How to Replace & Repair a Window Sill

Windowsill repairs should be made right away to avoid more damage

A windowsill is the horizontal board that rests underneath the sash. The sash is the frame of the window that holds the glass. Sills are exposed to weather and abuse. In the normal course of their lives they may become chipped or split or suffer rot from moisture. Repairing or replacing a sill helps to prevent damage to the interior frame of the window by sealing it from the outside elements.

Things You'll Need

• Wood filler
• Putty knife
• Caulking
• Primer
• Paint
• Flat pry bar
• Hammer
• Lumber for sill
• Circular saw
• Jig saw (optional)

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Instructions

Sill Repair

1. Repair small chips or splits with solvent-based wood filler. Scrape the surface of the sill to remove any debris from the damaged area. Fill the damaged area with filler using a flexible putty knife to level it off.

2. Allow the filler to harden completely before sanding the area smooth. Use fine-grit sandpaper by hand or with a power sander.

3. Seal any gaps in the seams around the sill with painter's caulk for painted windows or clear latex caulk for stained windows.

4. Prime a repaired sill with oil-based primer, and seal the repair with at least two coats of latex paint. Make sure the paint dries well before closing the sash onto painted areas.

Sill Replacement

1. Remove a badly damaged sill for replacement. Remove the horizontal trim underneath the sill with a hammer and flat pry bar. Work the nails loose gently to avoid splitting the trim if you intend to reuse it.

2. Open the window sash completely. Tap against the bottom of the sill with a hammer to release it from the nails, glue and paint that hold it down. If needed, use a pry bar to lever the sill out of the window.

   Cut stubborn sills with a circular saw set just deep enough to cut through the sill. Cut the sill in the center of the window, and remove the two pieces.

3. Use the old sill as a template to cut a new lumber sill. Choose or cut lumber to the correct thickness and width. Cut the board to length, and then add any notches or cut away areas on the ends, measuring the original carefully. Sand the new sill well, beveling the ends slightly for a smooth fit.

4. Tap the new sill into place with a rubber mallet or hammer and wood block. Nail the sill into place, and caulk around all sill edges on the top and bottom. Replace the horizontal trim under the sill. Caulk the seams around the trim. Cover the new sill with a coat of oil-based primer. Allow the primer to dry thoroughly.

5. Replace the horizontal trim under the sill. Caulk the seams around the trim. Cover the new sill with a coat of oil-based primer. Allow the primer to dry thoroughly.

   Apply two coats of latex paint over the dried primer, and allow it to dry thoroughly before closing the window.

References

• GSA.gov: Replacing a Wooden Window Sill
• Hammerzone: Fixing a Rotted Wood Window Sill