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.
A rotted window sill inside the house is usually the stool, the wood ledge in front of the window, soaked for years by condensation or by a leak at the sash. JMB Builders replaces water damaged window stools across the Twin Cities metro, checks the rough sill and the sash while the stool is out, and tells you whether the rot stops at the trim or the window is going. Priced per window, quoted from photos within one business day.
- Priced per window, stool and apron replaced
- Rough sill and sash checked before the new stool goes in
- Quote from photos in 1 business day
- Licensed MN contractor BC809005
Rot on the inside of a window almost always starts at the same place: the joint where the stool meets the bottom of the window frame. Water sits there. In Minnesota the water is usually condensation, running down cold glass every winter morning and soaking into the end grain of the stool through a failed paint or caulk line. Sometimes it is a leak: a sash that no longer seals, a cracked glazing bead, or an exterior sill that sends water inward instead of out.
The stool is the first thing to go because it is flat, it is end grain at the horns, and it is often pine or MDF. Pull it and the question is what is behind it. Most of the time the rough sill under the stool is dry and sound and the job is a new stool and apron. Sometimes the rough sill is soft too and gets cut back and sistered. And sometimes the bottom rail of the sash or the bottom corners of the window frame are soft, which means the window is failing and a new stool in front of it would rot again.
JMB Builders replaces rotted interior window sills across the Twin Cities metro and tells you which of those three you have before the new wood goes in. The new stool is solid wood sealed on every face, or PVC on a window that sweats hard, set on a sound rough sill and sealed at the frame with a joint that sheds water instead of holding it. Painting or staining is quoted as a separate line, or paint it yourself.
What you supply
- A photo of the whole window and a close-up of the rotted section, with a screwdriver or awl pushed into the softest spot
- A photo of the outside of the same window at the sill, if you can get to it
- Whether the window sweats in winter, and whether you have seen water come in when it rains
- Your paint or stain color if we are finishing it; painting is quoted as a separate line, or paint it yourself
What JMB supplies
- Removal of the rotted stool and apron, and of the casing if the rot has run up behind it
- Probing and repair of the rough sill and the bottom of the jambs — cut back, dried, sistered or replaced
- A new stool from thick solid stock sealed on all faces, or cellular PVC on a window that sweats badly, with a new apron
- Casing reset or replaced in a matching profile where it had to come off
- Sealant at the frame joint tooled so water runs off, not into it
- A straight answer on the sash and frame, with photos, before the new stool goes in
- Photos of the finished window in your job folder
- Painting or staining 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:
- Rot in the stool only vs. rot that has reached the rough sill or the bottom of the jambs
- Whether the casing has to come off and go back or be replaced
- Solid wood vs. PVC for the new stool, and paint-grade vs. a stain-grade species to match
- Mold cleanup and drying time before the new wood can go in
- A window over a counter, tub or radiator with tight access
- Finding the water source, when the outside sill or the sash is letting it in
- Several rotted sills in the same house
The screwdriver-in-the-soft-spot photo is the one that matters. It shows us how deep it goes and whether the sash is involved.
How long it takes
A rotted stool with a sound rough sill: about two hours per window. When the rough sill has to be cut back and rebuilt, half a day, sometimes with a day of drying between the demo and the new wood.
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
Stool and apron only
The rot stops at the trim. Old stool and apron out, rough sill checked and dry, new stool set level with horns fitted to the casing, new apron, sealed at the frame. The casing stays.
Rough sill repair under the stool
The framing under the stool is soft. We cut the rot back to sound wood, let it dry, sister or replace the rough sill, then set the new stool on it. This is the one where a screwdriver goes through the stool into nothing.
PVC stool on a sweating window
Basement, bathroom, over a sink, or a north-facing bedroom that runs with condensation every winter. A cellular PVC stool cannot rot, paints like wood, and ends the cycle. Solid hardwood sealed on all sides is the stain-grade version.
The rot is in the window
The sash bottom rail or the frame corners are soft. We can still put a new stool in, but we tell you it is a bandage. The right fix is a sash repair or a new window, and we quote that alongside the stool so you can choose.
Rotted Window Sill Inside? Stool Replacement and What the Rot Means: questions, answered
What causes a window sill to rot on the inside?
Standing water at the joint between the stool and the window frame. In Minnesota that water is usually condensation running down cold glass in winter, soaking into the stool's end grain through a failed paint or caulk line year after year. Less often it is a real leak: a sash that does not seal, failed glazing, or an exterior sill that slopes the wrong way. The photos and the pattern of the rot usually tell which.
Does a rotted interior window sill mean I need a new window?
Not by itself. The stool is trim, not part of the window, and most of the time the rot stops there. It does mean the window is being wet regularly, so while the stool is out we probe the bottom rail of the sash and the bottom corners of the frame. If those are firm, a new stool fixes it. If the sash or frame is soft, the window is failing and we say so before setting new trim in front of it.
Can a rotted window sill be repaired with epoxy instead of replaced?
A small soft spot at one horn can be consolidated and filled with a two-part wood epoxy and it will hold under paint. Rot that runs along the joint to the window, or that has gone through the stool, is past that. A new stool costs little more than a careful epoxy repair, does not need re-doing, and lets us see the rough sill underneath, which epoxy never does.
There is black mold on the wood sill inside my window. Is that dangerous?
It is a sign the wood has been wet for a long time, and it should not be sealed over. We remove the affected trim, clean and dry the framing behind it, and replace it with new wood or PVC. If the mold is on the wall below the sill or inside the wall cavity, that is beyond a trim job and we tell you what we found rather than closing it up.
What should the new sill be made of so it does not rot again?
Solid wood, sealed on every face before it goes in, with the joint at the window frame caulked so water runs off it. On a window that sweats hard every winter, cellular PVC is better; it cannot rot and paints like wood. Avoid MDF and finger-jointed pine, which are what usually failed. Running the humidifier lower in January keeps the glass drier than any material choice.
How do I tell if the rot is just the sill or worse?
Push an awl or a thin screwdriver into the softest part of the stool. If it stops within the thickness of the stool, the rot is likely in the trim only. If it keeps going, the rough sill under it is soft. Then press the bottom rail of the sash and the bottom corners of the window frame with your thumb; if they give, the window is involved. Send a photo of each of those and we quote from it.
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.
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Read morePage reviewed September 24, 2026 · JMB Builders LLC, MN license BC809005 · All window trim pages
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Based in Zimmerman, scheduling across the Twin Cities metro. Closest first: