Arched 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.
JMB Builders installs arched window trim across the Twin Cities metro — half rounds, eyebrows, full round tops and the arched transoms over picture windows. We build the curved jamb extension first, then run either flexible casing bent to the arc or segmented casing cut from straight stock, matched in profile and width to the straight casing on the rest of the window and the room. Priced per window, quoted from photos within one business day.
- Priced per window
- Flex moulding or segmented casing on the curve
- Matched to the straight casing profile in the room
- Quote from photos in 1 business day
- Licensed MN contractor BC809005
An arched window has two straight sides, a straight sill, and a curve that nothing in the trim aisle is made for. That is why so many of them end up with a drywall return on the arch and wood casing on the legs, or with a curve that was pieced together from short straight cuts and filled with caulk. From across the room the joints show, and every one of them opens up the first winter.
There are two honest ways to trim the curve. Flexible polyurethane casing comes in the common profiles, bends to the radius of the arch, and takes paint like wood. Segmented casing is the traditional route: straight stock cut into short pieces with matched angles, glued and fastened into a curve, then the edge cleaned up so the profile runs through. Flat craftsman-style casing can also be cut directly to the curve out of a wider board. Which one we use depends on the profile you have on the straight legs, whether the trim is painted or stained, and how tight the radius is.
Under the casing there has to be a jamb extension that follows the same curve. We build it from bendable plywood or kerfed stock so the drywall edge is covered all the way around, then the casing lands on a flat, continuous surface. New trim goes in primed, filled and caulked. Painting is quoted as a separate line — or leave it primed and caulked and paint it yourself.
What you supply
- A photo of the arched window straight on and one close-up of the trim on a straight window nearby
- The rough width of the window and the height of the arch from the spring line to the top
- Painted or stained: stain-grade means segmented wood, painted opens up flex moulding
- Paint or stain color if you want us to finish it — painting is quoted as a separate line
What JMB supplies
- Curved jamb extension built from bendable plywood or kerfed stock to the arc of the window
- Flexible polyurethane casing in a matched profile, or segmented casing cut from straight stock
- Straight casing on the legs and a stool and apron or a mitered bottom to match the room
- Glue, fasteners, filler and caulk so the curve reads as one piece
- Painting or staining of the finished trim when you want it done, quoted as its own line
- Photos of the finished window in your job folder
How it is priced: per window
Quoted in writing from your photos within one business day. What moves the number:
- Flexible casing on the arc vs. segmented wood casing, which takes more shop time
- Painted trim vs. stain-grade wood, where every joint on the curve has to be tight without filler
- Radius of the arch — a tight eyebrow or a full round top is harder to bend and fit than a shallow arc
- Whether the arch already has a jamb extension or is bare drywall and framing
- Matching an unusual existing casing profile vs. a stock profile
- Painting or staining, quoted as a separate line
A photo of the arch with a tape measure held across the widest point tells us the radius; the quote is firm from that and a photo of the trim on a straight window.
How long it takes
One arched window with flex casing: most of a day, including the curved jamb. Segmented stain-grade casing: shop time the day before, then a day on site.
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
Flexible moulding on the arch
Polyurethane casing in the same profile as the straight legs, bent to the arc and fastened to the curved jamb. Paint hides the material change completely. The right answer for most painted trim.
Segmented wood casing
Short pieces of the real casing profile cut with matched angles, glued and pinned into a curve, then the outside edge and the profile cleaned up so it runs continuous. What you want when the trim is stained and the wood has to match.
Flat stock cut to the curve
For craftsman or flat 1x casing, the arch piece is cut directly out of a wider board on the radius, so it is one piece with no joints. The straight legs butt into it. Simple, and it looks right on a bungalow.
Drywall return on the arch
If you like the no-trim look, the curve can be wrapped in drywall with flexible corner bead and only the sill gets a stool. We do these too; the corner bead has to be set tight or the arc goes wavy.
Arched Window Trim — Casing and Jambs for Round Top Windows: questions, answered
What is flex moulding and does it look like wood casing?
Flexible moulding is a dense polyurethane casing made in the same profiles as standard wood trim, but it bends to a curve without cracking. Once it is primed and painted alongside the wood casing on the straight legs, you cannot tell where one stops and the other starts. It does not take stain, so on stain-grade trim we build the curve out of segmented wood instead.
Can arched window trim match the straight casing on my other windows?
Yes. Send a close-up of the casing on a straight window and we match the profile and width. Most common profiles are available as flexible moulding for the arc; if yours is not, we segment the arc out of the same wood casing so the profile runs through. The straight legs on the arched window are trimmed with the same stock as the rest of the room.
How do you make a jamb extension for a curved window?
The extension has to follow the arc so the casing has something flat and continuous to land on. We build it from bendable plywood or from stock that has been kerfed on the back so it bends, fasten it to the window frame and the framing, and fill and sand the face. On a painted job it disappears; on a stained job we use a veneer that matches the casing.
Can an arched window be trimmed in flat craftsman-style casing?
Yes, and it is one of the cleanest ways to do it. The arch piece is cut out of a wider flat board on the radius of the window, so there are no joints on the curve at all. The straight legs butt into it or run under it, and a stool and apron finish the bottom. It fits a bungalow or farmhouse trim package without looking forced.
Can flexible moulding be painted or stained?
Painted, yes — prime it and paint it with the rest of the trim and it is invisible. Stained, no; it is a solid polyurethane and stain has nothing to soak into. If your trim is stained wood, tell us up front so we plan on segmented wood casing for the curve.
What photos do you need to quote trim for an arched window?
Three: the arched window straight on from across the room, a close-up of the arch showing whether it has a jamb or bare drywall, and a close-up of the casing on any straight window nearby. If you can hold a tape across the widest part of the arch in one of the photos, we can figure the radius from it and the quote is firm without a site visit.
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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Window trim near you
Based in Zimmerman, scheduling across the Twin Cities metro. Closest first: