeowner can do some simple window sill repairs with little mortar and paint, it is always recommended to hire a professional.

The damage may be worse than you think. In such circumstances, it can be better to replace the sill entirely.

Is a window sill structural?

YES. The surface or structure that extends horizontally from a window’s bottom is known as a sill. Window sills hold the window in place and provide structural support.

How much does it cost to replace a window sill?

Replacing a window sill ranges from $150 to $400 USD. The window sill, which is at the base of the window, aids in supporting and preserving the window’s structural integrity. A window sill is a typical window frame component that may require repair or replacement.

What is the difference between a window board and a window sill?

Even though the phrase “window board” is occasionally used to refer to the inside window sill only specifically, it actually refers to the long, flat boards made of uPVC, wood, aluminum, or stone that make up the sill, whether they are installed as internal or external sills.