How to Replace Outside Window Sills

A window won't seal properly if the sill is rotted.

Because of its location directly under the window, an exterior window sill is more vulnerable than any other part of the window casing. Even one made of rot-resistant wood like redwood or cedar, and covered with several coats of paint, will eventually deteriorate if it is in the sun, or a particularly rainy climate. Replacement is a straightforward operation that is only slightly more complicated if you have to do it from a ladder, and will take about two hours.

Things You'll Need

• Pry bar
• Hammer
• Reciprocating saw
• Tape measure
• 2-by-6-inch lumber
• Circular saw
• Pencil
• Jigsaw
• 3-inch galvanized nails
• 1-by-3-inch lumber
• 2-inch galvanized finish nails
• Wood filler
• Siliconized acrylic caulk
• Caulking gun
• Wood primer
• Paint
• Paintbrush

Instructions

1. Open the window and remove top plate off of the sill with a pry bar. Wedge the bar under the plate and tap the end with a hammer to work it underneath. Pry the plate up until the heads of the nails pop loose, then pull the nails out with a hammer. When all the nails are out, remove the plate.

2. Pry off the sill in the same way. Try to keep it intact so you can use it as a template to cut another one. If the ends of the sill are wedged under the window casing, pry it up a little and cut the nails holding the sill to the framing with a reciprocating saw. When you have cut all the nails, remove the sill by pulling it toward you.

3. Clean away all rotted material from the window frame. Inspect the wood framing for rot. If the sill was severely rotted, the rot may have spread to the framing, and you may have to make some structural repairs.

4. Measure the length of the sill with a tape measure and cut a new piece of 2-by-6-inch lumber to the same length with a circular saw. Lay the sill on top and outline the edges with a pencil. Cut along the outlines with a jigsaw.

5. Set the new sill in place in the window frame and push it into place. Tap it gently with a hammer if it fits tightly. Secure it to the frame by pounding in 3-inch galvanized nails under the location of the top plate.

6. Cut a new top plate from 1-by-3-inch lumber. You can use the old top plate if it is in good enough condition. Set the top plate into position and nail it in place with 2-inch galvanized finish nails. Sink the heads of these nails with a nail punch, and then cover the holes with wood filler.

7. Caulk around the top, bottom and sides of the sill and the top plate with siliconized acrylic caulk. Cover the new wood with wood primer, then paint it with the color of the window casing, using a paintbrush.

Tips & Warnings

• Because the sill has to withstand moisture, make it from a weather-resistant wood like redwood or cedar.

• If you are working from a ladder, take extra time to secure the ladder at the bottom to prevent it from sliding out, and at the top to prevent it from tilting. These precautions will prevent you from falling if you have to use force to remove the sill.

References

• HGTV: Replace Rotted Windowsill
• This Old House: Repairing Rotted Window Sills