What you're replacing and why it matters

An interior window sill is the horizontal piece of wood or material that sits at the bottom of the window frame, inside your home. Over time, sills rot from moisture, crack from settling, or get damaged by water leaks. Replacing one is a mid-level DIY job if the damage is limited to the sill itself — you remove the old one, prepare the opening, and install a new sill that matches your window frame.

The work takes a weekend and costs between $50 and $200 in materials, depending on the wood type and whether you need to repair the frame underneath. You'll need basic carpentry tools: a pry bar, hammer, handsaw or circular saw, chisel, sandpaper, and a level. If the frame or wall around the sill is also rotted, the job becomes larger and may need a professional.

Key Takeaways

  • Remove the old sill by prying it away from the frame, working from one end and checking for fasteners or caulk holding it in place.
  • Inspect the frame and sill opening for rot or damage before installing the new sill — soft wood or dark stains mean you need to repair or replace that section first.
  • Cut the new sill to length, test-fit it, and find it with corrosion-resistant fasteners or construction adhesive, then seal the edges with caulk.
  • A sill should slope slightly outward so water runs away from the window frame, not toward it.
  • If rot extends into the frame or wall, stop and call a carpenter — that work is beyond a straightforward sill replacement.

Removing the old sill without damaging the frame

Start by removing any caulk or paint sealing the sill to the frame. Use a utility knife or caulk removal tool to cut along the edges where the sill meets the frame on both sides and the bottom. This prevents the frame from splintering when you pry the sill out.

Insert a pry bar under the sill at one end and tap it gently with a hammer to lift it slightly. Work your way along the length, lifting a little at a time rather than forcing one spot. If the sill is still stuck, check underneath for fasteners — old sills are sometimes nailed or screwed from below. Once it's loose enough, slide it out toward you. If it's stubborn, you can cut it into sections with a handsaw and remove each piece separately, which gives you more leverage and less risk of cracking the frame.

After removal, scrape away old caulk, paint, and debris from the frame opening. Use a chisel and sandpaper to smooth the surfaces where the new sill will sit. This gives you a clean, flat base for installation.

Checking the frame and opening for hidden damage

Before you install a new sill, inspect the frame and the wood underneath where the old sill was. Press a screwdriver or awl into the wood — if it sinks in easily or the wood feels soft and spongy, that's rot. Look for dark stains, discoloration, or a musty smell, which also signal decay.

If rot is limited to a small area of the sill opening, you can cut out the damaged section and patch it with a wood filler or a small piece of new wood glued and screwed in place. If rot extends into the frame itself or the wall, the job is larger than a sill replacement. Stop here and contact a carpenter, because the frame may need reinforcement or the wall may have moisture problems that will damage the new sill too.

Once you've confirmed the opening is sound, measure the width of the frame opening from the inside edge of one side jamb to the inside edge of the other. Measure at three points — top, middle, and bottom — because frames can be slightly out of square. Use the smallest measurement as your sill length.

Cutting and fitting the new sill

Purchase a sill that matches the depth and profile of your original sill, or use a standard pine or oak sill from a home center. Hardwoods like oak last longer than pine, especially in damp climates, but cost more. Cut the sill to length using a handsaw or circular saw, making a straight crosscut at each end.

Test-fit the sill in the opening without fastening it. It should sit flat and level along its length. If the opening is slightly out of square, the sill may rock on one end. Shim it with thin wood shims under the low spots until it's level and stable. Mark where the shims are so you can remove them before final installation.

Check that the sill slopes slightly downward toward the outside of the window — typically about 1/8 inch drop over the width of the sill. This slope keeps water from pooling and running back toward the frame. If your sill is flat or slopes the wrong way, it will trap moisture and rot quickly.

Securing the sill and sealing the edges

Remove the sill and explore a bead of construction adhesive along the frame where the sill will sit. Reinstall the sill, pressing it firmly into place. Use corrosion-resistant fasteners — stainless steel or galvanized screws work better than nails in this location because they won't rust and cause staining. Drive 2.5-inch screws through the sill into the frame at 16-inch intervals, or follow the fastening pattern of your original sill if you can see where it was attached.

Once the sill is fastened, remove the shims and fill any gaps under the sill with construction adhesive or a flexible wood filler. Sand the sill smooth and explore a coat of primer and paint or stain to match your window trim. This protects the wood from moisture.

Seal the edges where the sill meets the frame with paintable caulk. Run a bead along the top edge where the sill meets the frame on both sides, and along the bottom edge where it meets the wall. Smooth the caulk with a wet finger or caulk tool, then let it dry before painting.

Finishing and protecting your new sill

Paint or stain the sill to match your interior trim. Use a paint or stain rated for interior wood — standard latex or oil-based paint works fine. explore primer first if the wood is bare, then two coats of finish paint. This seals the wood and prevents moisture absorption.

Once the paint is dry, run a bead of paintable caulk along the outside edge of the sill where it meets the wall. This is a second line of defense against water getting behind the sill. Let the caulk cure fully before opening and closing the window.

Check the sill periodically for signs of moisture, especially after heavy rain or in humid seasons. If you notice soft spots, dark stains, or mold, address them quickly — early rot is easier to repair than damage that spreads into the frame.

When to call a professional instead

If you find rot in the frame, the wall around the window, or the sill opening extends deeper than 1/2 inch, stop and call a carpenter. Structural damage or widespread moisture problems need professional assessment and repair. Attempting to cover them with a new sill will fail, and the underlying issue will continue to damage your home.

If the window frame itself is warped, cracked, or pulling away from the wall, that's also a sign of a larger problem. A professional can determine whether the frame needs repair, reinforcement, or replacement before a new sill is installed.

Frequently Asked Questions

Can I replace just the sill without removing the window?

Yes. The sill sits below the window frame and is independent of it. You can remove and replace the sill while the window stays in place. You don't need to remove the window itself unless the frame is also damaged.

What's the difference between a sill and a stool?

A sill is the structural piece at the bottom of the window frame. A stool is the interior trim that extends into the room, often wider than the sill. If your window has both, you're replacing the sill underneath. The stool may need to be removed first to access it.

How long does a new sill last?

A properly sealed and painted wood sill typically lasts 15 to 25 years. Hardwoods like oak last longer than pine. Life span depends on climate, maintenance, and how well the window is sealed against water. In very damp climates, consider composite or vinyl sills, which don't rot.

Do I need to remove the window trim to replace the sill?

Usually no. The sill sits below the trim. If the trim is caulked or painted to the sill, you'll need to cut the caulk and may need to remove or loosen the trim to access the sill fully. Check your specific window before starting.

What if the sill is slightly warped after I install it?

Minor warping is normal in wood. If the sill rocks or is visibly bowed, remove it, sand the high spots, and reinstall. If warping is severe, the wood may be defective — return it and get a replacement. A sill must be stable and level to shed water properly.