French Door Installation: Rough Opening, Shimming, and Weather Sealing
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French doors — a pair of hinged doors that open from the center — add light and access between rooms or between a room and an exterior space. Installing them is more demanding than a single door because both doors must align with each other and with the frame. If the rough opening is not plumb, level, and square, the doors will not meet in the middle and the latch will not engage. Here is the process from framing to final adjustment.
Rough Opening Requirements
The rough opening should be 1 inch wider and 1/2 inch taller than the outside dimensions of the door frame. This provides shimming space on all sides. Too tight and you cannot adjust. Too loose and the shims are bridging too large a gap.
For exterior French doors replacing a window or creating a new opening in a load-bearing wall, you need a properly sized header. This is structural work — the header carries the weight above the opening and transfers it to the jack studs. Consult a structural engineer or follow the IRC span tables for header sizing based on your opening width and the load above.
Interior French doors between rooms have simpler framing requirements because interior walls are usually not load-bearing. Verify by checking the direction of the floor joists above — if the wall runs parallel to the joists, it is likely not bearing load. If in doubt, open the wall and look.
Pre-Hung vs. Slab Doors
Pre-hung French doors come assembled in a frame with hinges, astragal (the center strip), and hardware pre-installed. You set the entire unit into the rough opening and shim it plumb and level. This is the approach for 95 percent of installations.
Slab doors (doors without a frame) require you to build or modify the jamb, mortise the hinges, drill for hardware, and align everything from scratch. This only makes sense when you are fitting doors into an existing jamb that is already in place and correctly sized.
When ordering pre-hung, specify the swing direction (inswing or outswing for exterior) and the active door (the door that opens first and contains the primary handle/lock). Changing this after purchase usually means returning the unit.
Setting the Frame
Remove the packaging and any shipping braces, but leave the doors latched to the frame. Set the entire unit into the rough opening on the subfloor or threshold.
Check the sill for level. If the floor is not level, shim under the low side of the sill until it reads level. This is the single most important measurement in the installation — if the sill is not level, the doors will swing open or closed on their own.
Shim behind the hinge locations on both jamb sides. Use pairs of tapered shims driven from opposite sides to create a flat, adjustable pack. Check plumb after each shim adjustment.
Shim behind the strike plate and latch locations. Check that the gap between the two doors is consistent from top to bottom — this should be about 1/8 inch. If the gap tapers, one jamb is not plumb.
Drive finish screws through the jambs and shims into the framing at each shim point. Do not over-tighten — the screw should pull the jamb snug against the shim, not bow the jamb inward.
Weather Sealing for Exterior Doors
Exterior French doors require flashing above the header to prevent water from entering the wall cavity. Install a drip cap (Z-flashing) over the head casing that extends behind the siding above.
Apply a continuous bead of exterior-grade sealant between the door frame and the house sheathing on all sides. Then install the exterior casing (brick mold or flat casing) over the joint. A second bead of sealant where the casing meets the siding provides the final weather barrier.
The sill pan is critical. Water that gets past the threshold needs a path to drain out, not into the subfloor. Many pre-hung exterior French doors include an integrated sill pan. If yours does not, install a formed metal or membrane sill pan before setting the door unit.
Adjust the weatherstripping after installation. French doors have weatherstripping on the meeting stile (astragal) and on all four edges of the frame. Close the doors and check for daylight gaps. Most weatherstripping adjusts with screws or by repositioning the strip in its channel.
Final Adjustments
Test the doors repeatedly — open, close, latch, lock. Both doors should swing freely without rubbing on the floor, the frame, or each other. The latch should engage without forcing.
If one door rubs at the top or bottom, the hinge jamb is not plumb. Remove the screws at the problem hinge, adjust the shims, and re-secure.
For interior doors, install stop molding after the doors are adjusted. The stop prevents the doors from swinging past the frame. Position the stop with the doors closed and latched — a playing card against the door face gives the right clearance gap.
Fill the gap between the jamb and the rough opening with minimally expanding foam (use the kind labeled for doors and windows — standard expanding foam exerts enough pressure to bow the jamb). Do not use excessive amounts. Trim the cured foam flush with the wall surface.
Frequently Asked Questions
Can I install French doors where a sliding door is now?
Yes, this is a common upgrade. Sliding door rough openings are usually wide enough for French doors. The height may need adjustment since French doors are typically taller than sliders. Check the rough opening dimensions against the French door unit you want to install before ordering.
How do you handle the center gap between the two doors?
The astragal — a vertical strip attached to one door — covers the gap and provides a weather seal and a latch surface for the other door. On pre-hung units, the astragal comes attached to the passive (non-latch) door. It overlaps the active door edge when both doors are closed.
Are French doors secure?
Standard French doors are less secure than a solid entry door because the glass can be broken. Upgrade to impact-rated or laminated glass for better security. Flush bolts (top and bottom of the passive door) and a multipoint locking system on the active door provide strong structural security for the door operation itself.