Introduction

Man installing new windowsill
Photo: Ryan Benyi
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Q: One of our windowsills appears to be rotting. Can it be replaced without buying a new window?
—Bill Bailey, Baldwin, Mo.

David Raymond, Raymond Design Builders, replies: Sticking out into the weather year after year, wood windowsills take quite a beating. They can last for decades if you just keep them clean and protect them with a coat of paint. But when water gets trapped in or against them, as happens with improperly flashed windows, rot can take hold.

A simple epoxy repair will do the trick if the rot is confined to a small area, but if more than 10 percent of the sill is rotten, the old sill should be cut off and replaced. For this project, I used a solid piece of Kleer cellular PVC sill that matches the profile and thickness of the other sills on the house. It costs less than a piece of clear, milled red cedar and is paintable, but no matter how many times it gets wet, it can't rot.
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    Tools List

    • circular saw
      Circular saw
    • detail sander
      Multitool
    • drill
      Drill/driver
    • hammer
      Hammer
    • flat prybar
      Pry bar

    Shopping List

    1. 8d stainless-steel ring-shank nails
    for casing

    2. 3½-inch deck screws for sill

    3. Two-part acrylic adhesive (The brand used here is Bond&Fill.)4. Cellular-PVC windowsill (The brand used here is Kleer Lumber, which has the most sill options. AZEK also makes a PVC sill.)