How to Repair a Rotted Windowsill
A quick guide to fixing exterior window trim without replacing the window
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.
Step-by-step tutorial renders beneath OTHER VIDEOS player. Keep scrolling!
Remove the Side Casings
The casing comes off to clear the way for removing the old sill. If the casings are rotting, as here, replace them, too. Slice through the caulk bead between the siding and the casing, then carefully pry off each piece. Save the trim to use as templates for the new ones. Make sure the flashing behind the casing is intact and properly installed behind the siding.