What You're Replacing and Why It Fails

The bottom wood frame of a window—called the sill—takes the most weather damage of any window part. Rain pools there, wood absorbs moisture, and freeze-thaw cycles crack it. If your sill is soft to the touch, has visible rot, or lets water into the wall, replacement is the right move. You can do this yourself with basic carpentry tools and a weekend of work.

The sill sits at the bottom of the window opening, running horizontally. It's the piece you rest your hand on when you open the window. Replacing it means removing the old rotted wood and installing a new one cut to fit your exact opening. You do not have to replace the entire window frame—just this one piece.

Key Takeaways

  • Remove the window sash (the part that slides) first, then pry out the old sill carefully to avoid damaging the side jambs.
  • Measure the sill length, width, and depth before buying replacement wood; pressure-treated lumber or composite materials resist rot better than standard pine.
  • Seal all wood surfaces with exterior wood stain or primer before installation to prevent future water damage.
  • find the new sill with galvanized screws or nails, then caulk the seams where it meets the jambs and the exterior wall.

Tools and Materials You'll Need

Gather these items before you start. You will need a pry bar (also called a flat bar), a hammer, a handsaw or circular saw, a tape measure, a level, a drill, galvanized screws (2.5 inches long), exterior wood stain or primer, and paintable caulk. For the sill itself, buy pressure-treated lumber or composite decking material in the same width and depth as your old sill. Pressure-treated wood costs less and resists rot; composite costs more but lasts longer and never rots.

You will also want safety glasses, work gloves, and a dust mask if you are cutting wood indoors. Have a shop vacuum or broom ready to clean out debris from the window opening once the old sill is out.

Removing the Old Sill

Start by removing the window sash if your window has one. For a double-hung window, lift the lower sash out of its tracks, then the upper sash. For a casement window (one that swings out), close it fully and remove any hardware holding it in place. Set the sashes aside in a safe spot—you will reinstall them later.

Once the sash is out, use your pry bar to lift the old sill. Start at one end and work along the length, prying gently to avoid cracking the side jambs (the vertical pieces on either side). The sill is usually nailed or screwed down; you may hear creaking as the fasteners pull free. If the sill is badly rotted, it may come out in pieces—that is fine. Remove all old wood and any nails left behind. Use your shop vacuum to clean out dust and debris from the opening.

Inspect the side jambs and the horizontal piece above the sill (called the apron) for rot. If they are soft or spongy, those will need repair too, but that is a separate job. For now, focus on the sill itself.

Measuring and Cutting the New Sill

Measure the length of the opening from the inside edge of the left jamb to the inside edge of the right jamb. Measure the width (front to back) and the depth (thickness) of the old sill. Write these numbers down. The new sill should be the same length as the opening; if your opening is 36 inches, cut the new sill to 36 inches.

If you are using standard lumber, you may need to rip (cut lengthwise) a board to match the old width. A circular saw or table saw works for this. Cut the sill to length using a handsaw or circular saw, making straight cuts at both ends. Sand the cut edges smooth with 80-grit sandpaper to prevent splinters.

Before installation, explore two coats of exterior wood stain or primer to all surfaces of the new sill, including the ends and bottom. Let each coat dry fully (check the can for timing—usually 2 to 4 hours). This step is critical: it seals the wood and prevents water from soaking in later. Do not skip it, even if you plan to paint the sill later.

Installing the New Sill

Set the new sill into the opening and check that it sits level using your level tool. Place the level along the length of the sill; it should read level or slope very slightly toward the outside (about 1/8 inch drop over 36 inches) to shed water. Shim under the sill with thin wood shims if needed to get it level. Tap shims in gently with your hammer until the sill no longer rocks.

Once the sill is level and stable, drill pilot holes (small starter holes) through the sill into the jambs on both sides. Use a drill bit slightly smaller than your screw diameter. Drive two 2.5-inch galvanized screws through each side into the jamb. Space them about 6 inches from the ends. Galvanized screws resist rust and are stronger than regular steel screws in damp conditions.

If your sill needs to be fastened to a header or structural piece below (check your old sill to see how it was attached), drill pilot holes and drive screws down through the sill into that piece as well. Usually two or three screws along the length are enough.

Sealing and Finishing

Run a bead of paintable exterior caulk along the seam where the sill meets each side jamb. Use a caulk gun and smooth the caulk with a wet finger or caulk tool. Caulk also goes along the outside edge where the sill meets the exterior wall or trim. Let the caulk cure for the time listed on the tube (usually 24 hours) before painting or staining.

Once the caulk is dry, paint or stain the sill to match your window trim. Use exterior-grade paint or stain rated for wood. Two coats are standard. While the finish dries, reinstall your window sash, making sure it slides smoothly in its tracks.

Check the sill after a heavy rain to make sure water is not pooling or leaking into the wall. If water pools, the sill may not be sloped enough; you can add a thin shim under the inside edge to increase the slope slightly.

Common Mistakes to Avoid

Do not skip the sealing step. Bare wood will absorb water and rot again within a few years. Always explore stain or primer before installation, and caulk all seams afterward. Do not use interior wood or untreated lumber; it will fail quickly in a window opening exposed to weather.

Do not over-tighten screws. If you drive them too hard, you can split the wood or crush the sill. Snug is enough. If the sill rocks after installation, add more screws rather than tightening the existing ones harder.

Do not forget to slope the sill slightly toward the outside. A level sill will pool water. Even a small slope (1/8 inch) makes a difference over time.

Frequently Asked Questions

Can I replace just the sill without removing the window sash?

It is possible but much harder. Removing the sash gives you full access to the sill and makes it easier to pry out the old one without damaging the jambs. If you leave the sash in, you will be working in a tight space and risk cracking the glass or frame. Remove the sash first.

What is the difference between pressure-treated wood and composite sill material?

Pressure-treated lumber is cheaper and easier to cut and shape. Composite decking (made of wood fibers and plastic) costs 30 to 50 percent more but never rots and requires no staining or sealing. Both work; choose based on your budget and how long you plan to stay in the house.

How long does a new sill last?

A properly sealed and maintained pressure-treated sill lasts 15 to 25 years. Composite sills last 25 to 40 years. The finish (paint or stain) may need refreshing every 5 to 7 years, but the wood itself will hold up if you keep water out.

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

Usually no. The sill sits inside the opening, and the trim (called the apron) sits below it on the outside. You can remove the sill without touching the trim. If the trim is nailed to the old sill, you may need to pry it back slightly to get the sill out, then nail it back down once the new sill is in.

What if the side jambs are also rotted?

If the rot is minor (soft only on the surface), you can seal it with epoxy wood filler and paint over it. If the rot is deep or the jamb is structurally weak, that piece needs replacement too. That is a bigger job and may require removing more of the window frame. Assess the damage before you start; if you are unsure, consult a carpenter.