A prehung French door unit is the easiest interior French door install: both doors come hinged in one jamb and already meet in the middle. Two slabs fitted into an existing jamb take more time per door. Either way, the job is judged at the meeting edge, where the two doors have to line up top to bottom.
That applies to any Twin Cities homeowner closing off a home office, adding a pair to a sunroom, or replacing a wide cased opening between two rooms. Below is how each install works, what sizes the doors come in, the tolerances that matter, the hardware choices, and how the labor is priced.
Prehung unit or two slabs?
A prehung French door unit comes with both doors hinged in one jamb and the astragal or meeting stop already set. Converting a cased opening, or replacing an existing pair where the casing is coming off? This is the way to go. The unit gets set plumb and shimmed, and the doors already meet correctly.
Two slabs into an existing jamb is the option when the frame and casing are worth keeping, common in older St. Paul and Minneapolis homes with stained oak or fir trim. Each slab is fitted to the opening, hinges are mortised into both door and jamb, and the two doors are trimmed so the reveal is even all the way around. It is careful work, which is why slab installs cost more per door.
What sizes do interior French doors come in?
Interior pairs are sold by the width of the two doors together. The common stock sizes are two 24" doors for a 48" pair, two 30" for a 60" pair, and two 36" for a 72" pair, all 80" tall, with 96" heights available for taller rooms. A prehung unit adds the jamb on each side, so the rough opening runs about 2" wider and 2" to 2-1/2" taller than the door pair itself. Measure the framed opening, not the old casing, before you order.
Two things catch people out. First, the jamb width has to match the wall: 4-9/16" for a standard 2x4 interior wall, wider for 2x6 or for walls with a layer of paneling or thick plaster. Second, the pair has to be ordered as a pair, with an astragal or a rabbeted meeting edge, or the two doors will not close against each other. Two single prehung doors set side by side is not a French door unit.
What has to line up on a pair of doors?
Most old openings are not square, and a French door pair shows it more than a single door. These are the tolerances that matter:
- An even 1/8" reveal across the top and down both hinge sides.
- A consistent gap where the two doors meet, top to bottom, usually 1/8" to 3/16".
- Both doors at the same height, with the tops in a straight line.
- Enough floor clearance for carpet, rugs or a threshold where flooring changes.
- Hinges mortised so each door sits flat and does not spring open or drift closed.
What keeps interior French doors closed?
Interior French doors usually do not latch like a regular door. One or both doors are held with catches. The options, from simplest to most secure:
- Ball catch. A spring-loaded ball in the top of the door that snaps into a strike in the head jamb. Simple, invisible, fine for most rooms.
- Roller catch. Same idea with a roller. Smoother and more adjustable, good for heavier doors.
- Flush bolts. Recessed bolts in the edge of the inactive door that slide into the head and floor. Used with a passage or privacy latch on the active door when one door needs to lock solid.
- Dummy handles. Fixed handles that do not turn, used with ball or roller catches. Buy them in pairs and match the rest of the house.
- Passage or privacy handles. A working latch on the active door with flush bolts on the inactive door. This is the setup for a home office or bedroom that needs to lock.
Which glass for privacy?
Clear glass gives the most light and suits offices and dens. Frosted, reeded or seeded glass adds privacy without blocking light, which is why it shows up on pantries, bathrooms off a bedroom and laundry rooms. Divided-lite doors (10-lite or 15-lite) match traditional trim. Single-lite doors suit newer homes. Glass in any door has to be safety glazing under the residential code, and manufacturers ship interior French doors with tempered glass as standard.
What does interior French door installation cost in the Twin Cities?
Published 2026 cost guides put labor for a prehung French door unit in a prepared opening in the several-hundred-dollar range plus casing. A slab pair fitted into an existing jamb with new hinges and hardware runs higher because of the fitting and mortising time. Converting a cased opening, adding a jamb, header work or drywall patching, moves the number up again. The doors themselves range widely, from hollow-core pairs to solid wood with specialty glass.
To get a firm number, send three photos: the opening straight on, one side showing the wall thickness and casing, and the floor at the opening. You hear back from us within one business day, with painting or staining as its own line.
What JMB Builders does
JMB Builders is a licensed Minnesota residential building contractor (BC809005) based in Zimmerman, working across the Twin Cities metro. One crew sets the jamb, fits the doors, mortises the hardware and cases the opening, prehung or slab, paint-grade or stain-grade. Painting is quoted as a separate line, or leave the doors primed and paint them yourself. Paid by check or direct deposit. Call 763-250-3621 or send photos through the site.
- Interior French door installation — jmbbuildersmn.com/services/finish-carpentry/interior-french-doors
- Single door to double door conversion — jmbbuildersmn.com/services/doors/double-door-conversion
- Cased openings, adding or trimming one — jmbbuildersmn.com/services/doors/cased-opening
- Bifold closet doors to French doors — jmbbuildersmn.com/services/doors/bifold-to-french-closet-conversion
- Labor-only installation of doors you bought yourself — jmbbuildersmn.com/services/finish-carpentry/door-installation-labor-only
- Trim and door painting, quoted as its own line — jmbbuildersmn.com/services/painting/trim-and-door-painting
- The finish carpentry hub — jmbbuildersmn.com/services/finish-carpentry
- Send three photos and we reply within one business day.
Frequently asked questions
Can I put French doors in a cased opening with no door?
Yes. A jamb is added to the opening, the doors are sized to fit, and the opening is cased. It is the most common French door conversion. A prehung unit is the usual choice because both doors arrive hinged in one jamb and already meet.
Should both French doors open?
Usually one door is the active door and the other is held with flush bolts or a catch, opened only when you need the full width. Both can be active with ball catches if you do not need a working latch. A room that has to lock needs one active door with a privacy latch and flush bolts on the other.
What holds interior French doors closed?
A ball catch or roller catch in the top of each door, snapping into a strike in the head jamb, is the simplest. Flush bolts on the inactive door plus a passage or privacy latch on the active door give a solid lock. Dummy handles go with catches; working handles go with a latch.
What size opening do interior French doors need?
Stock pairs are 48", 60" or 72" wide by 80" tall, made of two 24", 30" or 36" doors. A prehung unit needs a rough opening about 2" wider and 2" to 2-1/2" taller than the pair to allow for the jamb and shims. Measure the framed opening and the wall thickness before ordering, and send photos if the numbers land between sizes.
Can French doors replace a single interior door?
Yes, but the opening has to be widened, which means cutting the wall and framing a new header sized for the span. That is more work than fitting a pair into an existing double opening. Send photos of the wall from both sides and the quote will say what the framing involves.
Do interior French doors need tempered glass?
Yes. Glass in a door is a safety-glazing location under the residential code, so it is tempered and crumbles into small pieces instead of breaking into shards. Manufacturers ship interior French doors with tempered glass as standard, and the etched mark in the corner of each pane confirms it.
