Bay Window Trim Interior — Seat Board, Head Board and Jambs
A bay window has more angles in it than the rest of the room put together. We cut the seat board, head board and covers to fit them instead of caulking the gaps.
JMB Builders does bay window trim interior work across the Twin Cities metro: the seat board across the bottom, the head board across the top, mullion covers where the three windows meet, jamb extensions cut to the angle at each end, and casing around the whole unit. Bow windows get the same treatment across four or five panels. Priced per window, with the whole bay counted as one, quoted from photos within one business day.
- Priced per bay or bow window, whole unit
- Seat board, head board and mullion covers
- Jambs and casing cut to the bay's angles
- Quote from photos in 1 business day
- Licensed MN contractor BC809005
A bay window is three windows set at angles into one frame, pushed out past the wall. The window company sets the unit, insulates the head and seat cavities, and usually stops at the drywall. What is left is a deep, angled opening: a wide bottom that wants a seat board, a top that wants a head board, two joints where the windows meet, and two ends where the jamb meets the wall at something other than a square corner.
The seat board is the piece everyone sees. It runs the full width of the bay in one piece, follows the angles at the front, has a nosing on the room side, and needs to be flat and solid because things get set on it. The head board mirrors it above. Between the windows, where the frames meet at an angle, a mullion cover hides the joint and the shim space. At the ends, jamb extensions bring the frame out to the wall at the bay's angle, and then casing runs around the outside like any other window.
Bow windows work the same way with gentler angles and more panels, so the seat and head boards have more facets to cut and the covers are narrower. New trim goes in primed, filled and caulked. Painting or staining is quoted as a separate line — or leave it primed and caulked and paint it yourself.
What you supply
- A photo of the bay from across the room and one close-up of where the windows meet
- Whether the seat board should be painted, stained, or a solid surface like a countertop remnant
- The casing profile you want, or a photo of the trim on another window in the room
- Paint or stain color if you want us to finish it — painting is quoted as a separate line
What JMB supplies
- One-piece seat board cut to the bay's angles with a nosing on the room side
- Head board to match, or a drywall head with casing if you prefer it
- Mullion covers at each joint between window units
- Jamb extensions at both ends cut to the bay angle, not forced square
- Casing around the whole unit in a profile matched to the room, plus apron under the seat
- Painting or staining of the finished trim when you want it done, quoted as its own line
- Photos of the finished bay in your job folder
How it is priced: per window
Quoted in writing from your photos within one business day. What moves the number:
- Three-panel bay vs. a four- or five-panel bow, which has more angles to cut
- Painted trim vs. stain-grade wood, where the seat board has to be one clear piece with tight joints
- Depth of the bay — a deep seat takes a wider board and a support cleat underneath
- Whether the head and seat cavities are already drywalled or still open framing
- Casing profile: stock vs. matching an existing profile in the room
- Painting or staining, quoted as a separate line
Send a photo from across the room and a photo of the seat area from above so we can see the depth and the angles; the quote covers the whole bay, not piece by piece.
How long it takes
Most bay windows: one full day for seat, head, covers, jambs and casing. A bow window or stain-grade work can run into a second day.
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
Seat board
One piece across the full width, cut to the bay's angles at the front, with a bullnose or eased edge on the room side and an apron below. Poplar or MDF for paint, oak or maple for stain, or we set a countertop remnant you supply.
Head board
The same board mirrored at the top so the bay reads as a finished box. On some bays the head is a drywall return and only the seat gets a board; we do it either way to match the room.
Mullion covers
Where two window frames meet at an angle there is a joint and a shim space. A cover strip cut to the same angle hides it and matches the casing, so the bay looks like one unit instead of three windows shoved together.
Angled jamb extensions and casing
At each end the window frame meets the wall at the bay's angle. The jamb extension is cut to that angle so the casing sits flat on the wall, and the casing runs around the outside in the same profile as the rest of the room.
Bay Window Trim Interior — Seat Board, Head Board and Jambs: questions, answered
What is a bay window seat board and what is it made of?
The seat board is the wide, flat board across the bottom of the bay, running the full width in one piece and following the angles at the front. It works like an oversized window stool and gets a nosing on the room side and an apron underneath. For painted trim we use poplar or MDF; for stain we use a clear hardwood like oak or maple; some people have us set a countertop remnant so it cleans up like a kitchen counter.
Can the bay window seat be deep enough to sit on?
Only as deep as the bay projects, and most bays do not project far enough for a real seat. What we can do is make the seat board solid, with a support cleat under it, so it holds plants, a cat, or a kid leaning on it without flexing. If you want an actual window seat with a cushion, that is a built-in bench in front of the window, which is a different job we also do.
Do you trim bow windows the same as bay windows?
Yes. A bow is the same idea with four or five panels at gentler angles, so the seat and head boards have more facets to cut and the mullion covers are narrower. The pieces are the same: seat, head, covers, angled jambs at the ends, and casing around the outside. It takes a bit longer because there are more joints to fit.
What do the mullion covers on a bay window do?
Where two window units meet at an angle there is a joint between the frames and a gap where the shims are. A mullion cover is a strip cut to that angle that hides the joint and the gap and matches the casing. Without it you see caulk or bare frame at each corner, which is the most common thing left unfinished on a bay.
Why does my bay window trim have gaps at the angles?
Because someone cut the jamb extensions or casing square and tried to make them meet at the bay's angle, then filled the difference with caulk. Caulk shrinks and the gap comes back. We cut each piece to the actual angle of the bay, which is rarely a standard number, so the joints close on their own before any caulk goes on.
Can the bay window trim be painted or stained to match the rest of the room?
Yes. The trim goes in primed, filled and caulked, and painting or staining is quoted as a separate line — or leave it primed and caulked and paint it yourself. For stained trim, tell us the species and finish before we cut so the seat board and casing come out of the same wood.
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
Window Sill Trim — Stool and Apron Installation & Replacement
A window with no ledge, or a ledge that is split, swollen or painted into a lump — we install and replace window stools and aprons so there is a real sill to set things on.
Read moreJamb Extensions for Windows — Installation & Casing
The window frame stops short of the wall and there is nothing to nail casing to. Extension jambs fix that — sized to your wall, set straight, reveal even, cased the same day.
Read moreWindow Seat Built In — Bench Box With Storage Under a Window
The nook under the bay window or the wall between two closets is wasted space. A built-in window seat gives it a place to sit, storage under the lid, and trim that makes it look like it came with the house.
Read moreArched Window Trim — Casing and Jambs for Round Top Windows
The window company set the arch and stopped. The straight legs are easy; the curve is where most trim jobs fall apart, and it is the part you look at every day.
Read moreWindow Company Didn't Do Trim? We Finish Replacement Windows
The new windows are in and they look great from outside. Inside there is foam, a gap and no trim, and the window company says that was not part of the job. It is part of ours.
Read moreBasement Window Trim-Out — Half-Below-Grade & Egress Windows
Thick foundation walls, a sill that sweats every winter, an egress unit sitting in a window well — basement windows need trim built for a basement, not copied from upstairs.
Read morePage reviewed September 24, 2026 · JMB Builders LLC, MN license BC809005 · All window trim pages
Where we work
Window trim near you
Based in Zimmerman, scheduling across the Twin Cities metro. Closest first: