How to install French doors — a real walkthrough

A pair of French doors is two doors and one frame, and the difference between a great install and a mediocre one is almost entirely in the framing. The doors themselves arrive pre-hung from the factory and don’t need much from you. The rough opening, on the other hand, is where this job is won or lost.

Before you start: the rough opening must be right

The cardinal sin of French door installation is forcing a square unit into an out-of-square opening. The doors will sag, the astragal won’t meet, and the cremone bolt (if you have one) won’t engage. Spend an hour on the opening before you touch the unit.

Your opening should be:

• Height: door unit height + 1/2” (for shim space + adjustment)
• Width: door unit width + 3/4” (for shim space on both sides)
• Square: measure both diagonals; they must be within 1/8” of each other
• Plumb: the king studs (full-height studs flanking the opening) must be plumb in both planes — use a 4-foot level on each face

If the diagonals are off by more than 1/8”, you have rough framing that needs correction before you proceed. Don’t shim your way to square — the unit will fight you forever.

Step 1: dry-fit the unit

Pull the doors from the frame (most pre-hung French units allow this — the hinge pins lift out) and set them aside. The frame alone is much lighter and easier to handle.

Lift the frame into the opening. Center it left-to-right. Check the bottom: is the sill plate sitting flat on the subfloor? If not, you need to plane the high spots or shim the low ones now. A French door sill must be supported across its full length or it will flex and the doors will bind.

Step 2: shim the hinge side first

Always start with the hinge side. Set composite shims (cedar shims warp with humidity; composite or hardwood is the move) at three points: 6” from the top, at the middle hinge, and 6” from the bottom. Slide pairs of shims in from both sides until the jamb is plumb.

Check plumb in two planes. The jamb has to be plumb in the plane of the wall and perpendicular to the wall. A jamb that’s plumb left-to-right but leaning into or out of the wall will cause the doors to swing open or closed on their own.

Step 3: nail off the hinge side

10d finishing nails, through the jamb and shims, into the king stud. Three nails: one at each shim location. Set them but don’t drive them home — you may need to adjust on the strike side.

Step 4: rehang the doors and check the reveal

Put the doors back on. The gap between each door and the jamb (the “reveal”) should be uniform — about 1/8” — top to bottom on both sides. If it isn’t, your jamb is twisted, and you adjust by adding or removing shims at the middle hinge.

The two doors should meet at the astragal with about 1/8” gap, even, top to bottom. If they don’t, the jamb head or the strike side needs adjustment.

Step 5: shim and nail the strike side

Now repeat the shim process on the strike side, working in pairs at the same three heights. Check the reveal as you go. Once the reveal is even, nail off the strike side.

Step 6: shim and nail the head

The head jamb is the part above the doors. Shim it up against the rough framing — usually two pairs of shims, one above each side of the doors. Nail it off.

Step 7: trim, caulk, paint

Drive your finishing nails home with a nail set. Trim the shims flush to the jamb with a sharp chisel or utility knife. Caulk the gap between the jamb and the trim. Paint as needed.

The squaring step every tutorial skips

Before you touch the unit at all, square the rough opening. Most YouTube videos show installers slamming a unit into an opening and shimming it to plumb, ignoring that the opening itself is twisted. This is why their installs sag in two years. The doors are heavy; the frame transmits their weight into the rough framing; and a twisted opening puts asymmetric load into the unit, which racks slowly over time.

If you take one thing from this guide: spend an hour squaring the opening with sledge-and-block (or by sistering one of the king studs) before you do anything else. Your French doors will swing true for decades.

What can go wrong

• The astragal binds before the doors close: the strike-side jamb is twisted. Adjust middle-shim pairs.
• One door drags on the sill: the head jamb is sagging, or the sill is out of level. Re-shim.
• The cremone bolt won’t engage: usually a strike-plate alignment issue. Mortise the strike deeper by 1/16”.
• There’s a daylight gap at the top of the astragal: the doors are out of square in the frame. Check that both doors are hung true; re-set the hinge pin if one was inserted at an angle.

A note on the doors themselves

The Society has a collection entry on the Maplewood French, a 1924 Tudor revival pair built with twelve true divided lites and an integral astragal. Modern pre-hung French doors from a big-box store are not built to that standard. Don’t expect them to be. But a competently installed modern unit will still serve you for thirty or forty years, which is most of a house’s mortgage life.