How to Rebuild Wooden Window Sills

The simplest way to rebuild a wood window sill is to cut and install a new piece.

The sill is the horizontal plate that rests in the bottom of the window frame, directly below the bottom sash when it is closed. Sills typically extend into the interior space; some have adequate depth for holding plants and other small decorative items. Rebuilding a broken or rotted sill will consist of removing the old sill, then cutting a replacement piece and installing it. Choose wood in a matching grain for stained sills; pine works well for painted sills.

Things You'll Need

• Hammer
• Pry bar
• 1-by-10 pine lumber
• Jigsaw
• Sander
• Sandpaper
• Rasp
• Paintbrush
• Primer
• Stain
• Finish nails
• Paint
• Clear finish

Instructions

1. Lift the sash to the open position and prop it in place, if needed. Tap upward on the bottom of the sill with a hammer to loosen it. Insert a flat pry bar into the gap created by tapping the sill upward. Pry the sill up farther until one side can be lifted out of the frame. Use the hammer and pry bar to complete the removal of the sill.

2. Position the old sill on top of the 1-by-10 lumber and mark out the outline with a pencil on the face of the board. Cut out the new sill with a jigsaw. Sand the edges smooth with a power sander and 100-grit paper.

3. Sand both faces of the sill and apply a coat of primer, if the sill is to be painted, to all faces of the sill. Use a fine-bristle brush with long, straight strokes, working to apply the primer as smoothly as possible to prevent runs and drips. Allow the primer to dry for at least an hour before proceeding. Apply a coat of stain in a matching color for stained window frames. Use the same application technique.

4. Position the new sill in the window frame. Tap it into place with a rubber mallet. Use a rasp to trim down any edges that are too tight, for a clean fit. Nail the new sill in place with 2-inch finish nails, one every 6 to 8 inches.

5. Apply two coats of interior, semigloss latex paint to sills in painted frames, using a matching color. Apply two or three coats of clear finish in a matching sheen to stained sills. Use a fine-bristle brush to apply the finishes and follow label directions for best results.

References

• Smith & Company: Window Sill Repair
• "Ms Fix-its Door and Window Repairs"; Annette L. Bach; 1974