How to Install a Prehung French Door: Frame, Shim & Seal
By WholesaleDoorHub Team · May 2026
Direct answer: Installing a prehung French door takes 2–4 hours per opening for an experienced carpenter. The critical steps are: verify rough opening, set and plumb the unit, shim at hinges and strike, fasten through jamb, and install casing.
Tools and Materials Required
| Tool | Purpose |
|---|---|
| 4-foot level | Plumb and level check |
| Drill + 3" screws | Fasten jamb through shims |
| Shims (cedar) | Fill gaps, transfer load |
| Utility knife | Score and snap shims |
| Finish nailer | Attach casing |
| Tape measure | Verify rough opening |
| Pry bar | Adjust unit position |
Step-by-Step French Door Installation
Step 1: Verify the Rough Opening
Measure width at top, middle, and bottom. Measure height on both sides. All measurements should be within 1/8" of each other. If not, correct framing before proceeding.
Rough opening formula: Door unit width + 2" = rough opening width. Door unit height + 2" = rough opening height.
Step 2: Check for Square and Level
Use a level on the sill. If the sill is out of level by more than 1/4", shim the low side before setting the unit. An out-of-level sill causes the active panel to swing open or closed on its own.
Step 3: Set the Unit
Remove the shipping straps and temporary spreader bar (if present). Lift the unit into the rough opening. Center it side-to-side, leaving equal gaps on each side.
Step 4: Shim and Plumb the Hinge Jamb
Place shims at each hinge location. Drive a single 3" screw through the hinge jamb at each shim. Check plumb after each screw. The hinge jamb is your reference — get it right before touching the latch jamb.
Step 5: Shim and Fasten the Latch Jamb
With the hinge jamb plumb, close the active panel. Check the reveal (gap between door and jamb) — it should be consistent at 1/8" from top to bottom. Shim and fasten the latch jamb to achieve this.
Step 6: Check Door Operation
Open and close both panels. They should swing freely without binding. The latch should engage cleanly. If the door swings open or closed on its own, the hinge jamb is not plumb — adjust before proceeding.
Step 7: Install the Astragal
The astragal (center meeting stile) installs on the inactive panel. Follow the manufacturer's instructions for your specific unit. Verify that the active panel latches against the astragal cleanly.
Step 8: Fasten and Trim
Drive remaining screws through jamb at all shim locations. Score and snap protruding shims flush. Install casing on both sides, maintaining a consistent 3/16" reveal on the jamb.
Common Installation Mistakes
| Mistake | Consequence | Fix |
|---|---|---|
| Skipping rough opening check | Door won't fit or binds | Always measure before ordering |
| Hinge jamb not plumb | Door swings on its own | Re-shim and re-fasten |
| Screws through jamb without shims | Jamb bows, door binds | Always shim before fastening |
| Astragal installed before door operation verified | Must remove astragal to adjust | Verify operation first |
FAQ
Q: How long does it take to install a French door?
A: 2–4 hours per opening for an experienced installer. First-time installers should budget 4–6 hours.
Q: Do I need a special rough opening for a French door?
A: Standard rough opening is door unit width + 2" and height + 2". For units with sidelites, add 6" per sidelite to width.
Q: Can French doors be installed in an exterior wall?
A: Yes, but use an exterior-rated unit (solid wood or fiberglass core, weatherstripping, threshold). Interior-rated units are not suitable for exterior applications.
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