Step 1: Check the rough opening

Tom Silva taking measurements while installing a prehung door
Photos by: David Carmack
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Place a 4-foot level on the floor in the doorway. If the hinge side is lower than the latch side, slip shims under the level nearest the hinge jamb. Adjust the until the level's bubble is centered.

Tack the shims to the floor with a finish nail. If the latch side is lower, no shims are needed.

Check the walls and the trimmer studs for plumb using a level or plumb bob. Also, check the trimmers' faces with a framing square to see if they are square to the wall.

Finally, check that the trimmers are parallel by measuring between them at the top, bottom, and middle of the opening. If the wall is out of plumb, or the trimmers are out of plumb, out of square, or not parallel.
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    Tools List

    • hammer
      Hammer or nail gun,
      to attach casing and jambs to rough opening
    • four-foot level
      4-foot level,
      for checking plumb or level
    • drill
      Drill/driver,
      for fastening hinges
    • 16-foot tape measure
      Tape measure
    • chisel
      3/4-inch chisel,
      for fine-tuning mortises
    • Phillips screwdriver
      Phillips-head screwdriver,
      for mounting lockset
    • utility knife
      Utility knife (for trimming shims
    • plumb bob
      Retractable plumb bob,
      for plumbing jamb

    Shopping List

    1. PREHUNG DOOR WITH SPLIT JAMS

    2. SHIMS

    for adjusting and supporting jambs



    3. STRIKE PLATE

    to protect jamb from latch bolt



    4. LOCKSET

    to hold door closed



    5. 4D FINISH NAILS

    to tack shims in place



    6. 8D FINISH NAILS

    to nail through casing