Almost every door that gets returned was measured once, quickly, with a tape held against the old door. That number tells you the size of the slab currently hanging there. It does not tell you whether a new one will fit, and on an older Toronto house it frequently will not.
The decision between a pre-hung unit and a bare slab is really a decision about the frame. So the measuring has to start there.
What the two options actually are
A slab is just the door. No frame, no hinges, usually no bore holes unless you specify them. It hangs in the frame that is already in the opening.
A pre-hung unit is the door already mounted in a new frame, with hinges fitted and the strike mortised. The whole assembly goes into the rough opening after the old frame comes out.
Slab replacement is less work and less disruption. Pre-hung is more work but fixes problems a slab cannot touch.
The question that decides it
Is the existing frame square, solid and properly anchored?
Check three things. Close the door and look at the gap around it: it should be roughly even down both sides and across the top. A gap that tapers means the frame is out of square or the house has settled. Press a fingernail into the jamb near the bottom, especially on an exterior door, and see whether it is soft. Then look at whether the strike plate sits where the latch naturally lands.
Even gaps, solid wood, aligned strike: a slab will work. Tapering gaps, soft jamb, strike that has been moved or shimmed: a slab will fit badly no matter how carefully it is ordered, and the frame is what needs replacing. On an exterior entrance with rot at the sill, a pre-hung unit is usually the only honest answer, and that is entrance door replacement rather than a door swap.
Measuring for a slab
Take the door off, or measure it in place carefully.
- Width. Measure the slab itself at the top, middle and bottom. Use the largest number, but note all three, because a difference of more than a couple of millimetres tells you the old door was trimmed on site.
- Height. Measure the slab at both edges and in the centre.
- Thickness. Typically 1 3/8 inches for interior doors and 1 3/4 for exterior. Measure it rather than assuming, especially in an older house.
- Swing direction. Stand on the side the door swings toward you. If the hinges are on your left, it is a left-hand door. This single item causes more ordering mistakes than any measurement.
- Hinge positions. Measure from the top of the slab to the top of each hinge, and record the hinge size and whether the corners are square or radiused.
- Bore and backset. Distance from the edge of the door to the centre of the handle bore, plus the bore diameter and the height of the bore from the floor.
Order the slab to match, then expect light trimming. Most solid doors allow a small amount off each edge; hollow-core doors allow far less, and taking too much off exposes the void.
Measuring for a pre-hung unit
Here you are measuring the opening, not the door.
- Rough opening width and height. This is the framed opening behind the trim. It normally runs about two inches wider and two and a half inches taller than the door size, to leave room for the frame and shims. If the trim is staying on, measure the finished opening and note that the rough opening has to be confirmed once the casing comes off.
- Jamb width. Measure the wall thickness from the interior finished surface to the exterior finished surface, including drywall and sheathing. Common jamb widths are standard sizes, but older Toronto homes with plaster, strapping or added insulation often need a wider jamb or an extension.
- Sill and threshold condition. For an exterior door, check whether the subfloor under the threshold is sound. If it is not, that repair happens before the new unit goes in, not after.
- Square and level. Check the opening with a level on both sides and across the head. An opening that is out by a noticeable amount needs correcting during installation, and it changes how long the job takes.
Things that get missed until the door is on site
Trim. Removing a pre-hung unit means removing casing on both sides. Old casing rarely comes off intact, so budget for new trim and paint.
Flooring height. If flooring has been added since the original install, the undercut on a new slab may be wrong. Measure the gap under the existing door at the point of greatest floor height.
Weatherstripping and sill type. Exterior pre-hung units come with a specific sill and sweep arrangement. Matching it to the existing step or porch height matters for both water and drafts.
Hardware carry-over. Reusing an existing handleset only works if the bore, backset and thickness all match. Otherwise the hardware is part of the order.
When neither option is the answer
Sometimes the door is fine and the problem is elsewhere. A door that sticks only in humid months, catches at one corner, or has started rubbing the strike is usually reporting hinge sag or frame movement, not a failing slab. Those are adjustments and repairs, and buying a new door will not solve them. That is residential door repair territory, and it is considerably cheaper than a replacement.
When the measurements are unusual, the opening is out of square, or the house has original millwork worth keeping, a site visit before ordering saves the return trip. Most of the cost in door installation in Toronto comes from surprises found after the old door is out, and those surprises are almost always visible beforehand if someone looks.
Frequently asked questions
Can I install a pre-hung door myself?
It is possible on a square, sound opening with two people. It gets difficult fast if the opening is out of level, the floor slopes, or the exterior needs flashing and sealing. An exterior entrance that is not sealed correctly causes water problems that show up a year later.
How much can I trim off a new slab?
Solid wood and solid-core doors normally allow a small amount off each edge and a bit more off the bottom. Hollow-core doors allow much less, because the frame inside the door is narrow. Manufacturer guidance for the specific door is the number to follow.
Does the old frame have to come out for a pre-hung unit?
Yes. The new unit includes its own frame, so the existing jamb, stop and casing are removed. That is what makes it the right fix for a frame that has failed.
What is the most common measuring mistake?
Swing direction, followed by jamb width. Both are easy to record correctly and expensive to get wrong.