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Reinforcing Pillars in a Japanese Wooden House

Once the interior has been demolished, the next step is to replace or reinforce any damaged pillars. Pillars are one of the most crucial structural elements in a house, especially in traditional Japanese wooden homes, where the framework relies on pillars rather than walls for support. Their condition directly impacts the house’s durability.

In some cases, if a house was built over 100 years ago in a dry, well-ventilated location, its pillars and beams may still be in good condition with little to no deterioration. However, in areas with poor ventilation or high humidity, pillars are often damaged by termites or fungi. Even in well-ventilated spaces, Ie-Shiroari (Japanese subterranean termites, Coptotermes formosanus) can carry moisture from the ground to soften and consume the wood. This means that houses in humid environments are particularly vulnerable to termite damage.

Structural issues may also arise even if the pillars themselves are intact. A house might be sinking or tilting due to foundation problems, or the weight of the structure might be unevenly distributed due to design flaws or past renovations.

Before proceeding with repairs, check for:

• Termite damage in the pillars
• Tilted pillars or foundation
• Uneven weight distribution across the structure

If any of these issues are found, the affected pillars should be replaced or reinforced. Since this requires specialized knowledge and skills, consult an architect for structural calculations or hire a carpenter if you are unsure about handling it yourself.

If you are a Japanese citizen or have a Japanese partner, you may be eligible for government subsidies for seismic inspections and reinforcement. Check with the local government where the house is located for details.

Japan has a long history of advanced joinery techniques (kigumi, 木組). In some cases, rather than replacing an entire pillar or beam, only the damaged portion is cut out and replaced with a new piece of wood. While this method preserves more of the original structure, the jointed sections may be weaker than a fully replaced pillar.

There are ways to strengthen jointed pillars, such as using braces, metal reinforcements, or adding extra support columns. However, over-reinforcing in one area can create structural imbalances, so modifications should be carefully planned.

If a pillar is not structurally essential (e.g., if it’s only used to divide rooms), you may choose to leave it as-is or remove it if it’s not a safety risk. These less critical pillars can also be a great place to experiment with traditional Japanese joinery techniques.

Illustration of woodwork joints from a 1953 architectural reference book

Joints used in beams

After checking for damage, the next step is to examine how the pillars are secured to the beams and foundation.

Japanese earthquakes produce both horizontal and vertical shaking. Strong vertical motion, particularly near the epicenter, can cause pillars to separate from the foundation or beams, leading to a house collapse―even if the pillars themselves are undamaged.

To prevent this, modern Japanese homes use metal fittings to firmly connect pillars to both the foundation and beams. However, some older houses―especially those built over a century ago―were assembled using traditional woodworking techniques with little or no nails, let alone metal fittings. While this craftsmanship is impressive, it often doesn’t meet modern earthquake resistance standards. Even in homes that are not very old, outdated or improperly installed metal fittings may need to be replaced.

Rusty old fittings connecting a pillar and beams

You can purchase metal fittings from hardware stores or online and install them yourself by following the manufacturer’s instructions. However, if you’re unsure about which fittings to use or where to place them, it’s best to consult a professional.

Additionally, lateral reinforcement is essential for earthquake resistance. This can be achieved by installing braces and load-bearing walls. These must be positioned in a balanced manner―if they are concentrated in one area, weak points in the structure will be more vulnerable when an earthquake strikes.

Many older Japanese wooden houses were built with pillars and mud walls but no braces or load-bearing walls. Instead, they relied on horizontal wooden beams called nuki (貫), which connect multiple pillars and help stabilize the structure. When renovating, take care not to cut or remove nuki unnecessarily. However, in homes with very thin nuki, these may not provide sufficient support during a major earthquake, so reinforcing the house with additional braces or load-bearing walls is recommended.

Unfortunately, Japan has a growing problem with fraudulent and unscrupulous contractors in the remodeling, termite extermination, and earthquake reinforcement industries. To protect yourself, always obtain multiple quotes, review the proposed construction details carefully, and research the reputation of any company before making a decision.