Converting a Washitsu to a Western Room: Floor Height and Door Clearance

The number that makes or breaks a tatami-room conversion is 55mm: the thickness of the tatami you lift out. How you fill that 55mm gap decides the height of every door, the fusuma, and the threshold in the room. I have converted five washitsu across my rentals from tatami to a wood-look floor, and the first one taught me to plan the height stack on paper before I pulled a single mat.

Measuring the well before you start

I lift one tatami mat and measure three things: the mat thickness, usually 55mm but sometimes 60mm on older or thin-board mats; the depth from the top of the surrounding wood trim, the tatami-yose, down to the rough boards or joists below; and the joist spacing and condition. In a typical washitsu the subfloor sits about 55 to 60mm below the finished tatami surface, held on neda joists at 303mm or 455mm centres. If those joists bounce or the rough boards are stained, that gets fixed now, because everything else stacks on top of them.

Building the height back up

There are two clean ways to fill 55mm. The first is 12mm structural plywood laid over the existing rough boards, then 12mm engineered flooring on top, which comes to 24mm and leaves the new floor 31mm lower than the old tatami surface. The second is to add a fresh layer of 30mm packing battens or new joists, then 12mm ply and 12mm floor, landing within about 1mm of the original height. I choose based on the doorway: if the hallway outside is also being redone I take the lower finish and adjust the doors, and if the rest of the floor has to stay put I build up flush so the transition at the door is seamless. A third option some landlords use is a 40mm or 50mm placement-only floor system, the interlocking okiyuka panels sold for exactly this job, which drop straight onto the rough boards with no fixing and get you close to the original height in an afternoon, though they cost more per square metre and can feel slightly hollow underfoot. On a rental I usually go with plywood and glued-and-nailed flooring because it lasts longer between tenants.

The math that leaves you 3mm off

Getting this wrong by a few millimetres means a lip you catch your toe on or a door that scrapes. My stack sheet for the lower-finish method looks like this:

• Tatami removed: minus 55mm from the reference surface
• Existing rough boards: already there, 0mm
• New plywood underlayment: plus 12mm
• Engineered flooring plus underlay sheet: plus 13mm
• Net result: finished floor sits 30mm below the old tatami line, so plan for a 30mm transition and 30mm of door undercut

Door and fusuma clearance

Once the floor height is fixed, every panel that moves has to clear it with a 3 to 5mm gap. Fusuma and shoji I take off, mark a cut line, and trim the bottom rail with a fine saw and a guide, usually taking off around 31mm on the lower-finish method, then refit the paper edge. Hinged closet or entry doors get lifted off the hinges and run through the table saw, or planed if the cut is small. If a door was already tight to the old tatami, dropping the floor 30mm actually helps, but a door that swept the mats now has a visible gap that I close with a taller sweep or a threshold strip.

Insulation and the chance to fix the floor while it is open

A tatami floor with an open crawlspace below is often cold, and with the rough boards exposed during the conversion it is the one easy moment to fix that. I lay 30mm or 40mm rigid foam board between or over the joists before the plywood goes down, cut snug and foamed at the edges, which adds maybe 8,000 yen to a 6-jo room and noticeably warms the finished floor. It is also the time to deal with anything the tatami was hiding: a bouncy joist gets sistered, a stained board from an old roof leak gets checked and replaced, and I run a moisture meter over the framing before I close it up. Closing a soft or damp subfloor under a nice new floating floor just means tearing it all out again in three years.

Thresholds and the hallway transition

The shikii, the wooden threshold at the doorway, is the last piece. Where the new floor is 30mm low I fit a hardwood transition strip bevelled from the hallway height down to the new floor, screwed and glued, so there is no trip edge. Where I built the floor up flush, I just refinish or replace the existing threshold. I also run a 12mm scribed border plank around the room to close the gap the tatami trim used to hide, scribing it to the wall so an old out-of-square room still looks tidy. Materials for a 6-jo room, about 9.9 square metres, run me 45,000 to 65,000 yen with the insulation, and the job takes a weekend plus an evening for the doors. A converted room also lets me put a bed and a desk anywhere without worrying about tatami wear, which widens the tenant pool and, in my experience, adds about 2,000 to 3,000 yen a month to what the room will rent for.

Plan the height stack first, cut the doors to match, and a washitsu conversion goes from intimidating to a clean weekend. That is how I have done every one across my properties.

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