A hikimun-zukuri minka on Amami О̄shima.
On the subtropical Amami archipelago (Amami Guntо̄ 奄美群島) between Kyūshū and Okinawa, in particular on the main island of Amami О̄shima (奄美大島), minka employ a structural system called hikimun-zukuri (ひきむん造り). The minka of these islands are of the sebuntо̄-gata (分棟型, lit. ‘separate ridge form’) shо̄tо̄-bunritsu (小棟分立, lit. ‘small ridge separate standing’) form: small, structurally independent and often physically separate structures, each for a particular function, e.g. cooking, ‘living’, etc.; there are no huge, magnificent roofs as can be seen on the mainland. The simplest form is called sasuya-zukuri (さすや造り). Posts are closely set at around 0.5 or 1 ken centres, and linked at uchi-nori (内法, lit. ‘inner measure’) height by keta-uke-bari (桁受け梁) beams of the same dimension as the posts. The uchi-nori height is the ‘head’ height, i.e. the dimension from the top of the floor sill (the finished floor level) to the underside of the lintel. This dimension was based on the length of a tatami mat, which in turn was taken from the average height of an adult male in the Muromachi and Edo eras, around 154 to 156cm, and was eventually standardized at 5 shaku (尺) 7 sun (寸), or around 173cm. Thus in the oldest and most primitive Amami minka, the uchi-nori is too low to hang a ceiling, and the posts within rooms make the living spaces difficult to use.
In minka on the mainland, the nuisance of internal posts was eliminated as joinery became more sophisticated, and deep, tenoned beams and lintel beams (sashi-gamoi 差鴨居) capable of greater spans appeared; on these beams were erected roof posts (tsuka 束) to support the roof, turning the sasu-gumi (扠首組) structure into a wagoya-gumi (和小屋組) structure.
On the Amami islands, there was an almost identical development in the hikimun-zukuri, which takes its name from the fact that both the beams and lintels there are called hikimun (ひきむん or 引物). The name literally means ‘pull thing’ or ‘draw thing’, presumably because these beams are ‘drawn’ or ‘strung’ between or around the tops of the posts, at uchi-nori height. The hikimun became larger in section to achieve longer spans, and the structure also evolved to increase ‘head space’: the interior posts were increased in height, so becoming jо̄ya-bashira (上屋柱, ‘upper roof post’), and above the hikimun were added another set of beams, called kakure-hikimun (隠れひきむん, lit. ‘hidden pull thing’) because they are hidden above the ceiling now made possible by the increased post height.
There is major point of difference between the evolution of these two systems, however. Whereas on the mainland deepened beams were given correspondingly deep tenons and slotted into sides of the posts, on the Amami islands the hikimun were wider, and square vertical holes were cut into them, with lower beams having larger holes than upper beams. The beams were dropped from above onto the square posts, which are progressively stepped down in section to match the dimensions of the holes in the posts.
Drawings and photographs of the hikimun-zukuri framing system.
Drawing showing how the hikimun (ヒキムン) fit over the stepped jо̄ya-bashira (上屋柱) posts. The posts drop into the floor beams (neda ネダ) in a similar way, but pegged below the beams. The assembly has an almost toy-like appeal in its simplicity.
The transverse members (beams or lintel beams) are set onto the posts first, followed by the longitudinal wall beams/plates. The former have grooves planed into their soffits to receive sliding partitions. Above the ceiling there is another tier of beams (kakure-hikimun). At the highest level, at the top of the internal ring of posts, are wall plates called hon-geta (本桁, lit. ‘true wall beam’ though it should be noted that in honya-date construction they are not actually in the plane of the external walls) that support the principal rafters (sasu 扠首, here called sashiki さしき) and common rafters (taruki 垂木, here called kichi きち) and transverse beams called sasu-uke-bari (扠首受梁, lit. ‘common rafter bearing beam’). On the mainland this system is called honya-date (本家建て, lit. ‘true house construction’) which will be discussed in greater detail in future posts.
Section drawing of the Nakamura House, Amami О̄shima. This hikimun-zukuri structure has a distinct division between jо̄ya and geya, but in this example there is a curved hikimun beam spanning the full distance between perimeter geya posts.
Detail photograph of the Hikimun-zukuri construction of the Nakamura House. The post (hashira 柱) steps down in section as it rises; the wide hikimun (ひきむん) and kakure-hikimun (かくれひきむん) longitudinal and transverse beams are dropped onto the post from above. At the top of the post are the transverse sasu-uke-bari (扠首受梁) beams and the hon-geta (本桁) wall plates supporting the principal rafters (sashiki さしき) and common rafters (kichi きち). Note that in this example the principal rafter doesn’t directly bear on the sasu-uke-bari, but is offset to the left.
In later examples of hikimun-zukuri, the transverse hikimun at uchi-nori height might be omitted. Instead, only a grooved head (kamoi 鴨居) attached to a rail (nageshi 長押) or tie (nuki 貫) was used above the opening, and above that a bulkhead (ari-kabe 蟻壁 or 有壁) consisting of planks or boards.
Section drawing of the Furumi House, Amami О̄shima. An example of hikimun-zukuri construction where there is a transverse hikimun at the upper jо̄ya level but not at the uchi-nori level, where there is only a relatively shallow . The bulkheads (ari-kabe 蟻壁 or 有壁) above the kamoi consist of single wide planks of Japanese cedar (sugi, Cryptomeria japonica)
Photographic detail of the beam framing of the Furumi House.
Diagrammatised drawing of the photograph above. There are longitudinal and transverse hikimun (ひきむん) at uchi-nori height (内法高) and the transverse hikmun at this location is grooved to take sliding partitions; this is in contrast to the section drawing above, where there is only a kamoi and nageshi with no transverse hikimun. Above the transverse hikimun is a timber board (hame-ita 羽目板) bulkhead (ari-kabe 有壁) and above that are the ceiling boards (tenjо̄-ita 天井板). Above the ceiling are the kakutre-hikimun (かくれひきむん) beams, then at the top of the post the longitudinal ‘wall plate’ (hon-keta ほんけた) and transverse common rafter bearing beam (sasu-uke-bari 扠首受梁).
The ‘stepped’ or ‘tapered’ post method of construction is also seen in the floor framing of the raised-floor storehouses called taka-kura (高倉, lit. ‘high storehouse’) of Hachijо̄-jima (八丈島), an island in the Izu chain (Izu Shotо̄ 伊豆諸島) south of Tokyo, where the floor beams are dropped onto the posts from above.
A taka-kura on Hachijо̄-jima.
The same taka-kura as above from a different angle.
These are simple joints that have arisen from primitive tools and techniques. Yet the system may also be seen as a rational one: there are no tenons to twist or snap under lateral loads from the cyclonic winds that the islands are exposed to.