Exterior Window Sill Rot Repair — Sill Nosing Rebuilt, Window Kept
The sill outside the window is soft, checked and peeling, but the window itself opens, locks and is not fogged. We rebuild the sill and leave the window alone.
Exterior window sill rot repair means rebuilding the sloped sill or subsill under a wood window that is otherwise sound. JMB Builders does it across the Twin Cities metro: we cut the rot back to solid wood, fix the drip cap, flashing and caulk joints that let the water in, then set a new PVC or cedar sill nosing, or an epoxy repair when the damage is shallow. Priced per window, quoted from your photos within one business day.
- Priced per window, sill only or sill and jamb bottoms
- Window stays in; sash and frame are kept
- PVC, cedar or epoxy depending on how deep the rot goes
- Quote from photos in 1 business day
- Licensed MN contractor BC809005
This page is about the sill on the outside of the house: the sloped piece of wood at the bottom of the window frame that sheds rain, and the subsill or rough sill under it. The ledge you set a plant on inside the house is the stool, and that is a different repair. If your rot is inside, read the interior window sill page instead. If the paint on the outside sill has lifted, the wood is checked along the grain and a screwdriver sinks in at the corners, you are in the right place.
Exterior sills rot for a short list of reasons. There is no drip cap above the window, so water runs behind the head casing and comes out at the sill. The caulk joint between the sill and the siding or stucco opened up and started holding water instead of shedding it. The sill lost its paint and the end grain at the horns drank. Or the sill was made flat instead of sloped, which is common on older repairs. Every one of those has to be fixed before new wood goes in, or the new sill rots on the same schedule the old one did.
What we do not do is pull a good window to fix a bad sill. On most wood and clad-wood windows the sill nosing can be cut off flush, the subsill checked and repaired, and a new sloped sill set under the frame with the joint flashed and sealed. The sash keeps its glass, its balances and its locks. When the rot has run up into the jamb legs or into the frame itself, we tell you, with photos, and quote the sash repair or a new window alongside so you can decide with the whole picture.
What you supply
- A photo of the whole window from outside and a close-up of the sill, with a screwdriver pushed into the softest spot
- A photo of the top of the window showing whether there is a drip cap or metal flashing above the head trim
- The house siding type: lap siding, vinyl, stucco or brick, since it changes how the sill is flashed
- Your paint color if we are finishing the new sill; painting is quoted as a separate line, or paint it yourself
What JMB supplies
- Removal of the rotted sill nosing, cut back to solid wood on the subsill and jamb bottoms
- Probing of the rough sill and the sheathing behind it, dried out and sistered or replaced where soft
- A new sloped sill nosing in cellular PVC, or cedar when the house has stained or historic trim, with the horns fitted to the casing
- Two-part epoxy consolidant and filler for shallow rot where the sill is mostly sound
- Drip cap above the window and flashing tape at the sill joint when they were missing
- Sealant at the sill-to-frame joint and the sill-to-siding joint, tooled to shed water, with the bottom edge left open to drain
- Photos of the repair before it is closed up and of the finished sill in your job folder
- Painting of the new sill and any disturbed trim when you want it done, quoted as its own line
How it is priced: per window
Quoted in writing from your photos within one business day. What moves the number:
- Sill nosing only, or the subsill and jamb bottoms too
- PVC vs. cedar and whether we are matching an old sill profile
- Epoxy repair of a mostly sound sill vs. a full new sill
- Stucco, brick or lap siding at the sill, which changes the flashing and the caulk joint
- Second-story windows that need a ladder set or a lift
- Missing drip cap or head flashing that has to be added while we are there
- Several windows in the same visit are cheaper per window than one
Tell us how far the screwdriver went and whether the window still opens and locks. Those two facts decide whether this is a sill job or a window job, and the quote is firm from there.
How long it takes
One sill on a first-floor window: about half a day. Add time when the subsill has to be rebuilt or the wall needs a day to dry before new wood goes in. Painting is quoted as its own line.
How your project works
- 1
Send photos, get a written quote
Send three photos of the space and a sentence about what you want. You get a clear, itemized quote within one business day. We only come out to measure first when the job needs a tape measure — no pressure, no games.
- 2
Pick materials, or bring your own
We help you choose profiles and materials that fit the house and the budget, or match your existing millwork exactly — or you buy the door or trim yourself and we install it. Everything is confirmed before the crew is scheduled.
- 3
Clean, precise installation
The crew shows up on time, protects floors and furniture, and installs with tight joints and consistent reveals. Photos of the finished work go in your job folder.
- 4
Walkthrough and invoice
We walk the finished work with you. If anything is not right, we fix it before we call the job done. Then one invoice, paid by check or direct deposit — no card fees, no payment links.
Situations and options
Epoxy repair on a shallow spot
The rot is a soft corner or a check along the grain with solid wood behind it. We dig out the punky wood, soak the area with a liquid epoxy consolidant, fill with a two-part wood epoxy, shape it to the sill slope and prime it. Right for one bad corner, not for a sill that is soft end to end.
New PVC sill nosing
The sill is soft along most of its length. We cut the old nosing off flush, check the subsill, and set a new sloped nosing in cellular PVC that cannot rot. It paints like wood and the horns are cut to meet the casing. The usual fix on a painted house.
New cedar sill on a stained or historic house
Where the trim is stained or the profile matters, we mill a new sill from cedar to match the old one, prime or seal every face including the end grain before it goes in, and set it with the same slope and drip kerf the original had.
Subsill and jamb bottoms rotted too
The screwdriver went through the sill into soft framing. We cut the rough sill back to solid wood, let it dry, sister or replace it, splice the bottom of the jamb legs, then set the new sill on sound framing. Still no window replacement unless the frame itself is gone.
Exterior Window Sill Rot Repair — Sill Nosing Rebuilt, Window Kept: questions, answered
Can you fix a rotted exterior window sill without replacing the window?
Yes, in most cases. The sill nosing on a wood or clad-wood window is a separate piece that can be cut off and replaced while the frame and sash stay in place. We check the subsill and the bottom of the jambs while it is open and repair those too if they are soft. The window only has to come out when the rot has run up into the frame or the sash itself.
What is the difference between an exterior window sill and the interior sill?
The exterior sill is the sloped board on the outside at the bottom of the window that sheds rain away from the wall. The interior sill is properly called the stool; it is the flat ledge inside the room. They rot for different reasons: the outside sill from rain and missing flashing, the inside stool from condensation running off the glass. They are separate repairs and we quote them separately.
Should the new window sill be PVC or wood?
On a painted house we lean toward cellular PVC. It cannot rot, it holds paint, and it takes splash from a deck or a walkway without complaint. Cedar is the right call when the trim is stained or the house has a sill profile worth keeping, and it holds up well when every face is sealed before it goes in. Either way the drip cap and the flashing behind the sill matter more than the sill material.
Why did my window sill rot in the first place?
Water got behind or onto it faster than it could dry. The usual causes are no drip cap above the window, a caulk joint at the siding that opened and started trapping water, paint that failed at the end grain, or a sill that was set flat instead of sloped. We find the cause before the new sill goes in, because a new sill under the same leak rots the same way.
Is epoxy a real fix for a rotted window sill?
For a shallow spot with solid wood around it, yes. A liquid consolidant hardens the fibers and a two-part epoxy filler rebuilds the shape; done right it outlasts the wood around it. It is not a fix for a sill that is soft along its whole length or one with rot running into the framing. We tell you which one you have from the photos and the screwdriver test.
Do you paint the new sill?
The new sill goes in primed, or sealed if it is cedar for stain, with the joints caulked. Painting is quoted as a separate line on the same quote, or leave it primed and paint it yourself. On a PVC sill the paint is for color only; the sill does not need it to survive.
Recent JMB work, from our job log
59 installs recorded across 30 Twin Cities communities since Dec 2025. Window and door installs for homeowners and for the builders and dealers we sub for. Counts come straight from our job log; no customer names.
Get a written quote for this job
Three photos and a sentence is enough. You hear back within one business day. Or text the photos to 763-204-2099.
Related jobs we do
Rotted Window Sill Inside? Stool Replacement and What the Rot Means
The paint on the ledge has bubbled, the wood underneath is soft enough to push a screwdriver into, and there is black along the joint to the window. We find out how far it goes and fix it.
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Water stains at the top corners of the casing, paint peeling off the head trim, rot showing up at the bottom of the door: the house is missing a drip cap. We add one without tearing off the siding.
Read moreWood Window Sash Rot Repair — Epoxy, Dutchman Patch or New Rail
The bottom rail of the sash is soft where it meets the glass, the paint is lifting, and the corner joint has opened. The rest of the window is fine. We rebuild the rail instead of selling you a window.
Read moreStucco Window Trim Repair — Rotted Trim on a Stucco House
The wood trim around the windows on your stucco house is soft at the bottom and the caulk against the stucco has split. We replace the trim in PVC and seal it to the stucco the right way.
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The wood trim around the windows needs scraping and paint every few years and the bottom corners are starting to go. Capping it in aluminum coil ends the painting, as long as the wood under it is sound first.
Read morePage reviewed September 27, 2026 · JMB Builders LLC, MN license BC809005 · All exterior trim pages
Where we work
Exterior trim near you
Based in Zimmerman, scheduling across the Twin Cities metro. Closest first: