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How to Replace a Window Sill Yourself: Like a Pro

Need to remove and replace your window sill? We’ve got you covered with these easy-to-follow steps.

Is your window sill looking old, rotten, or worn? Or maybe just outdated? Then it’s probably time to consider replacing it. The costs involved in getting a professional to do the job may be putting you off.

So, we’re going to show you what you’ll need, what the steps are, and a few other important bits of information. With the confidence to replace a window sill yourself, you’ll have a new one in place in no time at all.

Key Takeaways

  • Remove old window sill by cutting caulk and prying trim from the wall.
  • Measure and cut the new sill to fit the window opening.
  • Install the new sill using nails and caulk to secure it in place.
  • Replace trims, fill nail holes with wood filler, and re-caulk seams and corners.

How to Replace a Window Sill

Replacing a window sill without removing the window isn’t that hard when you have the right steps. Let’s start with how to replace an interior window sill.

Interior Windows

Green plant in a modern home

What You’ll Need

  • Putty knife.
  • Pry bar.
  • Pliers.
  • Chisel.
  • Regular hammer.
  • Rubber mallet.
  • New sill.
  • Handsaw.
  • Table saw.
  • Nails.
  • Wood filler.

1. Removing the Caulk

Caulk is frequently used to seal seams and corners. Cut the trim board under the sill using an Xacto knife. This will make it easier to remove the wooden plate and minimize damage to the surrounding area.

2. Pry the Trim from the Wall

To get under the window sill, use a large putty knife to scrape away at the wall. Use a pry bar to carefully pry the trim from the wall by pressing it against the putty knife and beneath the trim with the pry bar’s edge.

The putty knife will function as a barrier, preventing the pry bar from puncturing the drywall. Use pliers to remove any nails from the back of the trim once you’ve removed it.

3. Removing the Sill

The side window trim can be easily removed with the help of a putty knife and a pry bar. Pry the sill with a pry bar and see if you can loosen the nails or the sill.

You can use a circular saw to cut to the sill’s depth if you need to. But make sure you don’t cut into the metal or wood of your window. Using a hammer and a chisel, you may be required to chisel close to the glass.

Remove the sill in two pieces and use pliers to remove any remaining nails.

Safety First

When doing a project like this, it’s important to protect yourself with the proper gear. Gloves, glasses, and steel-toed boots.

4. Measure the New Sill

On top of the new wood, set the new window sill you just bought. Make a precise trace of the sill’s contours. Using a handsaw and a table saw, carve out the shape. Use an orbital sander for sanding the surface. Fit the new sill and sand it until it’s just right.

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5. Fixing the New Sill in Place

Fix the new sill in place using a rubber mallet. Each back corner and every 12-inches along the back should be nailed with a finish nail. Finish nails should be nailed 1/2-inch from the inner window casing on both sides.

Trims should be reinstalled around the window and on the sill. Use a nail set to make your nails disappear. Replace all caulking around all corners, seams, and holes with wood filler.

Exterior Windows

Replacing an exterior window sill is a similar approach. But with some extra steps that we’ll discuss.

What You’ll Need

  • Same tools as before.
  • New window sill.
  • Nails.
  • Caulk.
  • Wood fill.

1. Removing the Old Sill

As indicated above, remove all exterior window trimmings and the sill. Remove any exterior trim, as well as the window. Cut the outer sill in the center using your circular saw. Discard the pieces and use them as a guide.

2. Installing the New Sill

Purchase an external window sill that matches your existing sill or trim your existing sill to fit. Loose insulation should be stuffed into any framing openings and covered with a vapor barrier.

Set the sill in place. Secure with nails and then seal cracks with caulking.

Install your new window sill using nails and caulk. Utilize the original sill as a reference point for nail placement.

3. Make the Finishes

Install your window and level it with shims. Then install the window sill and all trims on the interior side. Prepare your nails and fill the space with wood fill.

All corners, seams, and edges should be caulked.

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Should I Replace or Repair My Window Sill?

The sill is the protruding ledge at the base of your window. Replace this part if it appears to be loose, rotten, or has broken edges.

If the sill itself is in good shape and you just want to update the look, repairing is the way to go. Fill any cracks with putty, remove old caulk and re-caulk, and then add a coat of paint.

Not everyone knows how to repair a window sill. But in some situations, repairing isn’t even an option. Here are signs that you need to replace or repair your window sills.

Your Window Frames Have Developed a Discoloration

It is not uncommon for rotting window frames to show signs of mold and fungus, which can alter the color of the frames. Watch for peeling paint around the window frame, as this is a telltale sign of a water leak.

You Notice a Problem with the Miter Joints

The inside corners of your casing trim are your miter joints. If there is a bigger gap in the bottom corners than the top ones, the wood has been weakened by water and subsequently frozen.

You Notice Damage Around the Windows

While your window frames may appear to be in fine condition, they should be replaced if they allow water to enter. One of the most convenient ways to determine if they are leaking is to examine the wall surrounding the windows.

Is it apparent that it has been flooded? Then there is a possibility of a problem with the window sills.

Your Window Frames Feel Soft

If you are unsure whether your window frame is rotten, climb up and feel it. Wood should have a firm, strong feel to it. If the frames feel mushy, crumbly, or brittle, they are rotting and should be replaced.

How Much Does It Cost to Replace a Window Sill?

A simple window sill replacement typically costs between $240 to $524 in labor and parts.

Several elements influence the project’s cost. Removing an existing sill or repairing the frame beneath the window can increase the final cost. The cost of wood varies substantially as well. If the sill is just cosmetic, a highly decorative wood may be a good option for you.

A tougher wood with more strength could be better if it’s meant to hold heavier goods like potted plants. How much sanding or priming is required will also affect prices.

Replacement Window Sill Ideas

Here are a few great ideas and alternatives when replacing a window sill.


Stone window sills are the most luxurious and long-lasting solution for interiors. Whether made of natural or synthetic stone, they can last indefinitely.

You have a wide range of natural stone materials to pick from, including granite, marble, slate, and limestone.

They look great and are resistant to heat and moisture. Stone window sills have a long lifespan and require little upkeep because they are resistant to sunlight damage.


It’s a good idea to use tile window sills in the kitchen and the bathroom. Moisture- and sunlight-resistant, tile is a breeze to keep clean. In addition, the cost of tile is low, which is a crucial factor if you’re on a tight budget.


The most popular option is PVC window sills, which are available from a wide range of manufacturers and come in various widths, colors, finishes, and other features. A pre-made model or custom order can be chosen depending on the room’s overall design and window decor.


Here are some common questions surrounding the idea of replacing window sills.

Why Do Window Sills Need Replacing?

Window sills are also prone to rot and different types of damage. When not replaced, they won’t be able to hold the weight of your potted plants. And the damage can expand to the window frame and even the surrounding walls.

Can Rotted Window Sills be Repaired?

Rotted window sills need to be replaced. If the rot has advanced, it is seen simply by looking at it. Do not attempt to repaint or repair it because if it is rotting, it is irreparable.

Is It Easy to Replace a Window Sill?

You simply remove the old rotted sash window sill and replace it with a new one. You will then require priming and undercoating. Then paint it to match the rest of the window.

How Do You Replace a Window Sill on a Brick House?

Replacing a window sill that is made of bricks is a little more complicated, so you might want to call in an expert who knows how to handle masonry.

Where Can I Get Window Sill Replacement Parts?

Replacement window sills and sill parts should be available in hardware stores, but make sure you have the correct measurements before purchasing.

In Conclusion

Now that you know how to replace a window sill, all you need is to acquire the tools and get to work. First, determine whether your sill can be repaired or if you need a brand new one.

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About the Author

Candace Osmond

Candace Osmond is a USA TODAY Bestselling Author and Award-Winning Interior Designer. Using her years of hands-on experience, she now writes about design and DIY. She currently resides on the rocky East Coast of Canada with her family and slobbery bulldog.