Termite Risk in Japanese Wooden Houses: A Buyer's Guide
23 March 2026
What you are dealing with
白蟻 (shiroari) — termites — are the single most common cause of structural damage in Japanese wooden residential buildings. Unlike the dramatic visible damage of asbestos or flood, termite damage accumulates silently inside walls and subfloor timbers over years or decades. By the time it becomes visible, repair costs can be substantial.
For akiya buyers, this is not an abstract risk. The Ministry of Land, Infrastructure, Transport and Tourism (MLIT) conducted a national survey of approximately 5,300 wooden detached houses in 2013, measuring actual damage rates by building age. The findings are stark: in the untreated cohort, buildings aged 20–30 years show a 32.6% damage incidence rate. Buildings aged 30–50 years show rates above 50%.
Most akiya on the market were built between 1955 and 1985. The numbers apply directly.
The two species that matter in Fukuoka
ヤマトシロアリ — Yamato-shiroari (Reticulitermes speratus)
The dominant species nationwide. Present throughout Japan, it accounts for 92% of all termite damage in the MLIT survey. Colony size is moderate (50,000–300,000 workers). Treatment is straightforward and well understood by any licensed contractor.
イエシロアリ — Ie-shiroari (Coptotermes formosanus)
Restricted to warm coastal zones: Okinawa, southern Kyushu (including all of Fukuoka Prefecture), coastal Shikoku, and parts of Kanagawa and Shizuoka. Mature colonies reach up to approximately 10 million workers — ten to twenty times larger than R. speratus (mid-stage colonies: 1–5 million). A single established colony can span and simultaneously infest multiple adjacent buildings. Wood consumption in a mature colony runs approximately 400 grams per day.
Fukuoka properties sit in confirmed Ie-shiroari territory. Both species are present. The probability of infestation follows the same age curve as the national data; the severity of damage if an infestation occurs is substantially higher.
Why age is the primary risk factor
The MLIT 2013 survey bifurcated results by retreatment status. The Japan Termite Control Association (JSTCA / 公益社団法人日本しろあり対策協会) maintains a 5-year retreatment warranty standard, reflecting the effective chemical lifespan of standard soil treatment (土壌処理). The two cohorts:
| Building age | Untreated | Retreated (JSTCA 5yr) |
|---|---|---|
| 0–4 years | ~0.5% | ~0.2% |
| 5–9 years | 4.9% | 0.8% |
| 10–19 years | 18.9% | 5.6% |
| 20–29 years | 32.6% | 7.5% |
| 30–44 years | ~50% | ~19.9% |
The retreated cohort shows materially lower rates at every age bracket. The critical question when viewing any akiya is therefore: has this building been retreated within the last five years?
For akiya-registered properties, the honest answer is usually: unknown, and probably not. Properties that have passed through periods of vacancy and disengaged ownership are unlikely to have maintained a commercial retreatment cycle costing ¥100,000–¥200,000 every five years. Yudane’s risk panels apply the untreated cohort as a conservative default.
Construction era and ground contact
A second risk factor from Korean National Research Institute of Cultural Heritage (NRICH) factor analysis of 182 wooden heritage buildings (Kim et al., Forests 13(3):465, 2022) is the type of contact between ground and wooden structural members — the dominant predictor in the Korean dataset.
In Japan, ground contact type correlates with construction era:
- Pre-1950: Traditional 礎石 (post-on-stone) construction. Timber columns sit directly on stone bases. Moisture accumulates at the stone–wood interface. Highest risk.
- 1950–1979: 束立て (crawl space) construction. Elevated timber floor over a ventilated subfloor void — but high-humidity environment. High risk.
- 1980–1999: Concrete perimeter foundation (布基礎) increasingly common. Reduced direct ground contact.
- 2000+: ベタ基礎 (full concrete slab) widely adopted following 2000 Building Standards Law revisions. Substantially lower ground contact.
A 1968 farmhouse in Itoshima is carrying risk from two directions simultaneously: age-driven probability and construction-era ground contact. This is the typical Fukuoka akiya.
What to ask before making an offer
1. Do you have retreatment records? Ask for the most recent 防除施工証明書 (termite treatment certificate) or JSTCA warranty card. If the seller cannot produce one dated within the last five years, assume the untreated cohort rates apply.
2. Has there been a pre-purchase termite inspection? A 蟻害・腐朽診断 (termite and rot inspection) by a licensed JSTCA contractor takes 1–2 hours and typically costs ¥20,000–¥50,000 for a standard house. This is money well spent before committing to any wooden akiya. The inspector will probe subfloor timbers, examine the crawl space, look for mud tubes and swarm wings, and issue a written report.
3. What is the subfloor ventilation situation? Poor crawl space ventilation is a leading contributing factor. Ask whether vents are clear and whether the crawl space shows signs of moisture or past damage.
What treatment costs look like
If an active infestation is found, or if you decide to treat as a precaution regardless:
- Standard soil treatment (土壌処理): ¥150,000–¥300,000 for a 100㎡ building. Effective chemical barrier against subterranean termites. Requires retreatment every 5 years to maintain JSTCA warranty.
- Wood treatment (木部処理): ¥80,000–¥200,000. Borate or pyrethroid treatment applied to structural timbers, joists, and sill plates.
- Baiting system (ベイト工法): ¥200,000–¥500,000 installation + ¥30,000–¥60,000 annual maintenance. Effective against Ie-shiroari; eliminates the colony rather than just creating a chemical barrier.
- Structural repair after active damage: Highly variable. Mild damage to a few joists: ¥100,000–¥300,000. Severe damage requiring sill plate replacement and foundation work: ¥500,000–¥2,000,000+.
For a pre-1980 akiya in Fukuoka, budget ¥200,000–¥500,000 for initial treatment as a baseline assumption, and factor in ¥150,000–¥300,000 every five years for retreatment thereafter.
How to factor this into your offer
Termite risk is not a reason to walk away from a property — it is a negotiating input like any other. The practical approach:
- Get the inspection done before you commit. ¥30,000 for an inspection is the cheapest thing you will spend on this property.
- If infestation is confirmed or highly probable, obtain a contractor quote before finalising price negotiations.
- Deduct estimated treatment cost from your offer, or negotiate the seller completing treatment as a condition of sale.
- For Fukuoka properties specifically: budget for the Ie-shiroari premium. Even if R. speratus treatment alone is needed now, a baiting system capable of handling C. formosanus is worth the additional upfront cost.
The Yudane termite risk panel
Each Yudane listing page shows a termite risk panel derived from the MLIT 2013 survey data, building age, and construction era, with Fukuoka’s Ie-shiroari zone flag applied where relevant. The score reflects the untreated cohort by default — the appropriate conservative assumption for akiya.
This is a statistical estimate, not a substitute for a physical inspection. Use it to prioritise which properties warrant early inspection expenditure and to calibrate your cost assumptions. Where a seller can confirm retreatment within five years, the risk profile is materially better than the panel indicates.
A licensed JSTCA contractor in Fukuoka can complete a pre-purchase inspection and provide a written report. Ask your agent for referrals, or search the JSTCA contractor directory for registered operators in Fukuoka Prefecture.