Most bad door installations are not bad carpentry. They are bad measuring, done three weeks earlier by someone who took one number and assumed the rest. By the time the door is on the driveway and the old one is out, nobody wants to hear that the jamb depth is wrong, so it gets made to work — and “made to work” is why the door binds in July and lets a draft through in January.

Here is the full set of measurements a replacement door actually depends on, why each one matters, and what the usual shortcut costs you.

The measurements that decide the fit

Rough opening, not door size

The rough opening is the hole in the framing, before any jamb. A pre-hung door needs clearance all the way around for shimming and for the frame itself — not a lot, but a specific amount, and it has to exist on both sides and at the head. Measure the opening in three places across and three places up. Older Toronto houses rarely give you the same number twice, and the smallest of the three is the one that governs.

The shortcut people take is measuring the existing door slab and ordering that size. That tells you nothing about whether the frame it is sitting in is still square, and on a house that has settled it usually is not.

Jamb depth, which is the one that gets forgotten

Jamb depth has to match the wall thickness — framing plus sheathing plus interior finish. Standard depths exist, but a house with strapping, with an added layer of insulation, or with original plaster over lath will not match any of them. A jamb that is too shallow leaves the frame proud of the drywall and gives you a trim problem forever. Too deep and the frame sticks out past the exterior cladding, which is a water problem rather than a cosmetic one.

Measure it with the trim off at one spot, or at minimum from a window reveal in the same wall. Do not guess it.

Swing and handing

Which way the door opens, and which side the hinges are on. It sounds trivial and it is the most common ordering error in the business, because the convention depends on which side you are standing on. Agree the description in plain language — “hinges on the left as I stand outside looking at the door, opens inward” — and write that down rather than relying on a two-letter code.

Swing also has consequences the code cares about and that you will care about daily: where it lands relative to a landing, whether it clears a storm door, and whether it blocks a light switch or a stair.

Sill height and the step down

Measure from the finished interior floor to the top of the sill, and from the sill to the landing outside. These two numbers decide whether the new threshold sits flush inside and whether the step down outside stays reasonable. A new door with a taller sill assembly than the old one quietly raises the step every time, which is how a porch that was fine becomes a trip hazard.

Backset and bore for the hardware

If you are reusing a lockset — and plenty of people do, for good reason — the bore diameter, the backset from the door edge to the centre of the bore, and the door thickness all have to match. Older Canadian doors turn up with backsets that are not what a modern slab is machined for. Checking this before ordering is five minutes; discovering it on install day means either new hardware or a new hole in a brand-new door.

Whether the opening is square and plumb

Measure both diagonals of the opening. If they differ, the opening is racked, and a pre-hung unit set into it will either bind or show an uneven reveal. This is routine and fixable with shimming, but it has to be known in advance, because a badly racked opening sometimes means reframing. Spotting it early turns a surprise into a planned step of the door installation in Toronto.

The things that are not measurements but decide the outcome anyway

What the sill is sitting on

Pull the old threshold and look. Rot at the subfloor and the bottom of the studs is extremely common in houses where the sill flashing was never right, and it is invisible until that moment. Setting a new door on soft framing is the single most expensive mistake in the whole job, because the fix is the same work twice.

Flashing and drainage

The sill pan, the flashing laps and the slope away from the door are what keep the next twenty years dry. They cost very little during the install and are effectively unfixable afterwards without pulling the door back out.

Insulation and air sealing around the frame

The gap between the jamb and the framing is where a lot of winter draft comes from, and spray foam used carelessly here bows the jamb and makes the door bind. Low-expansion foam, or backer rod and sealant, applied after the door is shimmed and fastened — not before.

Repair, or replace?

Not every problem that feels terminal is. A door that drags, sticks seasonally or will not latch is usually a hinge, frame or sill issue, and residential door repair handles it for a fraction of a replacement. Replacement earns its keep when the slab itself is delaminating or split, when the frame has rot through its section, when the opening needs to change size, or when you want a different material or insulation level. For a tired front entrance that is sound underneath, a full Toronto entrance door replacement is a choice rather than a necessity — worth making deliberately.

What to have ready before anyone quotes

• Rough opening width and height, measured in three places each.
• Wall thickness at the opening, for jamb depth.
• Both diagonals of the opening.
• Swing and hinge side, written out in words.
• Sill to interior floor, and sill to landing.
• Door thickness, bore size and backset if hardware is being reused.
• Photos of all four corners, inside and out.

Frequently asked questions

Can I just order the same size as my current door?

Only if the frame is square and staying. The slab size tells you about the slab; the rough opening tells you about the house. On a settled older home those two have usually drifted apart.

How long should a front door installation take?

A straightforward pre-hung swap in a sound opening is typically a single day’s work. What stretches it is what is found behind the old threshold. Ask what the plan is if rot turns up, before the day starts.

Do I need a permit to replace a door in Toronto?

Like-for-like replacement in the same opening generally does not require one; changing the size of the opening or touching structure above it generally does. Your municipal building department is the authority on the specific case.

Why does my new door bind only in summer?

Either the jamb was foamed tight and bowed, or the frame is moving with seasonal moisture, or the hinge screws never caught solid framing. All three are adjustable. None of them are the slab’s fault.

The short version

Three numbers across, three up, both diagonals, the wall thickness, and the two sill heights. Write the swing out in words. Look under the old threshold before anything is ordered. A door that was measured properly rarely needs a second visit; a door that was measured by eye almost always does.