The previous seven posts or so were spent looking at the many varieties of minka roof beam framing (hari-gumi 梁組). But the hari-gumi is just one element of minka roof framing (koya-gumi 小屋組), and we are now finally ready to move on, and up, to consider the roof structure as a whole. It should come as no surprise that here we find perhaps even more variety than that presented by the beam framing, because the koya-gumi consists of the roof beams previously discussed, plus all of the structural elements within the interior of the roof or ‘roof space’ (koya-ura 小屋裏) that are above, and supported by, the beam framing: roof posts (tsuka 束), ties (nuki 貫), purlins/underpurlins (moya 母屋 or yanaka 屋中), principal rafters (sasu 扠首 or 叉首), common rafters (taruki 垂木 or 棰), etc.
In Japanese architectural terminology, elements on the external surfaces on the roof that are visible from the exterior, such as the roof covering (yane-buki 屋根葺き) or roof ornaments (yane-kazari 屋根飾り), are not considered part of the koya-gumi. The names for these elements customarily contain the characters for roof (yane 屋根), whereas internal structural elements, e.g. the transverse roof beams (koya-bari 小屋梁), are by contrast often associated with the word koya.
Koya-gumi can be broadly classified into four main types: sasu-gumi (扠首組 or 叉首組), lit. ‘principal rafter framing’), wa-goya-gumi (和小屋組, lit. ‘Japanese koya framing’), shin-tsuka-gumi (真束組, lit. ‘true post framing’), and nobori-bari-gumi (登り梁組, lit. ‘climbing beam framing’). The sasu-gumi and wa-goya-gumi (sometimes simply called wa-goya) were introduced in a previous post, but this and subsequent posts will go into far more detail.
The sasu-gumi is a primitive trussed or triangulated structure, formed by a pair of round-section log (maruta 丸太) principal rafters (sasu 扠首 or 叉首), which determine the pitch of the roof, and a koya-bari transverse beam as its lower chord. This framing arrangement is simple and strong, but not typically strong enough to bear roof tiles (kawara 瓦), and so it is closely associated with thatched roofs (kusa-yane 草屋根, lit. ‘grass roof’).
Sasu-gumi (叉首組) framing, showing the wall plates (keta 桁), transverse beam (koya-bari 小屋梁), principal rafters (sasu 叉首 or 扠首), purlins (yanaka 屋中), and rafters (taruki 棰).
In contrast, the wa-goya is highly representative of tiled roofs (kawara-yane 瓦屋根), and can also be seen under timber board or plank roofs (ita-yane 板屋根). In this method, vertical posts of around 1 metre in length, called tsuka (束), are set at intervals along the koya-bari; these posts support the underpurlins (moya 母屋) and the ridgepole (munagi 棟木) that run perpendicularly to the koya-bari, and transfer the live and dead roof loads down to the beams. A modernised form of wa-goya framing is the standard roof framing method even today in Japanese new-built houses.
Wa-goya-gumi (和小屋組) framing, showing the wall plates (keta 桁), transverse beam (koya-bari 小屋梁), crown post (shin-tsuka 真束), penetrating tie (nuki 貫), underpurlins (moya 母屋) and ridgepole (munagi 棟木), and rafters (taruki 棰).
A primitive agricultural shelter showing naturally forked perimeter posts supporting wall plates, central posts called muna-mochi-bashira (棟持柱, lit. ‘ridgepole holding post’), also forked, that run from ground to apex and support the ridge pole, and common rafters spanning from ridgepole to wall plates. This structural arrangement is thought to be the progenitor of the wa-goya-gumi, where the obstructive muna-mochi-bashira are replaced with crown posts (shin-tsuka) bearing on transverse beams (koya-bari).
In shin-tsuka-gumi framing, a longitudinal beam called a naka-oki (中置) is set along the centreline of the koya-gumi, and on this beam are erected crown posts (shin-tsuka) which support the ridgepole (munagi 棟木). Thick rafters span from the ridge to the eaves beams/wall plates (noki-geta 軒桁). This form of roof may be tiled, but is more commonly thatched, and is also common amongst the board/plank-roofed minka of the Shinshū region. Older minka with thatched roofs and shin-tsuka-gumi framing can be found in every region of the country; in these minka the rafters are mainly thick bamboo or timber poles, the central ridge-supporting crown post is called the odachi, among other names, and the method of construction is known as odachi-gumi. In some areas the type is known as burari-zukuri, because of the way the rafters hang or dangle down (burari-agaru ぶら下がる) from the ridge.
Odachi-gumi (おだち組) or shin-tsuka-gumi (真束組) framing, showing transverse beam (koya-bari, unlabelled), longitudinal beam (naka-oki 中置), crown post (odachi おだち), ridge pole (munagi 棟木), and bamboo rafters (kudari-dake くだり竹).
Nobori-bari-gumi framing was commonly employed when the roof space was required for storage or work, as it is a framing method by which this space can be opened up and/or made taller for those purposes. The transverse beams, here called zushi-yuka-bari (厨子床梁, lit. ‘zushi floor beam’; a zushi is a portable Buddhist shrine/altar) are tenoned into the posts at a level somewhat lower than the heads of the posts; these beams support a central longitudinal beam (naka-oki 中置) upon which are set crown posts (shin-tsuka 真束) a large longitudinal beam called a jimune (地棟) is run along the top of the shin-tsuka at the apex of the roof space and just below the ridgepole (munagi); inclined beams (nobori-bari 登り梁) at the same pitch as the roof span from the jimune down to the heads of the wall posts; purlins/underpurlins (moya 母屋) are set directly onto these inclined beams and themselves support the common rafters (taruki 棰).
Nobori-bari-gumi (登り梁組) framing, showing the transverse beam (zushi-yuka-bari 厨子床梁), tenoned (sashitsuke 差付け) into the posts, the central longitudinal beam (naka-oki 中置), crown post (shin-tsuka 真束), the beam brackets or ledges called makura-gi (枕木, lit. ‘pillow timber’, i.e. ‘sleeper’), rising beams (nobori-bari 登り梁), longitudinal apex beam (ji-mune 地棟), purlins or underpurlins (moya 母屋) and common rafters (taruki 棰).