Arched Cased Opening — Convert a Square Doorway to an Arch
The square opening between the living room and the hall wants to be an arch like the ones in the front room. We frame the curve, then drywall it or trim it, so it matches.
JMB Builders builds arched cased openings across the Twin Cities metro. We convert a square doorway to an arch by framing the curve with a plywood template and blocking, then finish it one of two ways: drywall with flexible corner bead for a plaster look, or a bent jamb and flexible casing for a trimmed look. Priced per opening, quoted from photos within one business day.
- Priced per opening; drywall arch or trimmed arch
- Curve framed to a template, both sides finished
- Quote from photos in 1 business day
- Licensed MN contractor BC809005
An arched opening starts as a square one. The header stays; the arch is built under it from two plywood templates cut to the curve, spaced with blocking to the wall thickness, and screwed to the header and the jack studs. The curve can be a half round, a shallow segment, or an ellipse, drawn to match arches already in the house or to fit the head height you have. Then the opening gets finished one of two ways.
The drywall arch is the usual choice in newer houses and anywhere the openings are wrapped rather than trimmed: flexible corner bead around the curve, drywall scored or a bendable strip on the underside, mudded and textured to match. The trimmed arch is the choice where the other doorways have casing: a jamb bent from kerfed or laminated stock follows the curve, and flexible casing in the same profile as the house trim goes around it, with straight casing on the legs. Both take a steady hand on the curve; a lumpy arch is the first thing anyone sees.
JMB Builders converts square doorways to arched cased openings across the Twin Cities metro. New trim goes in primed, filled and caulked. Painting is quoted as a separate line, or leave it primed and paint it yourself.
What you supply
- A photo of the doorway from each side, straight on, with a tape in the shot
- A photo of any arch already in the house you want to match, or the shape you have in mind
- Whether the other openings are trimmed with casing or wrapped in drywall
- Paint for the finished opening, or add painting as a line on the quote
What JMB supplies
- Plywood arch templates cut to the curve you choose, with blocking between them
- Removal of the door, jamb and casing when the opening had a door
- Drywall option: flexible corner bead, drywall on the curve, three coats of mud, texture matched
- Trimmed option: a kerfed or laminated jamb bent to the curve and flexible casing in the matching profile
- Straight casing on the legs, plinths or head blocks where the house has them
- Filler, caulk and primer on new trim; painting quoted as its own line
- Photos of the finished arch in your job folder
How it is priced: per opening
Quoted in writing from your photos within one business day. What moves the number:
- Drywall arch vs. trimmed arch with a bent jamb and flexible casing
- Shape: a segment arch is quick; a full half round or an ellipse takes more template work and more casing
- Head height: a low header may need to be raised to leave room for the curve, which is framing work
- Whether the opening is being widened at the same time
- Wall thickness and finish: stud wall with drywall vs. a thick plaster wall
- Matching an existing arch in the house vs. a new shape you choose
- Number of arches in the same visit
A straight-on photo with a tape held across the opening lets us draw the curve before we come. Add a photo of an existing arch if you want it matched.
How long it takes
Drywall arch: one day to frame and board, plus a return for finish coats and texture. Trimmed arch: one to two days depending on the casing profile.
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
Drywall arch with flexible bead
For houses where openings are wrapped, not cased. The curve is framed, drywall goes on both faces and a bendable strip on the underside, flexible corner bead wraps the edge, and it is mudded and textured to match the wall. The cleanest look with no trim.
Trimmed arch with flexible casing
For houses with casing on the doors. A jamb bent to the curve lines the opening, and flexible casing in the same profile as the house trim follows it, joined to straight legs. Painted, it reads as one continuous piece of trim.
Segment arch under a low header
When the header is too low for a full half round, a shallow segment arch gives the curve without losing headroom at the sides. The most common choice in a hallway or a basement.
Match an existing arch
Older Minneapolis and St. Paul houses often have one or two plaster arches already. We trace the shape from a photo with a tape reference, or from a template taken on site, and build the new one to the same radius and spring line.
Arched Cased Opening — Convert a Square Doorway to an Arch: questions, answered
Can a square doorway be converted to an arch?
Yes. The square header stays where it is and the arch is built beneath it from plywood templates and blocking, so there is no structural change. The opening loses a little height at the corners, which is why the head height matters: a standard door opening takes a shallow segment arch easily, while a full half round wants a taller opening or a slightly raised header. From a photo with a tape in it we can draw the curve that fits.
Should the arch be drywall or trimmed with casing?
Match the house. If the other openings are drywall-wrapped, do a drywall arch with flexible corner bead so it looks like the rest. If the other doorways have casing, a trimmed arch with a bent jamb and flexible casing keeps the trim continuous. Drywall arches are a little less work; trimmed arches show more detail. Either way the curve has to be smooth, and that comes from the framing template, not the finish.
How do you trim an arched opening with casing?
The jamb is bent to the curve, either from kerfed stock or thin laminated strips glued around a form, and set on the framed arch. Flexible polyurethane casing in the same profile as the straight casing is fastened around the curve and joined to straight legs at the spring line. It is filled, caulked and primed, and once painted the joint disappears. Stained arches are possible but the flexible casing is paint-grade, so a stained arch is built up from solid wood segments instead.
Can you match the arches already in my house?
Yes. Send a straight-on photo of an existing arch with a tape across the opening and we can work out the radius and where the curve starts. On site we can also make a template directly from the old arch. The new opening is framed to that shape, so it matches even if the two openings are different widths.
Does converting a doorway to an arch require a permit?
Not usually, because the header and the framing that carry load are not changed; the arch is added below them. If the opening is being widened at the same time, or the header is being raised, that is structural work and may need a permit and a header sized for the span. We tell you which when we quote.
How long does an arched opening take?
A drywall arch is one day to remove the old door or casing, frame the curve and hang the board, then a return visit for the finish coats, sanding and texture. A trimmed arch is one to two days depending on how the casing is built. Painting, if we do it, follows once the filler and mud have cured.
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 doors pages
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