Alright, let’s talk about framing out a rough opening for a prehung door. Did this recently for the basement room I’m finishing up. It’s not rocket science, but you gotta get it right, or the door just won’t hang properly. Learned that the hard way a long time ago.
Getting Started – The Door Itself
First thing I did was actually get the prehung door. Seems obvious, right? But you need the actual door unit there to measure. Don’t just go off the label size, like “30-inch door”. The whole unit, with the jambs already attached, is wider and taller than just the door slab.
I grabbed my tape measure and measured the width of the entire prehung unit, right across the outside edges of the side jambs. Then I measured the height, from the very bottom of the side jambs (not the door bottom, the jamb!) up to the top of the head jamb.
Figuring Out the Rough Opening Size
Okay, here’s the key part. The rough opening needs to be bigger than the door unit. Why? So you have wiggle room to shim it perfectly plumb (straight up and down) and level (straight across).
- Width: I took the actual width measurement of my prehung unit and added about 3/4 inch to an inch. Some folks say 1/2 inch is enough, but I like a bit more room, makes shimming easier. So, if the unit was, say, 32 inches wide, I aimed for a rough opening width of about 32 3/4 inches.
- Height: Same idea. I measured the unit’s height and added about 1/2 inch to 3/4 inch. You need space above the head jamb for shims, plus maybe a little clearance underneath depending on your flooring situation. If my unit was 81 1/2 inches tall, I’d shoot for a rough opening height around 82 to 82 1/4 inches from the subfloor. Remember to account for finished floor height if it’s not down yet! I usually measure from the subfloor.
Building the Frame
Now for the wood. A rough opening isn’t just a hole; it’s framed properly to support the wall and the door.
I started by figuring out where the door center would be. Marked that on the bottom plate (the horizontal 2×4 running along the floor) and the top plate (the one at the ceiling).
Then, the vertical pieces:
- King Studs: These are the full-length studs that go from the bottom plate to the top plate on the outer edges of where the opening will be. I put one on each side. They provide the main structural support.
- Trimmer Studs (or Jack Studs): These go right inside the king studs. They don’t go all the way to the top plate. Instead, they stop at the height needed to support the header. Their job is basically to hold up the header. The distance between the inside faces of these trimmer studs is your rough opening width. So I measured carefully before nailing them to the king studs.
The Header: This is the horizontal piece that goes across the top of the opening, sitting right on top of the trimmer studs. It carries the load from the studs above the opening down to the trimmers. For a standard interior door opening (not load-bearing), I usually just use two 2x4s nailed together with some plywood or OSB spacer in between to make it the same thickness as the wall studs (3.5 inches). For wider openings or load-bearing walls, you need something beefier – maybe 2x6s, 2x8s, or even engineered lumber. Better check your local codes or consult someone if you’re unsure. I slid the header into place on top of the trimmers and nailed it securely through the king studs and toenail it into the trimmers.
Cripples: Above the header, there’s a gap up to the top plate. I filled this space with short vertical studs called cripples, usually spaced 16 inches on center, just like regular wall studs. Nailed those in place between the header and the top plate.
Putting it Together & Checking
If you’re cutting into an existing wall, you first carefully cut out the drywall bigger than you need, locate the existing studs, cut out the section of stud(s) where the opening goes, and then build this frame within that space, tying it into the existing top and bottom plates. If it’s new construction, you just build this whole assembly as part of the wall framing.
Once I had the king studs, trimmers, header, and cripples all nailed together, I double-checked everything.
- Width: Measured the distance between the trimmer studs at the top, middle, and bottom. Made sure it matched my target rough opening width.
- Height: Measured from the subfloor up to the bottom of the header on both sides. Confirmed it matched my target height.
- Plumb: Used my 4-foot level to check if the trimmer studs were perfectly vertical (plumb). This is super important for the door to swing right.
- Level: Checked if the header and the bottom plate section under the opening were level.
- Square: Measured diagonally from corner to corner. The two diagonal measurements should be the same, or very, very close.
If anything was off, I adjusted it then and there. A little persuasion with a hammer, maybe adding a temporary brace. It’s way easier to fix the framing now than to fight with shimming a warped opening later.
And that’s pretty much it. Once everything was solid, square, plumb, and level, the rough opening was done. Ready for the prehung door unit to be set, shimmed, and nailed in place. Felt good standing back and seeing that perfectly framed hole, knowing the door would fit just right.