Doors #

Skill Level: Intermediate (Level 2) #

Estimated Time: 2-3 Hours #

Introduction #

Learning how to install french door pairs adds elegance and openness to any living space, creating a seamless connection between rooms while still providing the ability to close them off for privacy, noise control, or temperature separation. Unlike a standard single door, French doors make a design statement and allow more natural light to flow between spaces, especially when fitted with glass panels.

When you install french door pairs in a center-opening configuration — also called “double doors” — two door slabs that meet in the middle of the opening with no fixed mullion or center post between them. Both doors are fully operable, meaning either one can swing open. This distinguishes them from configurations where one door is permanently fixed and only one side opens.

In a center-opening pair, one door is designated as the “active” door — the one used for everyday passage that carries the lockset or passage latch. The other is the “inactive” or “fixed” door, which is secured in place with flush bolts at the top and bottom. The inactive door is opened only when you need the full width of the opening, such as when moving furniture. The active door latches into a T-astragal (a meeting strip) that is attached to the edge of the inactive door.

The key challenge when you install french door units is getting both doors to meet evenly in the center with a consistent reveal gap and proper alignment from top to bottom. This requires careful shimming of BOTH jamb sides independently, and verifying that the center gap stays uniform before locking anything down. Take your time with this installation — the payoff is a set of doors that look and operate beautifully.

Before You Begin #

Prerequisites #

• Rough opening is properly sized for your double door unit (typically the door unit width plus 1″ for shimming space on each side, and the unit height plus 1/2″ to 3/4″ above)
• Header is structurally adequate for the span of the opening — see Family Handyman French door guide for additional header sizing considerations
• Floor is level across the full width of the opening — any slope will cause the doors to hang unevenly and swing on their own
• Framing is plumb on both sides and the header is level
• Drywall or wall finish is completed up to the opening edges

What You Need To Know #

• French door units come pre-hung as a complete assembly — both door slabs are already hung on hinges within a single door frame, making installation similar to a standard pre-hung door but wider and heavier
• The T-astragal (meeting strip) attaches to the edge of the inactive door — it provides the surface that the active door’s latch engages when closed
• Flush bolts hold the inactive door in place — one bolt shoots up into a receiving hole in the head jamb, and one shoots down into a receiving hole in the floor or threshold
• The active door latches into the astragal on the inactive door — this means the inactive door must be secured first before the active door can close properly
• Order of operation matters — hang the entire unit, secure and plumb the inactive door first, then adjust the active door to match

Tools Required #

Power Tools #

• Drill/driver with assorted bits
• Miter saw (for cutting casing to length)
• 18-gauge brad nailer (for attaching casing)

Hand Tools #

• 4-foot level
• Tape measure
• Hammer
• Shims (cedar or composite)
• Pencil
• Utility knife
• Chisel (for flush bolt mortising if not pre-mortised)

Supplies #

• 3″ wood screws (for securing jamb through shims into framing)
• 18-gauge brad nails (1-1/2″ to 2″ for casing)
• Construction adhesive
• Low-expansion spray foam insulation

Materials #

• Pre-hung French door unit (center-opening pair) sized for your rough opening
• Flush bolts — top and bottom (usually included with the door unit)
• T-astragal / meeting strip (usually included with the door unit)
• Casing material (enough for both sides of the opening — four vertical pieces and two head pieces)

Install French Door: Step-by-Step Process #

Step 1: Prepare the Rough Opening #

Before you install french door hardware, verify that the rough opening dimensions match the requirements for your specific door unit. Measure the width at the top and bottom, and the height on both sides. The opening should be approximately 1″ wider and 1/2″ to 3/4″ taller than the overall door frame dimensions.

Use your 4-foot level to check the jack studs on both sides for plumb, and check the header for level. If the floor is not level across the opening, identify the high side — you will set the door unit on the high side first and shim the low side up to match.

Remove any debris, protruding nails, or old shims from the rough opening. Make sure the framing is clean and provides solid backing for shimming and fastening.

Step 2: Set the Door Unit in the Opening #

With a helper, lift the entire pre-hung assembly to install french door unit into the rough opening. Center the unit so there is approximately equal shimming space on both sides. If the floor is not perfectly level, set the hinge side of the active door on the high point first.

Place shims under the jamb on the low side to bring the head jamb to level. Check the head jamb with your level and adjust until it reads true. Temporarily tack the unit in place with a couple of finish nails near the top on the active door’s hinge side — do not drive them home yet. This holds the unit in place while you focus on plumbing and shimming.

Pro Tip: French door units are heavy and awkward. Always use two people. Set the hinge side of the active door first — this is your reference point for the entire installation.

Step 3: Shim and Plumb the Active Door Hinge Side #

When you install french door frames, start with the active door’s hinge jamb as your primary reference side and place shims behind each hinge location. Insert shims from both sides so they overlap and create a flat, solid bearing surface between the jamb and the framing.

Check the hinge-side jamb for plumb using your 4-foot level — check it both face-to-face (across the width of the jamb) and edge-to-edge (front to back). Adjust shims until the jamb reads perfectly plumb in both directions.

Once plumb, drive 3″ screws through the jamb and shims into the framing at each hinge location. For added security, remove one of the short hinge screws from each hinge and replace it with a 3″ screw that reaches through the jamb and shims into the jack stud. This anchors the hinge side solidly to the framing.

Close the active door and check the gap (reveal) between the door slab and the head jamb. It should be consistent — about 1/8″ — from the hinge side across the top. If not, adjust your shims.

Step 4: Shim and Plumb the Inactive Door Hinge Side #

The next phase to install french door pairs is the opposite jamb — the hinge side of the inactive door. Place shims behind each hinge location, just as you did on the active side. Check this jamb for plumb in both directions.

Before fully securing this side, close both doors and carefully examine the gap where the two doors meet in the center. This center gap should be consistent from top to bottom — typically about 1/8″. If the gap is wider at the top than the bottom (or vice versa), the inactive jamb side needs adjustment. Move the shims in or out until the center gap is even.

Once the center gap is consistent and the inactive jamb is plumb, secure it with 3″ screws through the shims at each hinge location, and replace one short hinge screw per hinge with a 3″ screw into the framing.

Pro Tip: Check the gap where the two doors meet in the center BEFORE fully securing the inactive side. If the doors don’t meet evenly, adjust the inactive side shimming until the center gap is consistent top to bottom.

Step 5: Install Flush Bolts on Inactive Door #

When you install french door flush bolts, they hold the inactive door in position when it’s closed. One bolt is installed in the top edge of the door (shooting up into the head jamb) and one in the bottom edge (shooting down into the floor). Many pre-hung French door units come with flush bolts pre-mortised — if yours does not, you will need to mortise them in with a chisel.

With the inactive door closed and held in position, mark the locations where the flush bolt rods will contact the head jamb and the floor. Drill receiving holes at these marks — typically 1/2″ diameter and about 1/2″ deep. Make sure the holes are positioned so the bolts engage cleanly without binding.

Install the flush bolts into the door edge according to the manufacturer’s instructions. Test the bolts by engaging and disengaging them. They should slide smoothly and seat fully into the receiving holes. The inactive door should feel completely solid and immovable when both bolts are engaged.

Step 6: Install T-Astragal and Adjust Active Door #

To install french door astragal hardware, attach the T-astragal (meeting strip) to the edge of the inactive door where the two doors meet. This strip provides the surface that the active door’s latch engages when both doors are closed. If the astragal is not pre-installed, attach it to the inactive door edge using the screws provided, making sure it is straight and properly aligned with the door edge.

Engage the flush bolts to lock the inactive door in place. Now close the active door against the astragal. The active door’s latch should engage smoothly with the strike plate on the astragal. If the latch doesn’t align, adjust the strike plate position on the astragal — you may need to move it slightly up, down, in, or out.

Verify that both doors sit flush with each other when closed — they should be in the same plane with no steps or offsets. Check the reveal gap all the way around both doors: between the doors and the head jamb, between each door and its respective side jamb, and between the two doors at the center. All gaps should be consistent at approximately 1/8″.

Step 7: Install Casing and Final Adjustments #

With both doors properly aligned after you install french door hardware, it’s time to install the casing (trim) around the opening. Shim any remaining gaps between the jamb and the rough opening — at a minimum, shim at the top, middle, and bottom on both sides, and at least two points along the head jamb.

Apply low-expansion spray foam insulation in the gap between the door frame and the rough opening. Use minimal-expansion foam only — high-expansion foam can bow the jambs and cause the doors to bind. Allow the foam to cure, then trim flush with a utility knife.

Measure and cut your casing pieces. For a French door, you will need casing on both sides of the wall (front and back) — that means four vertical (leg) pieces and two horizontal (head) pieces. Set a consistent reveal on the jamb edge (typically 3/16″ to 1/4″) and mark it with a pencil or combination square before nailing.

Attach the casing with 18-gauge brad nails, nailing into both the jamb edge and the wall framing. Set any nail heads slightly below the surface and fill with wood putty if painting.

Pro Tip: With French doors, the casing reveals must match on BOTH sides of both jambs. Take your time setting consistent reveals before nailing casing. Inconsistent reveals are immediately noticeable on a wide opening like this.

Quality Check #

• ✅ After you install french door pairs, both doors hang plumb and are even in the same plane
• ✅ Center gap between the two doors is consistent top to bottom (approximately 1/8″)
• ✅ Active door latches smoothly into the T-astragal on the inactive door
• ✅ Flush bolts engage and release easily without binding
• ✅ Inactive door is solid and immovable when flush bolts are engaged
• ✅ Casing reveals are consistent on all sides of both jambs
• ✅ No binding, rubbing, or dragging when opening and closing either door
• ✅ Both doors swing to full open position without hitting walls, stops, or each other

Install French Door Troubleshooting #

Doors Don’t Meet Evenly in the Center #

If the gap between the two doors is wider at the top than the bottom (or vice versa), one of the jamb sides is out of plumb. Identify which side is off by checking both hinge jambs with a level. Loosen the screws on the out-of-plumb side, re-shim to bring it back to plumb, and re-secure. Recheck the center gap after adjustment.

Active Door Won’t Latch #

If the active door’s latch won’t engage the strike on the astragal, first verify that the inactive door is fully secured with both flush bolts engaged. If the inactive door has any play, the astragal will move and the latch won’t catch. If the bolts are engaged and the door is solid, the strike plate on the astragal may need to be repositioned — adjust it up, down, or laterally until the latch engages cleanly.

Flush Bolts Won’t Engage #

If the flush bolt rods won’t seat into the receiving holes, the holes are likely misaligned with the bolt positions. Close the inactive door, engage the bolt partially, and mark exactly where the rod contacts the head jamb or floor. Re-drill the receiving holes at the corrected locations. Make sure the holes are deep enough for the bolt to fully extend and seat.

Doors Swing Open on Their Own #

If doors swing on their own after you install french door frames, the cause is almost always an out-of-plumb jamb. A door will naturally swing toward the side that leans away from plumb. Recheck both hinge jambs with a level, adjust shimming to bring them to true plumb, and re-secure. Even 1/16″ out of plumb over the height of the door can cause this problem.

Center Gap Is Too Wide or Too Narrow #

If the gap between the two doors is consistently too wide or too narrow (but even top to bottom), the issue is the overall positioning of the inactive door’s jamb. To narrow the gap, add more shimming behind the inactive door’s hinge jamb to push it toward the center. To widen the gap, reduce shimming to pull it away. Make small adjustments and recheck — the goal is a consistent 1/8″ gap.

Related Guides #

• How to Install a Pre-Hung Door
• How to Install a Cased Opening
• How to Case a Window