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Flooring

After repairing or reinforcing the pillars and other major structural elements, the next step is to start building the flooring. If the existing floor structure is still in good condition, it can be reused as it is. However, in many cases, the floor needs to be rebuilt due to issues like rot, unevenness, converting a tatami room into wooden flooring, or improving insulation in an old, cold floor.

First, the sleepers―the main structure supporting the floor―are cut to the required length and installed on the sills. In Japan, sleepers are typically sized at 90mm x 90mm or 105mm x 105mm.
The sleepers are attached to the sills either by making joints or by using metal fittings; using metal fittings is generally easier.

If a sleeper is bent, you can correct it by placing the curved side downward and pushing it straight using floor posts. Adjustable floor posts make it easier to fine-tune the height while checking the level with a laser. If you're using wooden floor posts, be sure to apply a termite treatment to prevent future damage.

If the house has a mat foundation, the floor posts are fixed directly onto the foundation.
If it has a strip foundation, concrete footings are installed to provide a stable base because the subfloor is soil. In that case, the floor posts are set between the footings and the sleepers.

In Japan, floor posts are often spaced at 910 mm intervals. This is because, although Japan officially uses the metric system, traditional building standards based on the old Japanese units of “尺/shaku" are still common.
For example, the standard size for plywood panels is 3 shaku × 6 shaku, which is about 910 mm × 1820 mm.
In the past, this 3-shaku × 6-shaku size was considered enough space for one person to sleep. While it’s a little cramped for modern Japanese people, this grid size is still widely used today.
However, older homes may have slightly different measurements depending on local construction practices.

When renovating, the flooring structure is usually built around this 910 mm × 1820 mm module.
Floor joists installed on top of the sleepers are typically spaced at 1 shaku (about 303 mm) intervals to properly support 3-shaku × 6-shaku plywood sheets. Joists are often made from 45 mm × 45 mm or 45 mm × 60 mm lumber.

Before attaching the plywood, insulation materials like styrofoam or glass wool can be installed between the floor joists to improve thermal insulation. Since insulation products are available that fit perfectly between the 303 mm-spaced joists, insulating the floor is relatively easy using these standard materials.

The joists are then covered with 12–15 mm thick plywood, and the finished flooring―either tatami mats or wood flooring―is installed on top.
Sometimes, flooring is laid directly over the joists without using plywood, or vinyl sheet flooring is applied over the plywood instead of wood flooring.
In Japan, tiles are not commonly used for flooring in wooden houses, except in specific areas like bathrooms, toilets, or entrances.

If you want to convert a tatami room to wood flooring at a low cost, one option is to install wooden flooring directly over the rough flooring that was under the tatami.
Since tatami mats are usually about 5-6 cm thick, and the room's threshold is set at that height, you can adjust the new floor height by adding joists.
By doing so, you can also add a thin layer of insulation (about 2–4 cm thick) if needed.