A quick guide to measuring for pre-hungs without the guesswork

I swapped out the bedroom doors in my 1940s colonial last summer and ran into exactly the issue of hidden framing secrets. I was convinced I had standard 30-inch openings, but once I pried off the old colonial-style casing, I found the framing was incredibly tight. The actual rough opening was barely 31.75 inches across the middle because of some bowed 2x4s. If I hadn’t pulled the trim to see the studs, those Jeld-Wen pre-hungs I ordered would have been a total nightmare to shim. One thing I did differently was checking the floor levelness along with the jamb depth. My house has a slight slope toward the hallway, and I realized I had to trim about a quarter-inch off the bottom of one side of the new jamb to get the head jamb perfectly level. Even with a 4 9/16 jamb, the old plaster-and-lath walls were closer to 5 inches thick in some spots. I ended up having to use some extension jambs anyway, but at least I wasn't surprised when the casing didn't sit flush. It’s definitely better to see the studs first rather than fighting a unit that’s physically too big for the hole.

I learned the hard way that a wide RO doesn't mean much if the studs are 'cross-legged'. Last time I did my laundry room, the width was fine, but the hinge-side stud was leaning into the room at the top. I had to fight to get the jamb plumb so the door wouldn't ghost-open.

Now I always run a 6-foot level across the face of the studs before I order. If the wall is leaning, I make sure I have an extra half-inch of wiggle room to plumb the frame up without hitting the drywall. It's a lifesaver for getting the door to acutally stay where you put it.

Back in the day we didn't have these fancy pre-hungs; we'd just hang the slab on a site-built jamb. But since you're ordering units, don't just trust your level. Take a framing square to the corners. I’ve seen guys get the RO width perfect only to realize the header is sitting at a cockeyed angle to the walls. If that opening is out of square, you better hope you’ve got an extra inch of wiggle room to shim it. I usally tell the young bucks to check the floor for a 'hump' right in the doorway too. If you don't account for a high spot, that door will drag on the rug no matter how plumb your jamb is.

Since you guys are dealing with those tight rough openings and bowed studs, you shold look into Masonite pre-hung doors. I’ve found thier factory-built jambs are usally more consistent than others when you're fighting a 1940s frame that’s slightly out of whack. If the opening is realy narrow, their molded panel series gives you a bit more flexibility for shimming without the jamb splitting. Just make sure you get the solid core version so the whole unit feels stable once you've wrestled it into those old studs.

Quick update — taking the framing square to the corners like someone suggested relly saved me. I found a header that was totally out of whack. I’m still struggling with those 'cross-legged' studs on the hinge side, though. Has anyone had luck using a power planer on the studs to get them plumb before setting the unit, or is that overkill?