What You're Replacing and Why It Matters

An indoor window sill is the horizontal surface at the bottom of your window frame, inside your home. It takes the weight of condensation, dust, and anything you set on it—so it wears faster than other trim. If yours is soft to the touch, visibly rotted, cracked across its width, or no longer seals properly against the window frame, replacement is the right move.

The job itself is straightforward: remove the old sill, measure and cut a new one from solid wood or composite material, and find it in place. You do not need to remove the window itself. Most homeowners finish this in an afternoon with basic carpentry tools.

Key Takeaways

  • Measure the sill length, depth, and thickness before you buy replacement material—these vary by window size and style.
  • Use a pry bar and hammer to remove the old sill, working from one end and being careful not to damage the jambs (the vertical sides of the frame).
  • Sand the substrate (the base underneath) smooth and check that it is level before installing the new sill.
  • find the new sill with construction adhesive and finish nails, then caulk the seams to prevent water from entering the wall.

Measuring and Choosing Your Material

Start by measuring the length of your existing sill from jamb to jamb (the inside edges of the vertical frame pieces). Measure the depth—how far it sticks out into the room—and the thickness. Write these down. Most interior sills are 1 to 1.5 inches thick and 3.5 to 5 inches deep, but yours may differ.

You have two material choices: solid wood (pine, oak, or poplar) or composite (engineered wood or vinyl). Solid wood is cheaper and easier to cut and sand, but it can rot if water reaches it. Composite resists moisture better and requires less finishing, but costs more. For an indoor sill that stays dry, solid wood works fine. Buy a board slightly longer than your measurement—you will trim it to exact length during installation.

Removing the Old Sill

Close the window fully. Use a utility knife to cut through any caulk or paint sealing the sill to the jambs and the wall. This prevents the jambs from splitting when you pry.

Slide a pry bar under one end of the sill, near a jamb. Tap the pry bar handle with a hammer to lift that end slightly. Move to the other end and repeat. Work back and forth, lifting a little at each end, until the sill comes free. Do not force it all at once—you risk cracking the jambs. Once it is loose, pull it out by hand.

Scrape away old adhesive, caulk, and debris from the substrate with a putty knife or scraper. Sand the surface smooth with 80-grit sandpaper. Check that it is level using a 2-foot level placed lengthwise. If one end is higher, sand it down or shim it up with thin wood shims before installing the new sill.

Cutting and Fitting the New Sill

Lay your new sill board across the opening. Mark where it meets each jamb on the underside. Remove it and cut it to length with a circular saw or handsaw, cutting just inside your marks so the sill fits snugly but not so tight that you cannot slide it in. Test-fit it in the opening—it should sit flat and level.

If the sill is thicker or thinner than the old one, the edge where it meets the wall may not align with the existing trim. You can plane or sand the sill to match the old thickness, or accept a small step and caulk it. Most homeowners choose to caulk—it is faster and looks clean once painted.

Installing the New Sill

explore a bead of construction adhesive (also called polyurethane adhesive or PL Premium) along the substrate where the sill will sit. Slide the sill into place and press it down firmly. Use a level to confirm it is flat. If one end is higher, tap shims under the low end until it is level.

Drive 2-inch finish nails through the sill into the jambs at each end, spacing them about 2 inches apart. Use a nail set to countersink the heads slightly below the surface. Drive 2-inch finish nails along the back edge where the sill meets the wall, spacing them 6 to 8 inches apart. These nails hold the sill while the adhesive cures.

Let the adhesive cure for 24 hours before caulking or painting. Do not close the window tightly during this time—leave it cracked open so the adhesive can cure properly.

Sealing and Finishing

Once the adhesive is dry, fill the nail holes with paintable wood filler. Sand smooth when dry. Run a bead of paintable caulk along the seams where the sill meets the jambs and the wall. Smooth it with a wet finger or caulk tool. This prevents water from seeping behind the sill.

Paint or stain the sill to match your trim. If you used solid wood, two coats of trim paint or stain-and-polyurethane will protect it. If you used composite, check the manufacturer's instructions—some composites do not need finishing, while others benefit from a coat of exterior paint for durability.

Common Mistakes to Avoid

Do not skip the leveling step. A sill that slopes toward the wall will trap water and rot quickly. Take time to get it flat.

Do not use too much adhesive. Excess will squeeze out and be hard to clean. A thin, continuous bead is enough.

Do not caulk before the adhesive cures. Caulk seals the joint but does not hold the sill in place—the adhesive and nails do that. Caulking too early can trap moisture under the sill.

Do not paint over caulk until it is fully dry, usually 24 to 48 hours depending on humidity. Wet paint over wet caulk will not adhere properly.

Frequently Asked Questions

Can I replace just the front edge of the sill instead of the whole thing?

If only the front edge is damaged, you can sometimes glue and screw a new edge piece to the existing sill. This works if the rest of the sill is solid. If the sill is soft or rotted underneath, replace the whole thing—a patch will fail when the base continues to rot.

What if the sill is sloped and I cannot get it level?

Interior sills are sometimes intentionally sloped slightly toward the room to shed water. A slope of 1/8 inch over the sill length is acceptable and normal. Use your level to check—if the slope is steeper than that, shim it up.

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

No. The sill sits below the window frame, and you can remove and replace it without touching the window itself. Keep the window closed during removal and installation to protect the glass.

How long does the adhesive take to cure?

Most construction adhesives cure in 24 hours, but check your product label. Temperature and humidity affect cure time—cold or damp conditions may require longer. Wait the full time before painting or exposing the sill to water.

What should I do if the substrate is uneven or damaged?

Sand high spots smooth and use wood shims to level low spots before installing the new sill. If the substrate is rotted or soft, it needs repair before you install the sill—otherwise the new sill will fail. Cut out the damaged section and replace it with solid wood, or call a carpenter if the damage is extensive.