JAPANESE MINKA XXVI - ROOF FRAMING 3: SASU FRAMING 2

In cold areas where the snow load on roofs is great, and in large minka where the principal rafters (sasu 叉首) are especially long, roof posts (tsuka 束) are employed at intermediate positions to support the sasu and prevent them from sagging or bending. These posts, in conjunction with the transverse ties or beams that sit on top of them, form a shape that resembles the Shinto torii (鳥居) gate, and so this reinforced sasu-gumi construction is known as torii-gumi (鳥居組). This is something of a hybrid style of construction, somewhere between true sasu-gumi, where the sasu are unsupported along their length, and true wagoya-gumi, where there are no sasu. The transverse elements also serve to prevent the sasu from thrusting outwards and spreading the walls that they terminate on.

A torii gate.

Torii-gumi construction.

Thanks to the trussed form created by the paired sasu and the transverse member that forms the base or bottom chord of the triangle, sasu-gumi structures are very strong in the transverse direction, but they are extremely prone to racking or tipping over in the longitudinal direction.

Various methods have been devised to compensate for this longitudinal weakness. The idea for sasu is thought to have originated in haza (稲架), the simple pole structures erected in paddy fields to dry harvested rice. Stability in these structures is achieved by adding a third leg to the ‘sasu’ pairs at each end to form tripods which brace the structure against longitudinal forces.

In minka roof framing, the equivalent ‘third leg’ members at the ‘gable’ ends (妻側 tsuma-gawa) or short sides of hipped roofs are called oi-sasu (追叉首, lit. ‘following principal rafter’) or mukau-sasu (向かう叉首, lit. ‘facing principal rafter’), in contrast to the ‘regular’ paired sasu in the long sides of the roof, which are called hira-sasu (平叉首, lit. ‘flat principal rafter’). The oi-sasu is tenoned into the underside of the ridgepole (munagi 棟木) just to the outside of the point where the end pair of hira-sasu cross.

Photograph showing how the oi-sasu (追い叉首) is tenoned up into the munagi (棟木) just to the outside of the paired hira-sasu (平叉首, here called sashiki さしき).

JAPANESE MINKA XXXV - ROOF FRAMING 2: SASU FRAMING

Last week’s post introduced the four main roof framing systems employed in Japanese minka. From today we consider the simplest of these systems, sasu-gumi (sasu-gumi 扠首組 or 叉首組) or ‘principal rafter framing,’ in more detail.

Typical example of sasu-gumi (叉首組) framing, showing four pairs of principal rafters (sasu 叉首) and transverse beams (koya-bari 小屋梁), ‘gable end’ rafters (oi-sasu 追叉首), wall plates (noki-geta 軒桁 and tsuma-bari 妻梁), cross bracing (hagai はがい), ridgepole (munagi 棟木), and common rafters (taruki 棰).

The paired principal rafters (sasu 扠首 or 叉首) after which sasu-gumi takes its name are not butt-jointed where they meet at the apex of the roof, but crossed at their upper ends so that some part of the rafters extend beyond the intersection; in the crotches thus formed sits the ridgepole (munagi 棟木). In the most primitive examples, this junction is secured with nothing more than rope, but in most cases some form of joinery is employed, in the form of either a cross half-lapped (aikaki 相欠き) and wedged (sen-uchi 栓打ち) joint, or a pegged mortise and long tenon (naga-hozo-sashi-hana-sen-uchi 長枘差し鼻栓打ち) joint. 　Whichever joint is employed, the ‘handedness’ of each sasu pair is alternated from bay to bay: first right over left, then left over right, and so on, giving greater strength to the whole roof structure.

Two joinery methods for connnecting the sasu at the apex of the roof: on the left, a cross half-lapped (aikaki 相欠き) and wedged (sen-uchi 栓打ち) joint; on the right, a pegged mortise and long tenon (naga-hozo-sashi-hana-sen-uchi 長枘差し鼻栓打ち) joint. The latter also shows how the ridgepole (munagi 棟木) is lapped into the crotch of the rafters.

The lower ends or ‘tails’ of the rafters are shaped down to a pencil-like point, and set into holes cut into either the transverse beams or the wall plates, then secured with wedges. This is a ‘pinned’ connection, meaning the sasu is theoretically only under axial loads and does not experience any bending, because it is unrestrained and free to rotate around the connection point in two axes, like the ball of the femur in the socket of the pelvis. This kind of joint is said to be more resilient than a fixed, rigid connection under loads from wind and earthquake.

Photograph of the apex of a roof showing the sasu joined with cross half-lapped (aikaki 相欠き) and wedged (sen-uchi 栓打ち) joints, and the ridgepole (munagi) carried in the crotches formed.

Two examples of joining the sasu to the beam structure: on the left, orioki-gumi (折置組) framing with the wall plate (keta 桁) over the transverse beam (koya-bari 小屋梁); on the right, kyо̄ro-gumi framing with the transverse beam over the wall plate, and the sasu ((叉首) secured in its hole with a wedge (kui 杭).

Photograph of a carpenter reassembling a minka roof. This is a kyо̄ro-gumi framed roof, with the sasu set into holes in the longitudinal beams (keta) which sit on the transverse beams (koya-bari), though this example has two tiers of both. On the left can be seen a new, pale wedge (kui) holding the much older and darker sasu in place. Diagonal braces (hagai) are also visible.

After the sasu and ridgepole are erected, purlins (commonly called moya 母屋 but here yanaka 屋中) are fixed horizontally to the sasu at around 60-80cm intervals with rope; in addition to restraining the sasu from tipping over, they form a subframe to tie the common rafters (taruki 棰) to. The rope used is regular straw (wara 藁) rope, but it is said that if the rope is made from straw that has first been exposed to winter snow it will last 40 or 50 years; this is comparable to the life of the reed (kaya 茅) thatch used as the roof covering, meaning that when the roof is rethatched the rope can be conveniently replaced at the same time.

JAPANESE MINKA XXXIV - ROOF FRAMING 1

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).

JAPANESE MINKA XXXIII - BEAM FRAMING 7

The previous post briefly mentioned the roof beam framing system known as honya-date (本家建て, lit. ‘true house construction’). In this system, the roof framing (and associated posts) consists of two elements or assemblies: the upper roof or jо̄ya (上屋, lit. ‘upper house’) and the lower roof or geya (下屋, lit. ‘lower house’). The jо̄ya and geya were defined and discussed in a previous entry as part of the series on posts, which can be read here. Today we will a look briefly at some of the variation seen within honya-date framing, as a way of concluding our series on the beam framing of minka. This post, and indeed this series as a whole, only presents representative examples of the main beam framing systems; in reality there is huge variety in these types, both from region to region and within regions.

A typical example of honya-date construction showing the jо̄ya (上屋), which consists of the inner, taller jо̄ya posts (jо̄ya-bashira 上屋柱) and transverse beams (koya-bari 小屋梁) that support the principal rafter framing (sasu-gumi 扠首組); and the geya (下屋), which consists of the outer, shorter perimeter geya posts (geya-bashira 下屋柱) and associated wall plates (keta 桁), which support the common rafters (taruki 棰) and roof covering that extend past the external wall plane to form the eaves. Note the principal rafters (sasu 扠首) have pointed ends which terminate at the junction between the koya-gumi and the longitudinal beam known as the kusa-keta (草桁, lit. ‘grass beam’).

The beam framing system of simpler, hut-like structures, which lack a geya, are called suya-date (素家建て, lit. ‘basic house construction’).

Two examples of soya-date construction: orioki-gumi (折置組) framing on the left, and kyо̄ro-gumi (京呂組) framing on the right.

The relationship between the jо̄ya and geya in the simplest honya-date structure can be seen in the section diagram on the left below (the Furui 古井 house). The outer, shorter geya posts (geya-bashira 下屋柱) and inner, longer jо̄ya posts (jо̄ya-bashira 上屋柱) are connected with thick ties called tsunagi-nuki (繋貫, lit. ‘connecting tie’). However, because the inner row of jо̄ya posts along the long sides of the building present a hindrance in the use of the internal spaces, various methods were devised so as to be able to omit some or all of these posts. Jо̄ya posts that do not ‘get in the way’, because they are in the plane of a partition wall along the line of the ridgepole or in the plane of other partitions, can be linked by inclined/raked beams to the lower geya posts in the plane of the external perimeter walls; roof posts (tsuka 束) erected on these beams support the upper jо̄ya beams above, forming a two-tiered beam structure (the section diagram on the right below). There are many regional variations on this method of construction.

On the left, the Furui house showing geya and jо̄ya posts connected with tsunagi-nuki (繋貫). On the right, an example from the Tо̄hoku region where the jо̄ya posts have been replaced with roof posts (tsuka) that bear on a pair of inclined beams (orioki 折置), which frees up the interior. Orioki framing (ori-oki-gumi 折置組) refers to the framing system where the transverse beams sit directly on the external wall posts, and the wall plates are above and supported on the transverse beams.

An example of orioki-gumi framing where the jо̄ya post is replaced by a short roof post (tsuka 束) that bears on the lower transverse beam (koya-bari 小屋梁) and supports the upper transverse beam and upper longitudinal beam (jо̄ya-geta 上屋桁) of a two-tiered beam system.

Examples of honya-date minka from the Kantо̄ region. On the left, an example of the structural style known as shikata-geya-zukuri (四方下屋造り lit. ‘four direction geya construction’) with six transverse posts. On the left, a style from the same region known as odachi-zukuri (おだち造り) where some of the obstructive internal posts can be omitted by the use of longer, curved transverse beams. The odachi is the crown post that supports the ridgepole and common rafters (taruki 棰); note that there are no principal rafters (sasu 扠首) in this system.

Examples of honya-date minka from the Chūgoku region. In the example on the right, the internal post is offset to the right in order to create a larger unobstructed space on the left; in the example on the left, the internal post is omitted altogether by the use of a large central longitudinal beam (nakabiki-hari 中引梁).

Section of a small minka on Amami О̄shima (whose unique minka are discussed in last week’s post). The construction method shown here is called sasuya-zukuri (さすや造り), where the principal rafters (here called sashiki さしき) bear directly and only on the wall plates (hon-geta 本桁) without any intermediate roof posts, beams or ties. A single tier of transverse beams called kyakuro (きゃくろ), of the same dimension as the posts, serve as both beams and head rails for the sliding partitions. The method is unusual in that there are inner and outer posts, but the common rafters terminate at the same point as the principal rafters, indicating that this is a suya-date (素家建て) rather than honya-date (本家建て) construction, as it lacks a geya.

In the famous gasshо̄-zukuri style of minka seen in Toyama and Gifu Prefectures, the geya is skilfully resolved by having the ends of hockey-stick shaped beams called chо̄na-bari (ちょうな梁, lit. ‘adze beam’, not because the beam is adzed but because its shape is like that of an adze handle) bear directly on the external wall plates and geya posts. In Toyama Prefecture, this method of construction is called maieya-zukuri (まいえや造り). Many of the minka of Shirakawa (白川村), Gifu Prefecture, have chо̄na-bari only on the facade side of the structure; the rear is either two-post honya-date, or suya-date construction. This hybrid style is called katagi-zukuri かたぎ造り.

Example of a gasshо̄-zukuri style minka showing a massive curved beam (chо̄na-bari ちょうな梁) on the right, spanning from the internal post to the external wall post (geya-bashira 下屋柱), allowing the omission of the internal jо̄ya post (jо̄ya-bashira 上屋柱), here called the chakuro-bashira (ちゃくろ柱).

Photograph taken during rethatching of a gasshо̄-zukuri minka, showing the curved transverse beam (chо̄na-bari ちょうな梁) bearing on the longitudinal wall plate (keta 桁) and geya post.