French Door Installation: Rough Opening, Shimming, and Weather Sealing

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 the frame for plumb and level. Too loose and the shims are bridging too large a gap, which creates a weak connection between the frame and the framing.

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 of the roof, floor, or wall above the opening and transfers it to the jack studs on each side. For a typical 6-foot-wide French door opening in a single-story home, a doubled 2x10 or 2x12 header is common, but the exact size depends on the span, the load above, and the species of lumber. Consult a structural engineer or follow the IRC (International Residential Code) span tables for header sizing based on your opening width and the load conditions.

Interior French doors between rooms have simpler framing requirements because interior walls are usually not load-bearing. Verify this by checking the direction of the floor joists above. If the wall runs parallel to the joists, it is likely not bearing load. If the wall runs perpendicular to the joists, it may be carrying their weight. If in doubt, open a small section of the ceiling and look at how the joists connect to the top plate of the wall. A load-bearing interior wall still needs a header, just like an exterior wall.

Standard French door widths are 48 inches (two 24-inch doors), 60 inches (two 30-inch doors), and 72 inches (two 36-inch doors). The 60-inch and 72-inch sizes are the most common for exterior applications. Order the door unit before framing the rough opening so you can build the opening to the exact required dimensions. Each manufacturer specifies the required rough opening size on the product data sheet.

Pre-Hung vs. Slab Doors

Pre-hung French doors come fully assembled in a frame with hinges, astragal (the center vertical strip), weatherstripping, and hardware pre-installed. You set the entire unit into the rough opening as one piece and shim it plumb and level. This is the approach for 95 percent of installations, and it is the only practical approach for exterior French doors because the weatherstripping, threshold, and sill pan are integrated into the frame.

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 approach only makes sense when you are fitting doors into an existing jamb that is already in place and correctly sized, such as replacing damaged doors in an otherwise sound frame. For new installations, pre-hung is faster, more reliable, and typically looks better.

When ordering pre-hung, specify the swing direction (inswing or outswing for exterior doors) and the active door (the door that opens first and contains the primary handle and lock mechanism). The inactive door, sometimes called the passive or fixed door, has flush bolts at the top and bottom that lock it in place, and the active door latches against the astragal on the passive door. Changing the swing direction or active door designation after purchase usually means returning the unit and reordering.

Popular exterior French door brands include Andersen, Pella, Marvin, and Therma-Tru. For interior applications, Masonite, JELD-WEN, and Simpson offer a range of panel configurations and glass options. Expect to pay $800 to $2,500 for a quality pre-hung exterior French door unit and $400 to $1,200 for interior units.

Tools for the Job

French door installation does not require exotic tools, but it does require precise ones. Having everything on hand before you start prevents interruptions while the frame is partially secured.

• 4-foot level - for checking plumb on the jamb sides and level on the head jamb. A 6-foot level is even better for tall door frames.
• Tape measure - for verifying rough opening dimensions and checking diagonal measurements for square.
• Composite shims - cedar shims work but composite shims do not compress or rot over time. Buy two bundles.
• Drill/driver - for driving finish screws through the jamb into the framing. Cordless drill guide.
• Finish screws - 3-inch trim-head screws at hinge locations, 2-inch screws at other shim points. Trim-head screws are less visible than standard screw heads.
• Caulk gun and exterior sealant - for weather sealing exterior installations. Use a polyurethane or silicone sealant rated for exterior use.
• Pry bar - for removing existing door frames, trim, and making fine adjustments.
• Oscillating multi-tool - for trimming shims flush with the wall and cutting nails in tight spaces.
• Low-expansion foam - for insulating the gap between the jamb and the rough opening. Use the type labeled specifically for doors and windows.

Setting the Frame

Remove the packaging and any shipping braces that hold the doors in alignment during transit, but leave the doors latched to the frame. The latched doors keep the frame at the correct width while you shim. Set the entire unit into the rough opening on the subfloor or threshold.

Check the sill for level with your 4-foot level. If the floor is not level (and in older homes, it rarely is), shim under the low side of the sill until it reads level in both directions. This is the single most important measurement in the entire installation. If the sill is not level, the doors will swing open or closed on their own due to gravity, and the gap between the two doors will taper from top to bottom.

Shim behind the hinge locations on both jamb sides. Use pairs of tapered shims driven from opposite sides to create a flat, adjustable shim pack. This opposing-shim technique lets you make fine adjustments by tapping one shim or the other. Check plumb after each shim adjustment using your level on the face of the jamb.

Shim behind the strike plate and latch locations on the head jamb and the latch-side jamb. Check that the gap between the two doors (the reveal) is consistent from top to bottom. This gap should be approximately 1/8 inch - wide enough for the doors to swing without rubbing, narrow enough to look tight and intentional. If the gap tapers (wider at top than bottom or the reverse), one jamb is not plumb. Adjust the shims until the reveal is even.

Drive 3-inch finish screws through the jambs and shims into the framing at each shim point, starting at the top hinge. Do not over-tighten. The screw should pull the jamb snug against the shim pack, not bow the jamb inward. After driving each screw, open and close both doors to verify they still operate freely. If a door binds after you tighten a screw, you have pulled the jamb too tight at that point - back the screw out slightly and re-check.

Weather Sealing for Exterior Doors

Exterior French doors are one of the most common sources of water intrusion in residential construction. The wide opening, the two-door configuration, and the number of joints create multiple potential leak paths. Proper flashing and sealing at the time of installation prevents expensive water damage later.

Install a drip cap (Z-flashing) over the head casing that extends behind the siding above. The flashing should run the full width of the door frame plus 2 inches on each side. Its purpose is to catch any water running down the wall above and direct it over the door casing rather than behind it. Integrate the drip cap with the house's water-resistive barrier by slipping the top leg behind the building paper or housewrap.

Apply a continuous bead of exterior-grade polyurethane sealant between the door frame and the house sheathing on all sides. Then install the exterior casing (brick mold, flat casing, or whatever trim profile matches the house) over the joint. Apply a second bead of sealant where the casing meets the siding to create the final weather barrier. This two-layer approach provides redundancy - if one seal fails, the other still protects.

The sill pan is critical for exterior installations. Water that gets past the threshold needs a path to drain outward, not into the subfloor or wall cavity. Many pre-hung exterior French doors include an integrated sill pan with drainage channels. If your unit does not include one, install a formed metal or self-adhesive membrane sill pan before setting the door. The sill pan must slope slightly toward the exterior (1/8 inch per foot minimum) and have turned-up side dams that extend at least 2 inches up each side.

Adjust the weatherstripping after installation. French doors have weatherstripping on the meeting stile (astragal), on the hinge-side jambs, on the head jamb, and at the threshold. Close both doors and look for daylight gaps along all edges. Run your hand along the closed door edges and feel for drafts. Most weatherstripping adjusts with screws that move the strip closer to or farther from the door face, or by repositioning the strip in its channel.

Final Adjustments

Test the doors repeatedly. Open, close, latch, lock, and unlock both doors individually and together. Both doors should swing freely without rubbing on the floor, the frame, or each other. The latch should engage without forcing. The flush bolts on the passive door should slide smoothly into their strikes in the head jamb and threshold.

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 behind it, and re-secure. Sometimes a single hinge needs adjustment, not the entire jamb. If the top of the door rubs on the frame, the top hinge is set too deep - add a thin cardboard shim behind the hinge leaf to push the top of the door away from the frame.

For interior doors, install door stop molding after the doors are fully adjusted and operating correctly. The stop prevents the doors from swinging past the frame. Position the stop with the doors closed and latched. Place a playing card against the door face, press the stop against the card, and nail the stop in place. The playing card creates just enough clearance that the stop does not bind against the door when it closes.

Fill the gap between the jamb and the rough opening with minimally expanding foam. Use the type labeled specifically for doors and windows - standard expanding foam (like Great Stuff original) exerts enough force as it cures to bow the jamb inward, which throws off all your careful shimming work. Apply the foam in thin beads, filling no more than one-third of the cavity depth. The foam will expand to fill the remaining space. Trim the cured foam flush with the wall surface using a utility knife or oscillating multi-tool, then install interior casing to cover the gap.

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 since both are designed for the same general opening sizes. The height may need adjustment since French doors are typically taller than sliding doors by 1 to 2 inches. Check the rough opening dimensions against the French door unit you want to install before ordering. You may need to add framing material at the bottom or top of the opening to adjust the height.

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 factory-attached to the passive (non-latch) door. When both doors are closed, the astragal overlaps the edge of the active door, creating a seal against weather, light, and sound. The active door's latch engages a strike plate mounted on the astragal.

Are French Doors Secure?

Standard French doors are less secure than a solid entry door because the glass panels can be broken to reach the lock. For improved security, upgrade to impact-rated or laminated glass, which holds together even when cracked. For the door hardware, flush bolts at the top and bottom of the passive door prevent it from being pried open, and a multipoint locking system on the active door engages at three or more points along the frame height rather than just at the handle. Combined, these upgrades bring French door security close to that of a standard entry door.

Related Reading

• Door Installation Guide
• Window Replacement Guide
• Weatherproofing Guide