Jamb Saw Guide: Undercut Door Jambs for New Flooring

What is a jamb saw?

A jamb saw is a tool made to undercut the bottoms of door jambs, casings, and sometimes baseboards so flooring can fit underneath. Undercutting lets you hide the rough edges of tile, laminate, vinyl, or hardwood and maintain the expansion gap that floating floors need. Jamb saws are available as dedicated power tools with adjustable cutting height or as hand tools for detail work.

Why undercut door jambs at all?

Cleaner look: Flooring slides under the trim instead of being scribed around it.
Faster fitting: You avoid fussy, tight cuts around curved or uneven trim profiles.
Better performance: Floating floors need space to expand; undercutting preserves this gap without a visible gap.
Easier transitions: Thresholds and reducers sit more flush when trim is undercut.

How a jamb saw works

Most power jamb saws have a flat, wide base that rests on the subfloor. A side-mounted blade cuts parallel to the floor at a set height. You adjust the base to match the finished floor thickness, then guide the tool along the wall or casing. The blade makes a horizontal cut through the trim so the bottom offcut can be removed, creating a slot for the new flooring.

Hand versions (often called flush-cut or Japanese pull saws) have flexible, fine-tooth blades that cut on the pull stroke. They’re used to finish tight corners, clean up cuts from a power jamb saw, or handle small jobs without plugging in a tool.

Common uses and applications

Installing tile, laminate, engineered wood, or LVP/LVT at doorways
Trimming casings around fireplace surrounds for stone or tile
Undercutting baseboards at transitions so reducers or thresholds fit better
Updating flooring thickness (e.g., carpet to wood) without removing and re-hanging doors and trim
Replacing or installing new jambs and needing a clean, even bottom edge

Types of jamb saws

Dedicated power jamb saw (undercut saw): Purpose-built tool with adjustable cutting height and a flat shoe that references the floor. Great for whole-house flooring jobs. Uses different blades for wood, MDF, or masonry.
Oscillating multi-tool with undercut blade: Extremely handy and precise; reaches tight spots and inside corners. Slower for large areas but excellent for detail, small projects, or metal jambs with the right blade.
Hand flush-cut (pull) saw: Flexible blade for trimming the last bit in corners, around door stops, or where power tools can’t reach. Also good if you only have a few cuts.
Toe-kick saw (related, but different): Designed to cut under cabinet toe-kicks to remove old flooring. Not used for door jambs, but often mentioned alongside jamb saws.

Choosing the right tool

Project size: Whole rooms or multiple doorways favor a dedicated power jamb saw. For a single bathroom or hallway, an oscillating multi-tool may be enough.
Material: Wood/MDF jambs use carbide wood blades; masonry or stone surrounds need a diamond or abrasive blade; metal jambs require a bi‑metal or carbide-grit oscillating blade.
Depth range: Make sure the saw can be set to your finished floor thickness (flooring + underlayment).
Dust control: Look for dust ports or plan for a vacuum. Undercutting creates fine dust near eye level.
Ergonomics: A wide, stable base and a front handle improve control and accuracy.
Budget/rental: Many home centers rent power jamb saws for a day, which is cost-effective for one-time projects.

How to use a jamb saw (step-by-step)

Confirm finished height: Stack a scrap of your flooring plus the underlayment you plan to use. If the flooring is floating, include the required expansion gap (often 1/4 inch) in your setup.
Set the cut height: Adjust the jamb saw so the blade just kisses the top of the stacked sample. For a multi-tool, place the stack beside the jamb and rest the blade on it as a guide.
Prepare the area: Score paint/caulk lines where trim meets the wall to reduce tear-out. Lay a drop cloth to protect existing floors and catch debris.
Check for fasteners: Run a stud finder with metal-detect mode or visually locate finish nails. Expect nails in the casing; let the saw work through them only if your blade is rated for it. Otherwise, pull nails after the cut loosens the offcut.
Make the cut: Hold the base flat on the floor, start the saw, and feed steadily into the jamb/casing. Keep the tool level and let the blade do the work. Avoid forcing; this reduces burn and wandering.
Finish corners: Inside corners and near door stops are easier with a flush-cut hand saw or an oscillating tool.
Remove the offcut: Pry the small strip free with a thin putty knife or oscillating blade. Clip or pull any remaining nails.
Test fit: Slide a scrap of flooring under the trim. It should fit with slight clearance. Adjust if tight.
Clean up: Vacuum dust and chips, and wipe the base of the saw before moving to the next doorway.

Safety and maintenance tips

Wear safety glasses, hearing protection, and a dust mask or respirator (especially for MDF and masonry).
Keep hands away from the blade path and maintain a two-handed grip.
Manage the cord to avoid cutting it or tripping.
Use the correct blade for the material; wood blades on masonry will glaze or shatter, and masonry blades on wood will burn.
Let the blade stop before setting the tool down.
Inspect blades for missing teeth or cracks and replace as needed. Clean pitch from wood blades to reduce burning.

Common mistakes to avoid

Cutting too low or too high: Too low can nick the subfloor; too high leaves a visible gap. Always set height with stacked samples.
Forgetting underlayment: Your finished height changes if you add foam, cork, or sound mat.
Skipping expansion clearance: Floating floors need room; don’t cut a slot so tight that the floor binds.
Using the wrong tool: A circular saw on its side is unsafe and imprecise. Use a jamb saw or oscillating tool.
Ignoring uneven floors: Sloped slabs or humps mean your undercut height must follow the floor. Re-check and adjust as you move.
Not finishing corners: A partial cut won’t accept flooring; complete the cut with a flush tool.

Related terms

Undercutting: Trimming the bottom of trimwork so flooring slides underneath.
Casing: Trim that surrounds a door or window opening.
Jamb: The vertical parts of a door frame the door closes against.
Threshold/Reducer: Transition pieces used where two flooring types meet.
Toe-kick saw: A different tool for cutting under cabinet toe-kicks.

Practical examples

Laminate in a hallway: You stack a laminate plank and foam underlayment, set the jamb saw height, undercut both sides of the doorway, and slide the planks under the casings for a tight look without shoe molding collisions.
Tile at a bathroom door: Using a diamond blade on an oscillating tool, you undercut the MDF casing and a bit of the door stop, ensuring the tile plus thinset fit under the trim while still maintaining an expansion joint with silicone at the transition.
Around a stone fireplace: A power jamb saw with a diamond blade undercuts the bottom edge of the stone surround so a new engineered wood floor can slip underneath, avoiding complex scribe cuts and improving the finish.

Quick buying checklist

Adjustable height range covers your floor build-up
Compatible blades for wood and, if needed, masonry
Stable base and comfortable grips
Dust port or plan for vacuuming
Consider renting for one-off projects

A jamb saw saves time and improves the finish of flooring projects by making your trimwork work for you. Set the height with samples, use the right blade, and take your time—your doorways will look factory-fit.