Door guides

Measuring. For a slab replacement, measure the existing door itself — width, height and thickness — at more than one point, because old openings are rarely square. For a prehung unit you need the rough opening, which is the framed hole behind the trim. If those two numbers disagree with what a supplier expects, stop and get the installer to measure. A mismeasured door is a restocking fee at best.

Handing. Doors are left-hand or right-hand, inswing or outswing. The convention varies by supplier, so describe it plainly in writing too: "stand outside facing the door; hinges on the left; door swings into the house." That sentence prevents a very common and very annoying error.

What a good install looks like. The opening is checked for rot and squareness, a sill pan or flashing goes in before the unit, the unit is shimmed plumb and level and screwed through the hinge jamb into framing, the shim gap is insulated (low-expansion foam or backer rod, not packed fiberglass everywhere), and the exterior is sealed and flashed so water is directed out, not into the wall.

Inspect before final payment. Even gaps all round, the door stays put when opened halfway, the latch engages without lifting the handle, weatherstrip contacts all the way around, and no daylight visible around the closed door. Check these with the installer still on site.

How to choose

Let the installer take the final measurements

If they measure, an error is theirs. If you measure and supply the door, it is yours. That alone is worth more than the small measuring convenience.

Put handing in words as well as codes

Write the swing direction and hinge side as a plain sentence in the contract. Handing conventions differ between suppliers.

Ask how the opening will be flashed

A sill pan and proper flashing are what keep the sill from rotting. A bidder who can't describe their method should not get the job.

Hold back final payment until you inspect

Walk the checklist — even gaps, no daylight, clean latch engagement, full weatherstrip contact — while the crew is still there.

Get the door and hardware model numbers in writing

You will need them for warranty claims and for matching hardware later. Ask for them on the invoice.