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Demolition and renovation of Japanese houses 2 (floor)

When demolishing a house interior, it’s usually easier to work from the top down―from the ceiling to the walls, and finally to the floor. This approach is especially helpful in older wooden houses where the subfloor isn't covered with concrete. If you start with the floor, you may have to set up a stepladder on uneven ground. However, this method might not apply if the floor is rotten or if you need to remove it before taking down the walls.

This image shows the underfloor of a 60-year-old building with a strip footing. Part of the foundation includes stone, and animal-brought garbage, likely from rats entering under the floor, is scattered at the far end. Since the 1995 Hanshin-Awaji Earthquake, concrete mat foundations have become mainstream for their high earthquake resistance. However, many older buildings were constructed with strip footings, where only the main frame was reinforced with concrete, leaving the rest of the floor exposed to bare soil. Some even older homes have isolated footings, where the structure isn’t anchored to the ground but rests on pillars set on stones instead of concrete.

As previously mentioned, the floor structure of a tatami-mat room is relatively simple, with wooden boards called ara-ita (荒板) placed under the tatami, and joists, lumber girders, floor posts, and post footings supporting them.

Dismantling is straightforward: after removing the tatami mats and underlying boards, the joists and lumber girders can be taken out. These can be reused if they’re still in good condition. For low-budget conversions to wood flooring, it’s common to keep the existing girders and joists, finishing the floor by laying square wood or plywood on top of the ara-ita once the tatami is removed.

However, if there’s high subfloor humidity or termite damage, it’s essential to improve the subfloor conditions and replace any damaged components.

In a Japanese-style room with tatami mats, once the tatami and half of the ara-ita are removed, you’ll find that the ara-ita boards are often nailed in place. However, in some cases, they are simply laid down without nails.

With all the ara-ita removed, the joists and lumber girders are now fully exposed. In a Japanese-style room with tatami mats, the spacing between the floor posts that support the joists and girders may be wider than what's typical for modern wood flooring. If needed, consider adding additional joists and floor posts to strengthen the floor.

Dismantling a wooden floor takes more time than removing a tatami floor, and reusing components is generally more difficult. If the floor is in decent condition but you just don't like its appearance, it's often quickest and cheapest to install a new floor directly over the old one without dismantling it.

Old solid wood plank floors, several decades old, are usually fine if not damaged by humidity or termites. However, in properties with multiple renovations, previous owners may have layered new floorboards over the old ones with the same approach. When dismantling, you may find additional layers beneath, making the process more labor-intensive than expected.

This issue is common with older decorative plywood or synthetic flooring materials. In Japan, the demand for wood rose sharply during post-World War II reconstruction, leading to the depletion of domestic wood resources, price hikes, and increased reliance on imported wood and plywood.

Additionally, some decorative plywood produced around the 1950s and 1980s had durability issues. Adhesives used in these layers often failed, causing the layers to separate and the plywood to deteriorate. In such cases, removing the old decorative plywood and installing a new floor is necessary.

(For context, in Japan, where fossil fuels were scarce, wood resources were used intensively, leading to overharvesting even before WWII. There has long been a culture of reusing wood, especially in homes built shortly after the war, which often included barracks structures made from reclaimed wood.)

Here’s an example of old floorboards found under a kitchen floor. The previous owner had renovated by hammering plywood over the original floorboards and then gluing linoleum tiles on top of the plywood. Over time, the adhesive on the plywood weakened, causing each layer to peel off easily.

Many older properties have tile-like plywood flooring similar to what’s shown in this photo. This type of plywood often separates into layers over time, making the floor feel soft and uneven underfoot.

In many older properties in Japan, carpets are often pasted over tatami mats or floorboards, which can be a hassle to deal with. If the carpet can be easily lifted, there’s no problem, but if it's attached with adhesive or solidified double-sided tape, it can be difficult to remove.

Leaving the carpet in place can also lead to hygiene concerns, such as mite infestations. The use of insulation in homes has only recently gained traction in Japan; until a few decades ago, few houses―except in some colder regions―were built with insulation in mind. Because humidity buildup under the floor speeds up a house’s deterioration, good subfloor ventilation is essential. However, without insulation, floors can become very cold in winter.

To combat this, many homes installed carpet to reduce the floor’s chill, and in apartment complexes, carpet is often used to reduce noise transmission to the floor below. If the carpet is glued down, the only option for removal is to scrape it off by hand, although a power tool, like a multi-tool, can make the job easier.

The carpet is attached to the wooden floor with double-sided tape. While removing carpet secured with tape is generally easier than removing a fully glued carpet, considerable time and effort are still needed to clean off the tape residue if you plan to reuse the original floorboards as-is.

If there’s no plan to reuse the floor joists and large beams beneath the floorboards, it’s easier to remove them by cutting them into sections rather than trying to take them out whole. While you can use a crowbar or hand saw for dismantling, a power tool like a reciprocating saw or chainsaw will make the process much easier.

During demolition, caution is essential as many accidents can occur, such as fingers getting caught in crowbars or lumber, or slipping while walking on joists and beams. Thick, long nails―called *gosun-kugi*, about 15 cm in length―are often used in joists and beams and can cause injuries if accidentally grabbed or stepped on.

Regular shoes can be easily pierced by these nails, so it's best to wear safety shoes with steel plates in the soles or use nail-protective insoles, which are available at home improvement stores. If you accidentally step on a *gosun-kugi* and it penetrates deeply, rinse the wound immediately with clean water and apply an antibiotic dressing if available. Otherwise, seek medical attention. Left untreated, such wounds can lead to serious infections, including necrosis or sepsis.

Lumber from demolition can be taken to a local government waste disposal facility if it's cut to a specified length. Since acceptable lengths vary by facility, it's best to check the guidelines on your municipal government’s website.

Waste wood can be reused as firewood, but avoid burning plywood or any wood treated with glue or chemicals at home, as these can release harmful substances when burned.