Window sill rot: when to repair it and when to replace the sill

A rotted window sill can usually be repaired if the soft spots are shallow and the sill still holds its shape. If a screwdriver sinks in more than about a quarter inch in several places, the rot has reached the ends, or the wood under the sill is soft too, replace the sill instead of patching it. On older Saratoga Springs houses the sill tends to be the first part of a window to go, since it's the one piece built to catch water and throw it off.

Why the sill goes first

The sill is the sloped piece of wood at the bottom of the outside of the window frame. Rain running down the glass and sash lands on it. Snow sits on it for weeks. After enough decades the slope flattens out in spots and meltwater pools there. Once the paint cracks, water soaks into the end grain and into the joints where the sill meets the side casings, and every winter freeze-thaw pries those cracks a little wider.

Peeling paint left for more than a season or two speeds it along. So does a sill that's lost its pitch, or failed caulk where the sill meets the casing and siding. Gutters that overflow dump water right onto the window below. And old storm windows with clogged weep holes trap water against the sill.

Sills take more water than any other trim, so they need paint a bit sooner than the casings around them. Our post on how often to paint exterior trim covers the schedule for the rest.

Test every sill with a screwdriver

Take a flat screwdriver or an awl and push it hard into the wood at several spots. Spend most of your time on the outer edge, the two ends and anywhere the paint has lifted.

If the tip barely dents the wood, it's sound. Scrape, prime and paint any bare spots and move on. Soft for an eighth to a quarter inch over a small area is surface rot, and that can be repaired. Soft and spongy across most of the sill is deep rot; plan on a new sill. If the wood crumbles or breaks off in chunks, the rot's advanced, and you need to look at the framing under it before anything else.

Then go inside. Peeling paint on the interior stool, a stain on the wall below, a musty smell under the window, or soft drywall or plaster beneath the sill all mean water is getting past the outside and into the wall.

Patching a sill that's mostly sound

Rot in one spot on an otherwise solid sill can be fixed with a wood consolidant and a two-part exterior epoxy filler. The order matters. Dig out every bit of soft wood until the tool hits firm fiber. Let the hole dry out completely, which in a damp fall can take days. Brush on a liquid consolidant, which soaks into what's left and hardens it. Then press in the epoxy, shape it to match the original profile, and sand it once it cures. Prime it and paint it with the rest of the trim.

Don't bother with ordinary wood putty. It's meant for nail holes indoors and falls apart outside.

A careful epoxy repair can last many years, because epoxy doesn't soak up water the way wood does. The weak spot is the line where filler meets wood, so keep the paint and caulk there in good shape.

If the middle of the sill is sound and one section is gone, a dutchman is the other route. You cut out the bad section and fit a piece of new wood in its place. It keeps as much of the original as possible.

Replacing the sill

When the rot runs the length of the sill, gets into the joints with the side jambs, or has spread to the framing under it, the sill comes out. A lot of older windows in Saratoga Springs were built with the sill let into the jambs, so it has to be cut out in pieces without wrecking the frame or the interior stool.

Make the new one from something that resists rot. That can be a durable species of solid wood, primed on all six sides before it goes in, or a cellular PVC or composite profile where the look allows. Either way it needs a proper slope away from the house, a drip groove cut on the underside so water drops clear instead of running back to the wall, and good flashing or sealant at each end.

With the old sill out, look at the rough opening underneath. Soft framing, or stains running down the inside of the wall, get fixed before the new sill goes in. Sometimes rot under a window means the whole unit is leaking, and rebuilding or replacing the window makes more sense. If the sash, jambs and glazing are in reasonable shape, though, a rotted sill doesn't mean you need a new window. The sill can be repaired or replaced on its own.

Lead paint and the historic districts

A lot of Saratoga Springs houses were built before 1978, so the trim may have lead paint under the newer coats. Scraping, sanding or cutting out a sill disturbs it. Test it first. If there's lead, the work should be done with lead-safe practices by someone trained in them.

In the city's historic districts, exterior changes that alter the look, like swapping a wood sill for a different material or profile, can need Design Review Commission approval. Like-for-like repair is generally simpler, but check with the city building department before you settle on a material. Our post on restoring an old house in Saratoga Springs talks about how we handle original trim more generally.

Stop the water or it'll rot again

Fix the cause at the same time or you'll be doing this again. Clean the gutters above the window and make sure they aren't overflowing onto it; gutter, soffit and fascia rot explains what to look for up there. Clear the weep holes in aluminum storm windows every fall. Touch up chips and cracks on the sills every year or two instead of waiting for a full repaint.

Recaulk the joints between sill and casing when the caulk shrinks or cracks. Never caulk the drip edge underneath, though. That's where the water is supposed to leave.

Knock snow off the ground-floor sills after big storms. Ice dams can also send water down the face of the wall onto upstairs sills. If stains show up under second-floor windows after a hard winter, read ice dams and winter damage.

Frequently asked questions

Can I just paint over a soft window sill?

No. Paint won't stop rot that's already started. Moisture trapped under fresh paint keeps the decay going, and the paint usually blisters within a season. The soft wood has to be dug out and repaired, or the sill replaced, first.

What is the best time of year to repair window sills?

In the Capital Region, late spring through early fall. Consolidants, fillers, primers and paint all need dry wood and moderate temperatures to cure. If a sill fails in winter, it can be covered to keep the water off and repaired when the weather turns.

Is wood filler good enough for sill rot?

Not regular wood putty, which is made for small indoor nail holes. Two-part exterior epoxy fillers, used with a consolidant, are made for this and bond far better to weakened wood.

Sills, casings and the rest of the outside trim on older houses fall under our restoration and built-ins work, and when the original profile should stay, we cut the new piece to match it.