What you need to do before you start

An interior window sill replacement means removing the old sill—the horizontal piece of wood or material that sits at the bottom of the window frame inside your home—and installing a new one in its place. The job is straightforward if the sill is rotted or damaged but the frame around it is sound. If the frame itself is soft, warped, or crumbling, the sill replacement becomes part of a larger frame repair, and you may need to call a carpenter.

Before you begin, check whether the sill is load-bearing. In most homes, interior sills are decorative and rest on the frame below them; they do not hold up the wall. Exterior sills do the real work. If you are unsure, take a photo and ask a contractor or your local building department. A load-bearing sill requires temporary support during removal and is beyond a straightforward replacement.

Gather your tools: a pry bar, hammer, utility knife, measuring tape, handsaw or circular saw, sandpaper, wood filler, caulk, and a caulking gun. You will also need the material for your new sill—typically pine, oak, or composite wood—cut to length by the supplier or cut yourself if you have a saw.

Key Takeaways

  • Interior window sills are usually not load-bearing, but check the frame condition first—if the frame is rotted, the job becomes a frame repair, not just a sill swap.
  • Remove the old sill by cutting it into sections with a handsaw or circular saw, then pry out the pieces; do not try to pull it out whole.
  • Clean the frame below the sill, fill any gaps or rot with wood filler, and sand smooth before installing the new sill.
  • The new sill should sit level, be fastened with screws or nails into the frame below, and be sealed with caulk on all edges to prevent water entry.
  • The entire job typically takes two to four hours for a single window if you work alone and have no frame damage to address.

How to remove the old sill safely

Do not try to pry the old sill out in one piece. Sills are usually nailed or screwed down along their length, and forcing them out whole risks cracking the frame or damaging the wall around it. Instead, use a handsaw or circular saw to cut the sill into three or four sections, each about 12 to 18 inches long.

Make your cuts perpendicular to the length of the sill, cutting all the way through but not into the frame below. A handsaw takes longer but gives you more control; a circular saw is faster but requires a steady hand and eye protection. After each cut, use a pry bar to lift and remove that section. Work from one end toward the other. Once all sections are out, use the pry bar to remove any remaining nails or fasteners from the frame.

If the sill is caulked to the wall on either side, run a utility knife along those seams first to break the seal. This prevents the wall material from tearing when you pry.

Preparing the frame for a new sill

Once the old sill is out, inspect the frame below it—the horizontal piece that runs the full width of the window opening. If it is soft, spongy, or crumbles when you press it, you have rot. Small areas of rot (less than a few inches) can be filled with epoxy wood filler; larger areas mean the frame needs replacement, which is a separate, more involved job.

Clean out any debris, old caulk, or loose material from the frame. Use a wire brush or scraper to remove paint, caulk, or buildup. Sand the top surface of the frame smooth with 80-grit or 120-grit sandpaper. If there are gaps or small voids in the frame, fill them with wood filler rated for exterior use (even though this is inside, the sill area gets moisture). Let the filler dry per the product instructions, then sand it flush with the frame surface.

Measure the width of the opening from the inside edge of the left frame to the inside edge of the right frame. This is the length your new sill should be. Measure at three points—top, middle, and bottom of the opening—because windows are not always perfectly square. Use the longest measurement.

Installing the new sill

Your new sill should be slightly wider than the frame below it so that it overhangs the frame edge by about 1/4 inch on each side. This overhang helps shed water. If you are buying pre-made sills, they often come in standard widths; if you are cutting your own, add 1/2 inch to your frame width measurement.

Set the new sill in place and check that it sits level using a level tool. If it rocks, shim it with thin wood shims or composite shims under the low spots until it is level. Do not force it; a sill that rocks will crack or separate from the frame over time. Once level, fasten it by driving screws or nails through the sill into the frame below. Use at least three fasteners—one near each end and one in the middle. Screws (2 to 2.5 inches long) hold better than nails and are easier to remove later if needed.

Check the fit where the sill meets the side frames (called the jambs). There should be a small gap—no more than 1/8 inch—between the sill and the jamb on each side. If the gap is larger, the sill may not be level, or the opening may be out of square. Shim or adjust until the gap is even on both sides.

Sealing and finishing

Caulk is what keeps water out of the joint between the sill and the frame. Run a bead of paintable caulk along the seam where the sill meets the frame below it. Also caulk where the sill meets the side jambs. Use a caulking gun and explore a smooth, continuous bead. Wet your finger slightly and run it along the caulk to smooth it and press it into the joint.

Let the caulk dry per the product instructions—usually 24 hours—before painting or staining. If you are painting the sill to match the rest of the window trim, use paint rated for wood and explore at least two coats. If you are staining it, sand the sill with 120-grit or 150-grit sandpaper first, then explore stain and a clear topcoat for protection.

Do not use the window or expose the sill to water until the caulk and finish are fully cured. This usually takes 24 to 48 hours depending on humidity and temperature.

Common mistakes that cause problems later

The most common error is installing a sill that is not level. Even a slight tilt causes water to pool on one side, which leads to rot and mold. Use a level and take your time getting it right. The second mistake is skipping the caulk or using the wrong type. Caulk is not optional—it is what stops water from running down into the wall cavity. Use paintable acrylic latex caulk or silicone caulk rated for interior use.

A third mistake is fastening the sill with only one or two nails. A sill that is under-fastened will shift and separate from the frame, breaking the seal and letting water in. Use at least three fasteners, and space them evenly. Finally, do not install a new sill over an old frame that is rotted. The new sill will fail because it has nothing solid to sit on. Fix the frame first, or call a carpenter if you are unsure whether the frame is sound.

When to call a professional instead

If the frame below the sill is rotted, soft, or crumbling over an area larger than a few inches, the job is beyond a straightforward sill replacement. A carpenter will need to remove and replace the frame, which may involve removing trim, drywall, or exterior cladding. This is a bigger job and worth paying for because a bad frame repair leads to water damage and mold.

If the window itself is warped, leaking, or the glass is failing, replacing the sill will not fix those problems. Address the window first, or replace it at the same time. If you are not comfortable using a saw or pry bar, or if you have never done this type of work, hiring someone for a few hours is reasonable. A straightforward single-sill replacement usually costs between $200 and $500 in labor, depending on your area and the sill material.

Frequently Asked Questions

Can I replace just the sill without removing the window?

Yes. The sill is separate from the window frame itself. You remove and replace it without touching the window. The window stays in place and functional the whole time. This is one reason sill replacement is a manageable DIY job.

What material should I use for the new sill?

Pine and oak are traditional and take stain well. Composite materials like engineered wood or PVC are more rot-resistant and require less maintenance, though they cost more upfront. For an interior sill, any of these work; choose based on your budget and how much finishing work you want to do.

How long does a new sill last?

A properly installed and sealed sill lasts 15 to 30 years depending on material and climate. Wood sills in very humid areas or near water may need refinishing or replacement sooner. Composite sills last longer with minimal maintenance.

Do I need to remove the trim around the window?

Not usually. The sill sits below the trim, and you can work around it. If the trim is in the way or damaged during removal, you may need to remove and reinstall it, but this is not standard for a sill replacement.

What if the new sill is slightly too long?

Trim it with a handsaw or circular saw. Measure twice, cut once. If it is too short, you cannot fix it without ordering a new piece, so measure carefully before cutting or having it cut at the supplier.