Q: Should interior doors be installed before or after the finish flooring is installed?

A: Nathan Nebbia, owner of Built Better by Nate, based in Berwick, Maine, responds:

We have installed pre-hung interior door units both ways—before the finish flooring is installed and after. Doing it before is easier for the door installer, but that can lead to problems installing the floor.

When the doors are installed before the flooring, we usually undercut the jambs with a Japanese pull saw, laying the saw on a scrap piece of flooring. This results in an undercut of the jamb with the thickness of the saw blade taken out of it. If the flooring material running through the doorway is all the same height, you’re in the clear to make that cut all the way across the jamb. It sounds simple enough to do, but I’ve seen some pretty sad attempts at it. It gets much harder if there are flooring transitions occurring in the doorway (hardwood transitioning to tile, for example). If the meeting floors are different in height, you have to cut the jamb at one height from one side and make a cut at the other height from the other side. And installing the floor gets tricky because you have to get it underneath the jambs. Using a threshold can help, but that’s not always what the architect or owner wants.

When I’m the one who’s doing everything, I install the floors first, when nothing is in my way. After that’s done, I scribe the door jambs to the floor. This adds some extra steps to the door install, but they are minimal, and it’s much easier to define the height of the gap you want under the door. I like to go with a 1/2‑inch gap, but sometimes that’s not large enough if there will be rugs on the floor.

The slideshow at left describes the steps I take once I know the gap size: I start by marking the distance of the gap on the hinge side of the jamb. Then I pull the hinge pins and detach the door so I can work the jamb.

Often, you need to cut the jamb down in length so the jamb can be stood in the doorway for scribing. I make all jamb cuts using a miter saw on the floor. Without the door, the jamb legs will flop around, so I attach a temporary stretcher 12 inches up from the bottom. Note in the photo below I am using a framing square as my stretcher—a fast solution that works for a 2'-0" door.

Before I do any cutting, I find the difference in level on the floor. (I use a laser, as it works with any size opening and helps to align multiple doors where trim runs together). With the hinge side as the high point, I cut the leg, leaving it as long as I can while still allowing the jamb to fit in the rough opening. With the jamb in the RO, and the head checked for level, I set scribes to the mark made on the hinge side and scribe both legs before making the final cuts.

When installing the jamb, I always set the head of the jamb flush with the drywall on both sides. If the wall is out of plumb, the legs will be out of alignment with the wall, but that problem is easier to hide with the casings than trying to fix it at the head. We use ball-bearing hinges, and ghost doors result when they are the slightest bit out of plumb.

Once the hinge side is screwed in plumb both ways, I add structural screws behind the hinges to take the weight of the door, then I hang the door again.

Next, I shut the door to the latch side of the jamb and align the top with the drywall, keeping an even reveal (the gap between the door and the jamb), then shim and screw it.

Make sure the door hits against the stops consistently from top to bottom. Once it does, shim and screw the bottom, leaving the proper reveal.

I then shim behind the strike plate until the gap is correct. You might need to shim in some more spots to keep a nice, even reveal all around the door. Just make sure to add a screw to keep all the shims in place. Finally, I shim and screw the head jamb to keep it from sagging.