I am skeptical about the recommended 2-1/2" addition to the door height for an interior door rough opening (assuming only subfloor). Consider the following estimated (meaning I am unsure if they are right) measurement stack:

  • 1/2" flooring cover thickness
  • 1/2" clearance b/ween flooring and door roughly
  • 1/8" clearance between door and jamb header
  • 3/8" jamb header thickness
  • TOTAL 1-1/2"

That provides for a whole inch of slack (2-1/2" recommended rough opening height minus 1-1/2" estimated allocation above). Isn't a whole inch a bit too much of a slack provision? I understand you can shim but it seems to me that a half inch to and inch would be too much of a gap for shims (OK, can use 1x lumber but still I think it is too much).

An alternative measurement stack, especially correction with my clearance provisions would be appreciated to dispel this phenomenon to me.

1 Answer 1


I might quibble about floor thickness (3/4 for wood strip flooring), jamb thickness (closer to 5/8) and you're missing a gap between jamb and header (1/2 to 3/4), so I would add 1/4 +1/4 +1/2 to 3/4 for a 2 1/2 to 2 3/4 RO

  • RE: missing a gap between jamb and header -- shouldn't that be 0 because the jamb goes directly against the header, i.e. no gap (kind of like between the jamb and framing on the sides)?
    – amphibient
    Jun 8, 2013 at 18:35
  • Not recommended, especially if header isn't level or RO otherwise not square. If you are installing a pre hung door, you'd be in a rough spot by making it that tight.
    – HerrBag
    Jun 8, 2013 at 18:41
  • 4
    Indeed. You want plenty of gap above AND on each side of the jamb so that when setting the jamb, pre-hung or not, that you have plenty of space to allow for leveling the top of the jamb, plumbing the sides of the jamb and also effectively making the top and sides square to one another. The whole idea of the shims, which should be done with pairs of wedge shaped wood pieces specifically made for this purpose, is to allow for adjustments to get the jamb into the desired positions. Studs and header locations in a rough opening are rarely ever able to be exact and accurate.
    – Michael Karas
    Jun 8, 2013 at 23:49
  • (continued from above). Rough opening variations come from framing material dimensional variation, uneven floors, rough cuts on studs, headers and plates, and the tendancy for joints to move around some when they get nailed together.
    – Michael Karas
    Jun 8, 2013 at 23:52
  • you are correct about jamb thickness. since i have a transom, i will have to make my own jambs as it turns out because the premade ones at Lowes/HD are only for 80" doors and i will need to use 1x lumber. so that doubles my initial 3/8" estimate for jambs
    – amphibient
    Jun 9, 2013 at 0:21

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