Can a rotted window sill be repaired, or does the whole window need replacing?

WEST SYSTEM, the marine epoxy maker, says the wood under a rot repair should be below 12 percent moisture content before any epoxy goes on. That one number tells you most of what you need to know about a rotted window sill: the soft wood is the symptom, and the water that softened it is the problem. Fix the wood without stopping the water and you'll be doing it again in a few years.

A lot of sill rot is smaller than it looks. Peeling paint and a dark corner stain can sit on an inch of soft wood or on a sill that's gone all the way through. A screwdriver tells you which in five minutes.

Probe it before anyone quotes it

Press a flat screwdriver or awl into the sill with firm hand pressure. Don't stab or pry. Start in sound wood near the middle so you know what solid feels like, then work toward the stain. Check both outside corners, the front edge, the joints where the sill meets each side jamb, and the bottom of each jamb. Those joints hold water longest.

For each soft spot, note how far the tip sinks, how long the soft area runs, and whether the wood under it is firm.

The half-inch line comes from Gougeon Brothers, the company behind WEST SYSTEM. Its window repair guide says that once a cavity is more than 1/2 inch deep and 1/4 inch wide, you cut and fit a wood plug to fill the bulk of it rather than filling it all with epoxy. The cost column is not from a primary source. It reflects common contractor pricing and moves with your region, the window's height, whether the sill is a separate piece or milled into the frame, and how much paint has to come off. Treat it as scale, not a quote.

Find the water first

Sills rot because water reaches them and can't leave. The usual sources fall into three groups: paint failure that lets rain soak the top of the sill, a missing or failed sill pan that sends water past the frame into the wood below, and indoor condensation running down the glass onto the interior stool.

Look at where the rot is. Rot on the top and front edge, with the inside dry, is weather. Rot that's worse underneath, or that shows up as a stain on the wall below the window, points to water getting behind the window. IRC Section R703.4 requires corrosion-resistant flashing at exterior window openings, applied to keep water out of the wall cavity and off the structural framing. A sill that's rotting from the bottom up usually means that flashing is missing or failing, and our window leak guide walks through how to tell. Rot on the interior stool with dripping glass on cold mornings is a humidity problem; the condensation guide covers it.

EPA's mold guidance says to dry water-damaged materials within 24 to 48 hours, and that cleanup without fixing the water source usually fails. Wood is no different: epoxy over a sill that's still getting wet seals the moisture in.

What a proper epoxy repair involves

A penetrating epoxy repair is a real fix, not a cover-up, when it's done in order. WEST SYSTEM's Wood Rot Repair Kit lists windowsills by name. Remove all loose and weathered fibers first, because debris blocks the epoxy from reaching sound wood. Dry the wood below 12 percent moisture. Apply the thin epoxy into cracks and over the surface until the wood stops absorbing it. Then thicken it with filler to rebuild the shape.

The details are where DIY repairs fail. WEST SYSTEM lists 30 to 40 minutes of working time at 72°F and a 50°F minimum, so a cold late-fall morning won't cure properly. Keep each layer under 1/2 inch thick; Gougeon's guide warns that too much epoxy at once can exotherm and smolder. After curing, wash off the amine blush, sand, and finish with a UV-stable topcoat.

The kit comes in 360 ml, 1.5 liter, and 6 liter sizes. One corner rarely needs more than the smallest.

Flat rule: if more than about a third of the sill's length is soft, stop patching. Epoxy rebuilds a corner well. It's a poor substitute for a whole sill, and once you've dug out that much wood, a new sill often costs less than the labor to fill it.

When the house was built before 1978

Scraping old window paint is exactly what EPA's lead rules target. Under the Renovation, Repair and Painting rule, a contractor paid to disturb paint in a pre-1978 home must be an EPA-certified firm using lead-safe practices. The minor repair exception covers jobs disturbing no more than 6 square feet of interior paint per room or 20 square feet outside, but EPA says it doesn't apply to window replacement or to prohibited practices. Ask any contractor bidding sill work on an older house for their RRP certification. Doing it yourself isn't covered by the rule, but the lead hazard is the same.

Repair or replace: three questions decide it

First, is the rot contained to the sill? If the jambs and framing below are firm, the window can usually stay. If they're soft, the window has to come out to repair them, and you're paying for most of a replacement's labor anyway.

Second, is the rest of the window failing too? ENERGY STAR's guidance on when to replace lists degraded sills and sashes, drafts and leaks, and double-pane glass that's cloudy between the panes. One of those is a repair. Two or three on the same window is a replacement. If glass is the other problem, our foggy window guide prices a glass-only fix.

Third, is it under warranty? Andersen's 400 Series coverage runs 20 years on glass and 10 on non-glass parts, against defects in materials and workmanship. Rot from unmaintained paint or a poorly flashed opening usually falls outside that. A recent clad window rotting behind intact cladding is worth a call to the installer and manufacturer before you pay anyone.

Our rule: rot in the sill alone, on a window that otherwise works, gets repaired. Rot that has reached the framing gets a full-frame replacement. A pocket insert set into a frame with soft wood around it seals the rot in. Our full-frame vs. pocket guide explains why that matters, and the quote guide shows how to get a rot allowance written onto the bid.

Where you stop and call a pro

A ground-floor sill you can reach standing up is reasonable to probe and repair yourself. Hire it out for any window above the first floor that needs a ladder, rot that has reached the wall framing, any window that has to come out of the opening, and paid work on a pre-1978 house. Pulling a window means working with the wall's water barrier and possibly the header, and a mistake there starts the next rot problem.

For labor scale, BLS puts the median carpenter wage at $28.51 an hour as of May 2024. A contractor's billed rate runs well above that once overhead and insurance are added, so even a half-day repair lands in the hundreds.

Research sources

• WEST SYSTEM: Wood Rot Repair Kit
• Epoxyworks (Gougeon Brothers): Wooden Window Repair, step-by-step with WEST SYSTEM Epoxy
• Epoxyworks (Gougeon Brothers): New WEST SYSTEM Wood Rot Repair Kit
• ICC: 2021 International Residential Code, Section R703.4, Flashing
• EPA: A Brief Guide to Mold, Moisture and Your Home
• EPA: How Is Window Repair or Maintenance Distinct From Removal Treated Under the RRP Rule?
• EPA: Window Repairs, Removing and Replacing Sash, Under the RRP Rule
• ENERGY STAR: Is It Time to Replace Your Windows?
• Andersen: product brochure with Owner-2-Owner warranty terms (PDF)
• BLS Occupational Outlook Handbook: Carpenters