Repair a rotted window sill

Why window sills rot

The sill is the sloped board at the bottom of the window frame, and on the outside it takes more water than any other part of the window. Rain sits on it, paint and caulk crack where it meets the frame, and condensation runs down the glass onto it from inside. Once water gets into the wood and stays, rot fungi break the fibers down until the wood turns soft and spongy.

Ordinary wood filler shrinks and cracks outdoors. Two-part epoxy fillers made for wood repair bond to the surrounding wood, don't shrink, and can be shaped and painted like wood, which is why they're used for sill repairs. A liquid epoxy consolidant soaked in first hardens slightly soft wood so the filler has something solid to grip.

Common causes of sill rot

Let's fix it

1. Find the soft woodPush an awl or screwdriver into the sill every inch or so, inside and out. Sound wood resists; rotted wood lets the tip sink in easily or feels spongy. Mark the edges of the soft area with a pencil. In a home built before 1978, test the paint for lead before scraping or sanding.
   Done

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2. Decide repair or replaceEpoxy repair works well when the rot is in a limited area and the rest of the sill is firm. If the soft area covers most of the sill, the rot goes all the way through, the sill moves when you push it, or the framing under the window is soft too, replacing the sill is the better fix, and usually a job for a carpenter.
   Done

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3. Dig out the rotUse a chisel and a 5-in-1 tool to remove all the crumbly, soft wood down to firm wood. Slightly soft wood at the edges can stay, since the consolidant will harden it. Undercut the edges a little so the filler locks in, and vacuum out the debris.
   Done

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4. Let it dryThe wood has to be dry for epoxy to bond. Cover the area from rain with plastic taped above it, and give it a few dry days, or use a fan. Rotted wood near the surface usually holds more water than it looks.
   Done

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5. Harden it with consolidantBrush or squeeze liquid epoxy consolidant into the cleaned-out area until the wood stops soaking it up, following the product's directions. It soaks into the remaining fibers and hardens them. Let it cure until it's tacky or hard, as the label says, before filling.
   Done

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6. Fill with epoxyMix the two-part filler in the ratio on the package until it's one even color. Press it firmly into the hole with a putty knife, working out air pockets, and overfill slightly. For a deep hole, build it up in layers. Shape it roughly to match the sill's slope and edges while it's still soft.
   Done

   Something went wrong?

   • The filler sags out of the hole Build it up in thinner layers and let each set.

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7. Sand and shape itOnce the filler has fully cured, usually overnight, sand it flush and smooth with 80-grit, then 120-grit paper on a sanding block. Keep the sill's slope so water runs off it. Wipe off the dust.
   Done

   Something went wrong?

   • The filler is still soft Give it more time or warmth; check the mix ratio next time.

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8. Prime and paintPrime the repair and any bare wood with an exterior primer, then apply two coats of exterior paint over the whole sill. Paint seals the wood against the water that caused the rot.
   Done

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9. Caulk and keep water offCaulk the joints where the sill meets the window frame and the siding or trim, but leave any weep holes or drip gaps open. If condensation on the inside glass was soaking the sill, we have a guide for condensation on windows.
   Done

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Watch it done

How to Fix Rotted Wood with EpoxyBy This Old House on YouTube. Their method may vary a little from ours.

Watch on YouTube

If it's paint or condensation

If water forms on the inside of the glass, see the guide for condensation on windows.

If the paint is peeling but the wood underneath is sound, check our guide for fixing peeling paint.

Rotted window sill questions