How to Install French Doors: Step-by-Step Guide

A pair of French doors can transform a dark dining room or a closed-off den, but a double door is far less forgiving than a single one. If the frame is even 1/8 inch out of plumb, the two slabs will meet crooked at the center, rub at the top, or leave a gap you can slide a credit card through. Learning how to install french doors properly means focusing on the rough opening, the sill, and the center astragal, then taking your time with shims. This guide covers both prehung exterior units and interior pairs, with the tools, timing, and checks a finish carpenter uses.

Part of our Windows & Doors guide — explore the full guide →

How to Install French Doors: Planning Before You Start

Most homeowners buy a prehung unit, meaning both slabs arrive hung in a frame with hinges, sill, and weatherstripping in place. That is the right choice for almost every DIY install. Decide these details before ordering:

• Exterior or interior: Exterior units include an insulated sill, weatherstripping, and usually a multi-point lock. Interior units are lighter and have no sill.
• Swing direction: Inswing is standard for most US homes; outswing resists wind-driven rain better and saves interior floor space.
• Active and passive leaf: One door is active with the handle; the other is held with flush bolts top and bottom. Choose which side opens first.
• Rough opening: Typically the unit width plus 1/2 inch and height plus 1/2 inch. Confirm the exact spec on the product sheet.

A prehung exterior pair costs roughly $900-$3,500 depending on material, glass, and hardware. Professional installation typically runs $500-$1,500 when the opening already exists, and more if a wall must be opened and a header added.

Tools and Materials

• 4-foot and 6-foot levels, or a laser level
• Tape measure, speed square, and pencil
• Composite or cedar shims in bundles
• Drill/driver, 2-1/2- and 3-inch exterior-grade screws
• Pneumatic or cordless finish nailer with 2- to 2-1/2-inch nails
• Flexible self-adhered flashing tape (4 and 9 inches wide) and a sill pan or pan flashing
• Exterior sealant and low-expansion window-and-door foam
• Pry bar, oscillating multitool, utility knife
• A helper; a pair of exterior doors can weigh 150-250 lbs

Measuring and Ordering the Right Unit

Measure the existing opening after removing interior casing so you can see the framing. Take the width between jack studs at three heights and use the smallest number. Take the height from the subfloor to the underside of the header on both sides. Also measure wall thickness from interior drywall to exterior sheathing; standard jambs are 4-9/16 inches for 2×4 walls and 6-9/16 inches for 2×6 walls. Ordering the wrong jamb depth means adding extension strips later, which is doable but adds a day.

Lead times for custom exterior units can stretch 4-10 weeks, so order before you remove the old door. Store the new unit flat or upright on a padded surface in a dry space until installation day.

Step 1: Prepare the Rough Opening

Remove the old door and trim, and inspect the opening. Measure the width at the top, middle, and bottom and the height on both sides. The opening should be within 1/4 inch of square; measure the diagonals and compare. Check the subfloor with a level. A sill that sits on an out-of-level floor causes most French door headaches, so shim or plane the floor now if it slopes more than 1/8 inch across the opening.

If you are creating a new opening in a load-bearing wall, you need a properly sized header, jack studs, and usually a permit. A 6-foot opening in a typical bearing wall might require a doubled 2×10 or engineered LVL header; confirm with your local code or an engineer.

Step 2: Flash the Sill and Opening

Water intrusion at the sill ruins more subfloors under exterior doors than anything else. Install a sill pan, either a preformed plastic pan ($40-$90) or one built from flexible flashing tape. The pan should have upturned back and end dams and slope slightly toward the exterior. Run flashing tape up the jack studs 6 inches.

Apply three continuous beads of sealant across the sill pan where the door sill will sit, leaving the front edge open so any water that gets in can drain out. On the exterior, integrate the weather-resistive barrier so upper layers lap over lower layers, like shingles.

Step 3: Set and Plumb the Frame

Remove any shipping braces only after the unit is in position if the manufacturer instructs it. With a helper, tilt the unit into the opening from the outside, setting the sill into the sealant first. Center it side to side.

1. Drive one screw through the hinge-side jamb near the top hinge, without fully tightening.
2. Plumb the hinge jamb in both directions with a 6-foot level, adding shims behind each hinge.
3. Replace one short screw in each hinge with a 3-inch screw that reaches the framing. This locks the jamb to the stud.
4. Shim the opposite jamb at the flush bolt and strike locations, top, middle, and bottom.
5. Check that the head jamb is level and the reveal between the doors and frame is even, around 1/8 inch.

Never over-shim. Pushing a jamb inward bows it and makes the doors rub. Close both doors and look at the center meeting line. The astragal edges should be parallel top to bottom.

Step 4: Adjust the Astragal and Hardware

The astragal is the vertical strip on the passive door where the active door closes. Install the flush bolts or surface bolts into the head jamb and sill strike locations. Drill for the strike plates exactly where the bolts meet, typically with a 1/2- or 5/8-inch bit.

Install the handle set and deadbolt per the instructions. If the unit has a multi-point lock, check that all three latch points engage without forcing the handle. Tight latching usually means the jamb or head is slightly out of alignment; adjust shims before adjusting strikes. Most modern hinges allow small adjustments by turning a screw, which can move a slab 1/16 inch or so.

Step 5: Insulate, Seal, and Trim

Once the doors swing freely and latch cleanly, fill the gaps between the jamb and framing with low-expansion window-and-door foam. Standard expanding foam can bow jambs, so use only the product labeled for doors and windows. Fill about halfway; the foam expands to fill the rest. Let it cure, then trim flush.

On the exterior, install brick mold or trim and seal the joint to siding with a high-quality exterior sealant. Leave the bottom sill-to-pan joint open to drain. Inside, install casing and caulk the seams, then fill nail holes and paint or stain.

Installing Interior French Doors

Interior pairs follow the same steps but skip the sill pan and weatherproofing. Leave a 1/2- to 3/4-inch gap under the doors for flooring and air movement, more if you plan on thick rugs. If you are installing new hardwood, tile, or LVP soon, set the door after the floor so you do not have to trim the bottoms. Interior prehung pairs usually include ball catches or roller catches rather than flush bolts, and they are lighter, making them a reasonable one-day project for most confident DIYers.

Common Mistakes and Troubleshooting

• Doors meet unevenly at the top: One jamb is out of plumb or the head jamb sags. Re-shim before blaming the hinges.
• Air leaks at the center: The astragal weatherstrip is not compressing. Adjust strikes and check that both doors sit flush in the same plane.
• Water at the sill: Missing or reversed flashing, or sealant blocking the drainage path at the front of the pan.
• Doors swing open on their own: The hinge jamb is out of plumb toward the room. Adjust the shims at the top hinge.
• Sticking in summer: Wood doors swell with humidity. Seal all six sides of wood slabs, including top and bottom edges.

How Long It Takes and When to Hire a Pro

An experienced DIYer can install a prehung interior pair in 4-6 hours and an exterior pair in a full day, plus time for trim and paint. Call a professional if you need to cut a new opening in a load-bearing wall, if the floor is badly out of level, or if the door is heavy fiberglass or steel with a large glass area. Understanding how to install french doors the right way means respecting the sill, the plumb jambs, and the center line. Get those three right and the doors will swing smoothly, seal tightly, and look sharp for decades.