Thatch (茅葺)

Kayabuki is a type of roof structure using kaya (grass) (Japanese pampas grass or cogon) as the material for thatching and is also called kaya-thatched roof. It is sometimes called straw-thatching or grass-thatching depending on the material used.

Thatch is seen worldwide, including England and Germany.

Summary

Kaya is an alternative name for susuki or Japanese pampas grass, but it is sometimes as a general term for chigaya (Imperata cylindrica). It is said that thatch is the most primitive roof in various areas of the world. It is also thought that dwellings using only a thatched roof were built in the Jomon period of Japan. The possibility that buildings in the Nara period onwards might have been kokera (bark tiles)-roofed or bark-roofed has been investigated; but restorations of the roofs of pit dwellings from Yayoi or earlier periods ruins (for example, Toro Ruins) are usually thatched.

Plant material for roofs quickly rots if they are used in a moist condition. Therefore, such material is usually collected in the winter after they die. They are used after sufficient drying until spring but are sometimes smoked before being use to increase their durability. When a kamado (kitchen range, cooking stove) and/or a fireplace (a sunken hearth) is used within a building, it increases the durability of such a roof. However, shrine buildings are shorter-lived than private houses because fire is rarely used inside the building.

Steep roofs are used to prevent rain from seeping in. However, if plant materials with thick stalks are used, the gaps between the stalks are larger and tend to let the rain seep in. Thus, the roof must be made steeper to prevent rain from seeping in. The roof has advantages such as good air permeability/heat retention abilities and also reduces the sound of falling rain. However, its disadvantage is its short life and the fact that it may easily catch fire from neighboring fires.

In Japan, as communities grew larger and developed into densely-packed urban areas, its flammability was a serious disadvantage, therefore, such roofs disappeared rapidly in urban areas, and town houses along roads to be replaced by tiled roofs. Thatch was prohibited in some urban areas of Edo. On the other hand, it was easier to obtain the material such as Japanese pampas grass, cogon or rice straw in villages and it was possible to jointly gather these materials and repair roofs during the agricultural off-season. Therefore, many thatched houses existed in villages in mountainous areas until around the middle of the 20th century in Japan. Compared to mountainous areas, thatched roofs decreased earlier in areas where typhoons are severe because thatched roofs cannot withstand strong winds.

After the Second World War, thatched roofs disappeared rapidly because of the decrease in population in farming areas made it difficult to jointly re-thatch roofs, introduction of regulations that complicated new buildings and the price of wood, such as Japanese cedar, temporarily increased and kaya-growing areas were converted to artificial forests. At present in the 21st century, most of the thatched roofs have been replaced by metal roofs, such as tin roofs; but a small number of buildings with thatched roofs still remain. Newly built thatched private houses are extremely rare, but many temple/shrine buildings, such as the Shogu (main shrine) and Betsugu (sub-shrine) of Ise-jingu Shrine maintain thatched roofs according to old customs. Thatched roof designs are sometimes used in commercial facilities to give character to their exterior, but some use reinforced plastic that look similar to thatch instead.

Ishinomaki City of Miyagi Prefecture relaxed the regulations for thatched roofs based on Clause 1, Article 22 of the Building Standard Law starting on November 1, 2006, opening the way for building new thatched roofs. This has led to stimulation of local industries through the use of reeds, a specialty of the Kitakami-gawa River, which flows through the city. It also is attracting attention because it would induce people who wish to live in rural areas to live in Ishinomaki City by allowing new buildings with thatched roofs, which are of high cultural value. Ishinomaki City initially applied to the national government for a special area to allow new buildings with thatched roofs. The national government expressed the view that the city could deal with deregulation to allow the construction of new thatched roofs by redesignation of nonflammable materials for preventing the spread of fire specified in Article 22 of the Building Standard Law.

Outside of Japan, use of such roofs in private houses has decreased in West Europe (such as Germany, Denmark and Holland). However, newly built thatched houses are still relatively numerous because it is a symbol of wealth among the rich and Japanese craftsmen visit these countries for training.

Type of roof shapes

Kirizuma-zukuri style (an architectural style with a gabled roof): The roof has a book opened facedown shape and is thought to have developed in Japan from takayukashiki-soko (warehouse on stilts). Gassho-zukuri (a house built of wooden beams combined to form a steep thatched roof that resembles two hands together): This type is usually classified as Kirizuma-zukuri, but is sometimes classified as irimoya style (building with a half-hipped roof) when the eaves of the gable are large. Shinmei-zukuri style used in temple/shrine buildings require kirizuma-zukuri, but the use of thatched roofs is optional.

Yosemune-zukuri (a square or rectangular building, covered with a hipped roof): This style is found throughout Japan and is thought to have developed from prehistoric pit dwellings.

Irimoya style: A structural style where a gable and large eaves are integrated into one with windows for taking in light and letting smoke out are added to yosemune-zukuri. Other than private farm houses, many examples can be found amongst temple/shrine buildings.

Ridges

Takesumaki structure: The kaya ridge is wound around by woven bamboo.
Found in various areas in Japan

Okichigi structure: Wood is assembled so as to cover the ridge. Found in mountainous areas where plenty of wood materials are available.

Kogaimune structure: Kaya is piled on the ridge and is fastened to wood protruding from the roof.

Shibamune structure: Trees and grasses are grown intentionally to use their roots to prevent the ridge from loosening.

Re-thatching

Depending upon the location and usage, the entire roof is commonly renewed around once every 20 to 40 years. However, periodic repair is required around the top area because there is more damage to the thatch.

Regarding securement of material, Japanese pampas grass fields called kayaba existed in the periphery of villages in the past. These Japanese pampas grass fields were maintained by stopping the progress of transition (biology term) by cutting the grass regularly as food for domestic livestock. However, use of the fields has been abandoned and most of them lost with the change in lifestyle since the end of the Second World War. However, due to the introduction of the acreage reduction policy and decrease in farming, areas that have become kayaba are on the rise. Therefore, it is said that finding material for such roofs has become easier than the past. Some building companies have started to train young craftspeople in thatch-roofing, and therefore, the decrease of craftspeople has begun slow. Since ancient times, kaya has been collected at various places around the Ise-jingu Shrine, such as Kamiji-yama Mountain. However, the shrine acquired land during the Taisho period in Watarai-cho, Watarai-gun especially for growing kaya (kayaba for the shrine). At this kayaba, Japanese pampas grass is collected in late autumn by volunteers from the local residents.

The edge of many thatched roofs are cut cleanly in a plane to make it look beautiful. However, it is also said that water drainage is better if the edge is not cut cleanly.

If the resident himself is thatching the roof, it is usually sakabuki (literally, reverse thatching), where the ears of grass are placed downwards. When the work is entrusted to a professional, it is usually honbuki (literally, real thatching), where the ears of grass are placed upwards. The former method uses only a half or third of kaya compared with the latter method and is easier because kaya does not slide down. However, it is less durable because the ear part, which has a low oil content, is exposed to rain.

Although the cost depends on the area, it is said that the typical market price is roughly around \120,000 per 3.3 square meters (around \5 M in total).

Locations with many thatched building

Gassho-zukuri-concentrated areas are in Shirakawago and Gokayama (located in Gifu Prefecture).
- World Heritage (cultural heritage) Site
Ouchi juku (Shimogo-machi, Minami Aizu-gun, Fukushima Prefecture)
Kayabuki-no-sato (thatched house village): Kitamura (Miyama-cho, Nantan City, Kyoto Prefecture)
EDO TOKYO OPEN AIR ARCHITECTURAL MUSEUM
Nihon-minka-en (exhibits of transitional private houses in Japan) (located in Kawasaki City, Kanagawa Prefecture)
Nara Prefectural Museum of Folklore (located in Yamatokoriyama City, Nara Prefecture)
Sanshu Asuke Yashiki (located in Asuke-cho, Toyota City, Aichi Prefecture)