Kokerabuki is a roofing method in which thin wood boards are used as the roofing material. It is also used to refer to Itabuki (shingle roofing).
It is a traditional roofing method used from ancient times in Japan, and is used for the roofs of many cultural properties.
In a broader sense, it is a kind of Itabuki, and Itabuki includes the following types depending on the thickness of the boards used:
Thinnest boards (kokera boards) are used. The thickness is two to three millimeters.
Boards thicker than kokera boards are used. The thickness is four to seven millimeters.
Thickest boards (tochi boards) are used. The thickness is one to three centimeters.
The mokoshi (small roof-like structures attached to walls), unique to the main hall of Horyu-ji Temple, have roofs made by placing a board overlapping the adjacent board. The roof of this type is called a Yamatobuki roof.
It should be noted that the shape of the character "杮" (kokera) and that of the character "柿" (persimmon) are quite similar but different. "杮" (kokera) indicates a piece of wood or wood shavings as in "kokera" in "kokera-otoshi" (literally, dropping kokera and actually, meaning the opening of a new theater: this term originates in that pieces of unnecessary wood are dropped after the building work completed). However, use of these terms has been mixed (for more information, refer to "Kokera-otoshi").
Water-resistant wood with a pronounced grain that is easy to shape, such as Japanese cypress, Japanese false cypress, Japanese cedar and Japanese hackberry, is used.
Kunugi (sawtooth oak) and tokusa are not such materials.
The boards used are made by cutting wood along the growth rings.
The history of itabuki is old, second only to that of kayabuski (thatched roofs).
Itabuki is used for the roofs of the five-storied pagoda in Horyu-ji Temple, and therefore it is thought that use of boards for roofs started in the Kofun period.
Later on, as tools improved, methods for using thinner boards were developed, and use of kokerabuki, utilizing the thinnest boards, started in the early Heian period.
Until the Edo period, tochibuki and tokusabuki had also been used in shrine buildings and in temple buildings. However, because thicker boards are used in these methods, they only worked on roofs with nearly straight lines. Therefore, kokerabuki with thinner, flexible boards has gradually become more widely used.
There are many buildings, including Hondo Hall of Sanbutsu-ji Temple, in which tochibuki or tokusabuki was initially used for the roof but was replaced with kokerabuki later.
Therefore, most extant cultural properties with itabuki roofs use kokerabuki, and few of them use tochibuki or tokusabuki, so it is hard to find craftsmen who can work on these methods.
Itabuki roofs commonly use boards 9-10cm wide and 24-30cm long. These boards are placed horizontally so that a board overlaps the adjacent board, and are fixed with bamboo pegs. Sometimes, logs are placed over these boards horizontally, and only stones are used for holding them in place.
Small gaps are made between these boards, generating air flow from the area under the ridge, and thereby increasing the durability of the wood.
It is said that kokerabuki roofs ordinarily have a life of around 40 years.
However, board making has been mechanized recently, resulting in cutting of the wood fiber. Therefore, the lifespan of roofs made with these boards is shorter than that of those made with hand-made boards.
Fore this reason, nowadays, waterproofing treatment is given to the surfaces of these boards or preservative treatment is given to the boards themselves.
Typical buildings using itabuki
The golden (main) hall of Muro-ji Temple
The main hall of Kamosu-jinja Shrine
The Kinkaku (literally golden tower) of Rokuon-ji Temple
The Ginkaku (literally, silver tower) of Jisho-ji Temple
The Ko-shoin of Katsura Rikyu Imperial Villa
The Kyakuden of Shugakuin Imperial Villa
The main hall of Sanbutsu-ji Temple (tokusabuki was used when the hall was built initially, and kokerabuki is used now). The Emadokoro (a hall where special plates for prayer are placed) of Kitano Tenman-gu Shrine (tokusabuki was used when the hall was build initially, though a tiled roof is used now).
The Kobo-do hall of Rengebu-ji Temple
The San-mon gate of Zenko-ji Temple (the roof had been replaced with hiwadabuki, but the intial tochibuki roof has been restored).
The Nio-mon gate of Komyo-ji Temple (located in Ayabe City)
Shiramizu Amidado Hall