French door replacement

Why two doors are harder than one

A single door has one job: meet a frame on three sides. A pair of French doors has to do that and also meet itself, in the middle, along a seam that runs the full height of the opening. Every source of movement a single door shrugs off, seasonal humidity in the jambs, a house settling a sixteenth of an inch, a hinge that has taken a slightly different set than its mate, shows up at that center meeting point as a gap, a rub, or a latch that won't throw. There is nowhere for the error to hide.

That is the honest reason French doors cost more to install well than two single doors of the same size would. The opening has to be built plumb and square with less tolerance than a single door forgives, because both leaves are referencing each other as much as they are referencing the frame.

The astragal and the leaf that isn't supposed to move

On most French door pairs, only one leaf is the active door you actually walk through day to day. The other is the inactive leaf, and it is supposed to stay put. It gets there through an astragal, a vertical strip fixed to the edge of the inactive leaf that the active leaf closes against, and a set of flush bolts, usually one at the top and one at the bottom, that drive into the head jamb and the sill or threshold to lock that leaf in place.

Flush bolts are simple and they work, but they rely on someone actually throwing them, and a bolt that only gets engaged occasionally tends to seize or get skipped. A multipoint lock changes that. Instead of two separate bolts a homeowner has to remember, a single handle motion on the active leaf drives locking points into the head and sill simultaneously, and on many systems it also engages the inactive leaf through the astragal. It costs more up front and it is worth it on a door you actually want to rely on for security, not just for looks.

Whichever hardware set is on the door, the astragal itself has to be weatherstripped on both faces, because it is doing the sealing job that a single jamb does on a normal door. A worn or poorly fitted astragal seal is the single most common place a French door pair drafts.

In-swing or out-swing, and why it matters more here

The same in-swing and out-swing tradeoffs that apply to any entry door apply to French doors, but the stakes are higher because you are managing two leaves instead of one. An in-swing pair needs a genuinely clear arc inside the room, roughly the full width of both doors, or furniture ends up rearranged around it. An out-swing pair needs that same clearance outside, and it puts both sets of hinges on the exterior, which means non-removable pins or security studs matter more here, simply because there are twice as many hinges exposed.

Out-swing has one real advantage specific to French doors: it lets the wind load push both leaves into their seals instead of away from them, which matters on an exposed patio or deck. In-swing is still the more common choice, mostly because it keeps the hardware and hinge pins protected and plays better with a covered porch.

Hinge loading on a leaf that is doing more work than usual

A standard exterior door leaf is roughly three feet wide. A French door leaf is usually narrower, often two and a half feet or less, because the opening is being split between two of them. That sounds like it should make hinge loading easier, and in terms of raw weight per hinge it does. Where it gets harder is that a French door leaf is frequently glazed edge to edge, sometimes full lite, which puts more weight into a narrower panel and shifts where that weight sits relative to the hinge line. A glass-heavy leaf that isn't hung with hinges rated for the actual weight will sag at the lock side over a few years, which is exactly what throws the meeting point at the astragal out of alignment and starts the drafting and sticking that gets a French door pair a bad reputation it didn't need to earn. Three hinges per leaf, sized to the door's actual weight rather than a generic spec, is the fix that costs nothing extra if it's specified correctly the first time. A pair that's already sagging at the lock side is often a hinge adjustment, not a reason to replace the unit outright.

Interior French doors are a different job

Interior French doors, between a dining room and a den, say, or closing off a home office, share the double-leaf mechanics but drop most of what makes an exterior pair demanding. There is no threshold to flash, no sill to worry about water at, and no weatherstripping doing real work against weather, just sound and draft between rooms. Because there is no exterior load to resist, interior French doors can also run lighter, with thinner stiles and simpler hinges, and the astragal hardware is often just flush bolts rather than a full multipoint system, since security isn't the point.

What still matters on an interior pair is the same thing that matters on every double door: a square, plumb opening, because the two leaves still have to meet each other cleanly. A slightly out-of-square interior opening is more forgiving to live with than an exterior one, since nothing is trying to get in around the gap, but it still shows as a visible reveal that isn't even side to side, which is the kind of detail people notice in a dining room even if they can't say why it looks off.