Reclaimed Lumber Day (Kozai no Hi) — 5/31, from "ko(5)-za(3)-i(1)"

What Is Reclaimed Lumber Day?

May 31st is "Reclaimed Lumber Day" (古材の日, Kozai no Hi) in Japan. The date comes from a wordplay on "ko (5) - za (3) - i (1)." It was established by Vintage Aimoku Co., Ltd. (now Aesthetics Japan Co., Ltd.), a company in Matsuyama, Ehime Prefecture, that works to reuse old timber, and it is registered with the Japan Anniversary Association. Aiming to build a "culture of reclaimed lumber," the company created the day to share the appeal of old timber and deepen interest in reusing it. It is a chance to see the beams and pillars salvaged from demolished old buildings not as waste, but as resources steeped in history.

Why Reclaimed Lumber Is Being Re-evaluated

Reclaimed lumber reflects both Japan's wooden architecture techniques and its spirit of "shimatsu" — making full use of resources without waste.

• A tradition of reusing timber: In Japan, materials from dismantled buildings have long been reused elsewhere — for example, when shrines and temples were repaired or relocated. Before World War II, new timber was expensive, so old timber was often reused.
• Postwar change: During the postwar construction boom, cheaper kiln-dried and imported lumber came into wide use. As a result, the high-quality domestic timber in older houses is now being re-appreciated as reclaimed lumber.

What Makes Reclaimed Lumber Special

• Seasoned over many years: Having dried naturally over a long time, old timber is said to warp less. Research on Japanese cypress (hinoki) is also known for finding that its strength increases for some time after felling.
• Carbon storage: Wood stores the carbon that trees absorbed as CO2 while growing. Reusing old timber instead of burning it keeps that carbon locked in the wood.

Interesting Facts About Reclaimed Lumber

• Defining "reclaimed lumber": The term generally refers to domestic timber salvaged from traditional houses built before 1950 (Showa 25) or more than 50 years old.
• The marks of the "chōna" adze: The distinctive uneven surface of old timber comes from being shaped with a chōna, a type of adze, in the days before machines — a trace of handcraft that many people find appealing.
• Blackened by the hearth: The dark, glossy sheen of beams in old farmhouses comes from years of smoke and soot from the sunken hearth (irori), which is also said to have helped protect the wood from insects and decay.
• House lifespans: According to Japan's Ministry of Land, Infrastructure, Transport and Tourism, houses demolished in Japan are on average only about 30 years old — much younger than in Western countries. Reusing timber supports the idea of using buildings and materials for longer.

Tips for Spending Reclaimed Lumber Day

• Visit a cafe or old house that uses reclaimed lumber and enjoy the color and texture of aged wood.
• Look at your own belongings and think about which ones can be used for a long time or repaired.
• Try a small DIY project, such as a shelf, using offcuts of reclaimed lumber.
• Look into examples of traditional Japanese architecture and old-house restoration.

A Day to Appreciate the Warmth of Aged Wood

May 31st is a day to see old things not as an ending, but as the start of a new use. The marks and colors on timber that supported a house for many years carry the history of the people who lived there. Why not take a closer look at the wooden furniture and buildings around you today?