How to undercut door jambs and casings in an older Seattle house

Older Seattle houses tend to have wide fir door casings, sometimes with a square plinth block at the bottom, under decades of paint. Butt a new plank up against that and every doorway ends in a wobbly scribe line and a bead of caulk. Cut the casing so the plank slides underneath and the joint disappears. Many floating-floor instructions ask for it too, telling you to undercut wood casings where you can. It takes a few minutes per door once the setup is right, and it has to happen before the first plank goes down.

Check these before you cut

Lead paint. If the house was built before 1978, treat the casing paint as lead until a test says otherwise. Undercutting makes fine dust right at floor level, and a lab test of a paint chip settles the question. If it is lead and you still want to do the cut, EPA says to seal off the work area, keep children, anyone pregnant and pets away, and finish with a HEPA vacuum and a wet wipe-down. Hired work on pre-1978 homes falls under the state's lead-safe renovation rules, which the Washington Department of Commerce runs.

What is under the old floor. Old vinyl tile, sheet vinyl or black adhesive at a doorway in a house from before about 1980 can contain asbestos. Keep the blade off it and have it tested first. Old floor removal and subfloor prep explains what comes next.

The final floor height. Finish your subfloor work first. A plywood overlay or a leveling patch in the doorway raises the floor, and the cut has to match the finished height.

The door itself. Close the door, hold a scrap of plank and underlayment under it, then swing the door through its whole arc. If it rubs, the door needs trimming too. A hollow-core door has only a short solid rail at the bottom, and a door to the garage may be fire-rated, so check before cutting either one.

What the frame is made of. Metal door frames and stone or tile casings do not undercut the same way. Cut the plank to fit and follow your product's instructions for finishing the gap.

Tools

• An oscillating multi-tool with a flush-cut blade. Use a bimetal blade in an old house; it survives finish nails that a wood blade will not. This is the most controllable tool for trim with some history.
• A jamb saw, a low-profile saw built for this job. It is faster and more aggressive, but messier.
• A flush-cut handsaw. Fine for a door or two.
• A scrap of new plank and underlayment, a sharp utility knife, a small chisel and some thin cardboard.
• Safety gear: eye and ear protection, plus a P100 respirator and a HEPA vacuum where old paint is involved.

Step by step

1. Protect the room. Vacuum the doorway. If the paint might contain lead, tape plastic over the floor and close off the doorway before you start.
2. Build the gauge. Lay the underlayment on the subfloor, then the plank on top, face down so it does not get scratched. For a floating floor, add a layer of thin cardboard on top.
3. Score the line. Slide the gauge tight against the casing and run the utility knife along its top edge, across the front and sides of the casing, the jamb and the door stop. Scoring keeps paint and old fir from splintering.
4. Cut in one pass. Rest the blade flat on the gauge, let the tool do the work and cut straight through the casing, jamb and stop in one continuous pass. Starting in one spot and then moving to another leaves a jagged cut.
5. Mind the back. Stop when you reach plaster, drywall or framing. If the blade hits a nail, ease through it or change the blade.
6. Stop where the floor stops. If the new floor ends at this door, it typically ends under the closed door, so cut the jamb only that far.
7. Clear the slug. Lift it out with a chisel, vacuum the pocket behind it and wipe with a damp cloth.
8. Test with a real plank. It should slide in and out without force. If it pinches, slide it in until it stops and trim that spot with the tool.
9. Seal raw wood if the floor runs damp. Many Seattle houses sit over crawl spaces that run damp from October to May, and a coat of primer on the cut ends is cheap protection.
10. Repeat at every door, closets included, before you start the first row.

Keep the floor floating

An undercut hides the expansion gap; it does not replace it. Planks should run under the casing and stop short of the wall or jamb behind it, leaving the gap your product's instructions call for. Do not nail, glue or caulk through to the plank. Transitions sit in the doorway, fastened to the subfloor rather than to the floating floor, and if a reducer or T-molding will not tuck under the jamb, cut it to butt against the jamb instead. Our page on stairs, transitions and trim shows how those pieces come together.

What it costs

These are typical Seattle-area ranges for planning. They vary with the tool, the job and the installer, and your written quote is the real number.

The Seattle flooring cost guide puts these next to product and prep costs.

When to call a professional

Hand this one over when:

• The paint tests positive for lead, or it is flaking and young children or someone pregnant lives in the house. Ask for a firm certified under the state's program.
• The casing is original, carved or hard to replace, and you are not confident of a clean cut on the first pass.
• Old vinyl or black adhesive shows at the doorway.
• Floor heights change from room to room, so a transition needs planning.
• The door needs trimming and it is solid, fire-rated or hard to rehang.
• You have a whole floor's worth of doors. Preparing for installation day shows what to handle yourself and what to leave to the crew.

An installer looks at every doorway at the in-home measure and lists the undercuts on the written quote. Call (844) 543-5757 to book one; coverage by neighborhood is on the Seattle areas page.