A replacement door that arrives hinged on the wrong side is not a small annoyance. A prehung unit cannot simply be flipped: the jamb is bored, the weatherstripping is mitred for one direction, the threshold has a slope, and the lockset bore is drilled on one face. Getting the hand wrong usually means sending it back and waiting again.

Handing is one of those things that seems like it should be obvious and then produces two confident, opposite answers from two people standing in the same doorway. There is a convention, it is not intuitive, and it is worth learning properly.

The convention: stand on the outside

Handing is determined from the outside of the door, which for a residential entry door means standing outdoors looking in. For an interior door, the “outside” is the side the door swings away from — the side you push from. Some trades call it the security side or the key side, which is a useful memory aid on an entry door: the side with the keyway is the side you stand on.

From that position:

• Hinges on your left and the door swings away from you: left-hand (LH).
• Hinges on your right and the door swings away from you: right-hand (RH).
• Hinges on your left and the door swings toward you: left-hand reverse (LHR).
• Hinges on your right and the door swings toward you: right-hand reverse (RHR).

“Reverse” means outswing. On a residential entry in Ontario, inswing is far more common, mostly because an outswing door’s hinges are exposed outdoors and because snow banking against an outswing door is a real problem. Outswing entries do exist, and they have genuine advantages in a tight entryway or where water management is difficult — but they need non-removable hinge pins and a threshold designed for the direction.

The shorthand that causes the arguments

“It opens to the left” means nothing without a reference point, which is why two people disagree. Neither are hinge photographs taken from the inside, which is what most people send. If you are describing a door to a supplier, say where you were standing and which side the hinges were on. Better still, give them the hand in the four-letter form above.

Slab or prehung, and why it changes everything

A slab is just the door. A prehung unit is the door already hung in a new frame, with hinges mortised, weatherstripping fitted, and usually a threshold attached.

If you are replacing a slab into an existing frame, handing still matters but the frame does the deciding — you need a slab with hinge mortises and a lock bore that match what is already there, which in an older house frequently means a non-standard backset or a mortise lock pocket rather than a cylindrical bore.

If you are replacing a prehung unit, you are replacing the frame too, and now the rough opening, the jamb depth and the sill all have to be right as well. That is usually the better repair when the frame itself has moved or rotted, and it is the normal scope for an entrance door replacement.

The four measurements people get wrong

Jamb depth

The thickness of the wall the frame has to span, measured from the inside face of the interior trim to the outside sheathing or masonry. Standard depths exist, but a house with strapping, added insulation, or a brick veneer over an older frame wall rarely matches one. A unit with the wrong jamb depth can sometimes be extended, but it is a compromise.

Rough opening, not the door size

The rough opening is the framed hole, not the door. You measure it with the old unit out, or you measure the existing frame and add the known allowances. Measuring the old door and ordering that size is how people end up with a unit that will not go in.

Square, plumb and level — measured, not eyeballed

Measure both diagonals of the opening. If they differ by more than a few millimetres, the opening is racked, and a prehung unit is going to need shimming that eats into the reveal. Check the sill with a level across its width as well: a sill that slopes along its length will put the door out of square the moment it is fastened down.

Threshold height against the finished floor

Especially where flooring has been replaced since the original door went in. A new threshold that sits proud of a floor that was raised 12 mm by underlay is a trip hazard and a door that drags.

Hardware details that have to match the hand

• Lever handles. Most modern residential levers are reversible. Many are not. Check before assuming.
• Mortise locks and multi-point systems. Handed, and not casually convertible. On a multi-point unit the gearbox and the strike plates are both direction-specific.
• Deadbolt and lock bore backset. The distance from the door edge to the centre of the bore. Older doors commonly use a backset that modern off-the-shelf hardware does not offer without an adaptor.
• Thresholds and sweeps. An inswing threshold and an outswing threshold shed water in opposite directions. Fitting the wrong one is a leak waiting for the first driving rain.

Where swing direction is not a preference

On a residential entry you can usually choose. There are places you cannot. Doors serving exits in commercial and assembly occupancies generally have to swing in the direction of travel, and a door’s required swing can also be driven by the space available on either side — a door that opens into a stair landing or blocks a corridor is a problem whatever the homeowner prefers.

Closer to home, the swing also decides whether a door is usable. Measure the floor space on both sides before changing a swing direction. A door that now opens into a hallway two feet wide looks fine on paper and is a daily irritation in practice.

Why the wrong door still arrives

Three reasons, in rough order of frequency. The hand was described in words instead of in the convention. The hand was taken from a photo shot from the inside. Or the hand was correct and the jamb depth or rough opening was not, so the unit is right-handed and still will not fit.

The way to avoid all three is boring and effective: write the hand in its four-letter form, note which side you were standing on when you determined it, record the rough opening width and height, the jamb depth, and both diagonals. Photograph the hinge side, the lock edge and the threshold. That set of information answers almost every question a supplier will have.

It is also worth being honest about when a new door is the wrong answer. A slab that sticks, a latch that misses, or a frame that has moved slightly is frequently a residential door repair rather than a replacement — hinges, strike position and frame adjustment solve a surprising share of what looks terminal. Where the door genuinely is past fixing, handing and measurements decided before ordering make the installation a single visit instead of two.

Frequently asked questions

Can a prehung door be re-handed on site?

Rarely worth it. On a simple interior unit with a reversible latch it is sometimes possible. On an exterior prehung unit with weatherstripping, a sloped threshold and a bored lock, no — you would be rebuilding the unit.

Does handing matter if I am only replacing the slab?

Yes, because the hinge mortises and the lock bore are cut into one face and one edge. A slab for the opposite hand has them on the wrong sides.

How do I hand a double door?

You identify the active leaf — the one that opens first and carries the lock — and hand that one. The inactive leaf is described in relation to it. Pairs are also where coordinators and astragals come in, which is a separate set of decisions.

My opening is out of square. Do I need the frame rebuilt?

Not always. Small amounts are absorbed by shimming. Larger amounts mean the door will only ever latch in one position and will worsen with seasonal movement, and the opening should be corrected before a new unit goes in. The diagonal measurement is what tells you which situation you are in.

Should I match the old door’s size exactly?

Match the rough opening, not the old door. Old doors were sometimes trimmed on site, and copying a door that was planed down to fit a settled frame reproduces the problem in a new unit.