Laminate

Undercutting Door Jambs and Casings

Undercutting door jambs and casings so new flooring slides underneath: setting blade height, choosing a jamb saw, and avoiding the cuts that show.

Updated · 2 min read

Undercutting the casing and jamb lets the new floor slide underneath, so there's no caulk line or scribed notch around the door. It's faster than scribing. Undercut when the floor will butt a casing and you want no visible gap or caulk line.

Set the height from the actual floor

The cut height has to match the finished floor exactly, so build it from real material.

  1. Stack a scrap of the new flooring on a scrap of the underlayment (and a trowel's worth of adhesive thickness if it's glue-down or tile).
  2. Lay the stack against the casing. That's your cut line.
  3. Set the saw so its blade rides just above the stack. Most jamb saws adjust by height plates, a depth knob, or by resting the saw on the scrap itself.[1][2]
  4. Leave a hair of clearance so the flooring slides under without forcing. Floating floors need to move freely; see Floating floor layout and expansion gaps.

Picking the saw

The right saw depends on how many doors you're doing and what they're made of.

Tool Best for
Jamb saw (e.g. Roberts 10-56, 10-46) Residential door casings; light and quick to set up
Heavy-duty undercut saw (e.g. Crain 835, 575) Commercial jambs, metal-framed openings, lots of doors
Flush-cut saw (e.g. Cuz-D Straight Flush) Cutting flush along walls and cabinets, not just door trim
Oscillating multi-tool Tight corners and the last bit a round blade can't reach
Hand undercut saw A few doors, no power

Step by step

With the height set, the cut is quick. Work through these steps in order.

  1. Remove the door, or mask it if it stays.
  2. Remove existing flooring at the opening.
  3. Set the blade height with your flooring stack.
  4. Cut the casing and jamb in one pass with the saw held level and resting on the scrap stack, so the blade does not touch the subfloor.[2]
  5. Knock out the waste with a chisel or multi-tool and vacuum the cavity.
  6. Test-fit a scrap of flooring before you install.

Common mistakes

Undercut problems show up at the last row or in the gap.

  • Setting height without the underlayment. Skipping it leaves the floor jammed at the last row.
  • Cutting too high. The gap shows above the flooring.
  • Tipping the saw. The cut angles and you get a wedge-shaped gap.
  • Forgetting dust. Many jamb saws take a dust bag or vacuum port; use it indoors.[3]
  • Metal door frames. Metal frames need a saw and blade rated for metal. If yours is not, scribe and caulk the frame instead.

Tools we carry

Stock for every job type above is available from PreFloor, including blades and blade changes.

References

  1. Crain No. 835 undercut saw manual
  2. Roberts 10-56 jamb saw manual
  3. Crain No. 837 dust cover manual

Linked PDF documents are archived copies saved by PreFloor Tools so they stay available if the publisher moves or removes them. Check the publisher's website for the latest revision.

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