How to Undercut Door Jambs and Casing

Why this matters

The doorway is the first thing a customer looks at when they walk onto a new floor, and it is where the shortcuts show. A plank notched around the casing leaves a visible gap that fills with caulk, then cracks. A floating floor butted hard against a jamb has no room to move, and the doorway is exactly where it peaks. Undercutting the jamb and casing lets the floor slide under them, so the trim hides the cut edge and the floor keeps its expansion space where nobody can see it. It takes a few minutes per door, and it is the step that separates a floor that looks fitted from one that looks cut in.

This card covers undercutting wood jambs and casing for a new floor. The expansion space itself (how much, and why) belongs to 'Expansion Space at Walls and Fixed Objects'; the shelf default is per the maker, commonly about 1/4 to 1/2 inch, hidden by base, shoe or trim and never pinned by a fastener. Fitting the last row and the planks at the door for a floating floor is 'How to Fit the Last Row and Door Jambs When Floating'.

Before you start

• The exact build-up. A scrap of the new flooring and a piece of every layer that goes under it: underlayment pad, vapor retarder film, uncoupling membrane, plywood underlayment. For tile, a tile set in thinset on a scrap of the substrate the day before, so the guide includes the mortar bed as it will actually be.
• The saw. A powered undercut saw with a height-adjustable blade, or a flush-cut handsaw laid flat; an oscillating multi-tool with a flush blade for tight spots, and with a metal-cutting blade for the nails the undercut saw should not be asked to cut.
• The old floor out. Undercut from the substrate the new floor will sit on, not from the top of the old floor, or the cut lands at the wrong height.
• A check of the house age. In a home built before 1978, painted casing and jambs may carry lead paint, and cutting them disturbs it; treat it as covered by 'EPA Lead RRP Rule for Contractors' and 'The Lead-Safe Floor Work SOP' unless a test says otherwise. Any old resilient floor or black adhesive still at the doorway is presumed to contain asbestos until tested, and is not cut or scraped to make room for the guide.

The cut, in elevation

ELEVATION, foot of casing and jamb, old floor removed

   drywall  | casing | jamb |       door opening
            |        |      |
            |        |      |
  cut -> ===+========+======+=== kerf 5/64 in
            | waste  | waste|[ plank   15/32 in ] guide
            | 19/32  | 19/32|[ foam pad 1/8 in  ] stack
 ========================================================
                        slab
 Blade rides flat on top of the guide stack.
 Stack 15/32 + 1/8 = 19/32 in. Waste below the kerf
 comes out; new floor slides in with 5/64 in clearance.

Step 1: Stack the guide exactly as the floor will be built

Lay the underlayment scrap on the substrate tight against the casing, and the flooring scrap on top of it, face down so the face does not get marked. The stack has to be the same layers in the same thickness as the installed floor. What breaks if you skip a layer: leave out a 1/8 inch pad and the floor will not slide under the jamb at all; add a layer that is not in the floor and the cut leaves a visible gap above the finished surface.

For glue-down floors the adhesive adds only a trace, and the guide can ignore it. For tile it does not: the bed under a tile is real thickness, which is why the guide is a tile already set in mortar, not a dry tile on a scrap.

Step 2: Check for metal and the cut path

Look and feel for finish nails low in the casing and jamb, and for a metal threshold anchor or a weatherstrip on an exterior door. A nail in the kerf dulls or breaks the blade and can throw the saw. Pull it, set it above the cut line, or cut it with a metal-cutting blade on the oscillating tool first. Note what is behind the jamb: on an interior wall, shim space and then the trimmer stud.

Step 3: Set the blade on the guide and cut

Rest the saw so the blade lies flat on the top of the guide stack (with a powered undercut saw, set its blade height to the stack). Hold the guide stack tight to the trim with your free hand well away from the blade path, or clamp a scrap of the stack to the floor. Cut through the casing and the jamb leg full width, feeding steadily, with the blade guard working and the cut moving away from your body.

Cutting trim makes fine wood dust, and on painted trim, paint dust. Run the saw's dust port to a vacuum (a HEPA vacuum on pre-1978 trim), wear eye protection, and HEPA-vacuum the cut area rather than sweeping it.

Cut both jamb legs and both casings on the side the floor runs under, and the door stop where it reaches the floor.

Step 4: Clear the waste

Split the waste out with a chisel or the oscillating tool, working the shim space too if old shims or drywall run down to the substrate where the plank needs to go. Vacuum the slot. What breaks if you skip it: a sliver of waste left at the back of the slot holds the plank up by its thickness, and the plank rides up and opens its joint right at the doorway.

Step 5: Test-slide a scrap under every cut

Slide a scrap of the flooring, on its pad, into each undercut until it touches the framing behind the jamb. It should go in with light friction and no lifting. Mark the scrap at the face of the jamb, pull it out, and measure: that distance, minus the expansion space, is how far the installed plank can run under the jamb. If the scrap binds, recut at the same stack height; do not shave the plank to fit.

Worked example: a bedroom door over a slab

Laminate going into a bedroom over a slab: a 12 mm laminate plank, which measures 15/32 inch, on a 1/8 inch foam pad. The laminate maker's instructions, read for this product, call for 3/8 inch expansion space at all walls and fixed objects. The doorway is a standard interior door in a 2x4 wall, painted casing, house built in the 1990s, carpet removed.

The stack. 15/32 + 1/8 = 15/32 + 4/32 = 19/32 inch. That is the height of the waste that comes out, from the slab to the bottom of the kerf.

The kerf. This crew's undercut blade measured 5/64 inch with calipers. The cut bottom sits at 19/32 inch, and the new floor's surface also sits at 19/32 inch, so after install the jamb clears the floor by the kerf, 5/64 inch. That reads as a shadow line, not a gap, and it is clearance a floating floor needs: the floor slides under the trim without the trim pressing on it.

How far under. The jamb leg is 11/16 inch thick, and behind it is 1/2 inch of shim space before the trimmer stud, so from the jamb face to the stud is 11/16 + 8/16 = 19/16 inch. The test scrap slides in until it touches the stud and is marked at the jamb face: 1-3/16 inch, which is 19/16, matching. With 3/8 inch (6/16 inch) of expansion space kept to the stud, the installed plank runs 19/16 - 6/16 = 13/16 inch under the jamb face. 13/16 inch of plank edge hidden under the jamb, 3/8 inch of open space behind it, and nothing visible at the doorway.

What the door needs now. The carpet that came out was thicker than the new floor, so the door clears easily. Had the new floor been higher than the old, the door would have to be trimmed, and that changes one more thing: in a house where the bedroom has no return grille of its own, the gap under the door is the return air path. Losing it can make the room hard to heat or cool and the door whistle; tell the customer before the door is cut down, and route an air question to whoever services the HVAC.

What changes the cut

• Tile. The guide is a tile set in thinset on the substrate, cured overnight. The cut height is the tile's finished height, and tile is not floating, but the perimeter still needs its movement joint, so the space at the framing behind the trim is kept clear of mortar and finished with sealant, never grout; see 'Movement Joints in Tile Floors'.
• Nail-down solid hardwood. The floor is fastened, not floating, but solid wood still swells and shrinks across its width with the seasons. The same stack-and-cut gives the board edge somewhere to go under the jamb instead of pushing on the trim, and it keeps the expansion space hidden the same way.
• Vinyl over plywood underlayment. The underlayment panel is part of the stack, so the guide is a scrap of the panel plus the vinyl; undercut after the underlayment is down if the panel scrap is not to hand, using the installed panel as the base of the guide.
• Exterior doors. The sill or threshold is part of the weather seal and is not undercut. The floor stops at it with its expansion space, finished with a transition profile or a sealant joint.
• Steel, stone and labeled fire-rated frames. A steel frame is not undercut with a wood blade, and a labeled fire-rated frame is not cut at all without checking what the fire door rules adopted where you work permit; scribe the floor to the frame and leave the expansion space visible under a profile or sealant.
• Casing that is coming off anyway. If new trim is going in after the floor, the old casing can come off and the new casing sits on the finished floor, which beats an undercut.

Finish test

At every door: the floor slides under the casing and jamb with light friction and no lift, the measured run under the jamb leaves the maker's expansion space to the framing, the jamb sits just above the finished floor with only the kerf showing, and no plank is notched around trim. The slot is vacuumed clean before the floor goes in.

References

• See related: Expansion Space at Walls and Fixed Objects
• See related: How to Fit the Last Row and Door Jambs When Floating
• See related: EPA Lead RRP Rule for Contractors
• See related: The Lead-Safe Floor Work SOP
• Flooring maker's installation instructions for expansion space at door frames and fixed objects.