JAPANESE MINKA XXIX - ROOF FRAMING 6: GASSHO-ZUKURI FRAMING 2

In gasshо̄-zukuri (合掌造り) framing, diagonal bracing members (sujikai 筋違) are known as hagai. The roof structure of the Wakayama House in the Shо̄-gawa region, shown below, is a good illustration of these members. There are the о̄-hagai (大はがい, lit. ‘large hagai’), which, as discussed last week, can be thought of as ‘corner principal rafters’ (sumi-sasu 隅叉首) of a hipped roof (yose-mune-zukuri 寄棟造り) that have been repurposed into major diagonal bracing elements within the gable-roofed gasshо̄-zukuri. They run from the middle section of the ridgepole down to the four corners of the structure. Then there are the ko-hagai (小はがい, lit. ‘small hagai’), slender diagonal members which run between principal rafters to further brace the roof structure against longitudinal lateral forces.

The roof structure of the Wakayama House in the Shо̄-gawa region. The о̄-hagai (大はがい), otherwise known as sumi-gasshо̄ (隅合掌), and ko-hagai (小はがい) are labelled in this gasshо̄-zukuri (合掌造り) roof. The ko-hagai shown are high up near the ridge, but would also be found in the lower part of the roof at the feet of the sasu (principal rafters); presumably they have been omitted for clarity. The dashed lines indicate the external form the roof would take were it a hipped-and-gabled roof, or iri-moya-zukuri (入母屋) in Japanese.

The Wakayama House not a typical example of gasshо̄-zukuri framing, however. Thought to have been built in the early Hо̄reki (宝暦) era (1751 - 1764), it is a rare and extremely valuable survival from the period when the minka of the Shо̄-gawa region were transitioning from hipped roofs to gable roofs, necessitated by the widespread adoption of sericulture in the area. It is thought that the Wakayama House started out as a hipped roof construction, with the gable roof simply being built over it at a later date. This ‘double structure’ gives the roof great stability. The corner and end principal rafters no longer serve as the substructure for purlins (yanaka 屋中), common rafters (taruki 垂木) and thatching, as they would in a hipped roof. One of the end principal rafters (oi-sasu 追い叉首) has further been repurposed as a log ladder (kiza-hashi 刻梯子) by having steps cut into it; this is used to access the upper attic floors.

Whereas the ko-hagai are tenoned diagonally into the long-side principal rafters (hira-sasu 平叉首), the о̄-hagai are often lashed with rope to the principal rafters. Over time as joinery became more sophisticated, the pairs of principal rafters were cross-lapped together at the apex, and in some instances the о̄-hagai developed into a kind of thick penetrating tie that was run through mortises in the principal rafters.

A gasshо̄-zukuri roof in the process of being reassembled, with most of the transverse tie beams (kainagi) in place, and the diagonal ko-hagai clearly visible between the lower halves of the principal rafters (sasu). The pale bands on the principal rafters indicate where the ropes used to tie the purlins (yanaka) to the rafters were.

The same roof fully reassembled and waiting to be thatched. All the transverse members are in place, as are the purlins (yanaka) and rafters (taruki). There are no о̄-hagai on this roof.

Hagai are common in minka built before around 1800 but are not found in minka built more recently than this; instead, thick horizontal penetrating ties (nuki 貫) through the principal rafters are used, just as they are used through the posts in walls. The reason usually given for this change is that advances in techniques for erecting the roof structure made it more logical, but given that hagai are in the planes of the roof faces and therefore present no obstruction to use of the roof space, and also given the obvious structural superiority of diagonal bracing members over horizontal, it does seem to add weight to the idea that the Japanese simply have a deep-seated aesthetic preference for the orthogonal over the diagonal in timber architecture. Regardless, the presence, absence or style of hagai in a minka is one clue available to researchers in estimating its age.

Whereas mortise-and-tenon joints were used in gasshо̄-zukuri framing to connect major structural members such as principal rafters and transverse horizontal ties/beams (kainagi かいなぎ), the purlin — principal rafter and common rafter — purlin connections were only rope-tied. Neso (ネソ, also known as shide-zakura (シデザクラ, Amelanchier asiatica) saplings softened by soaking in water were wrapped tightly around the members to be joined; to this joint was added rope made by twisting the bark of the shinanoki (Tilia japonica). Rice straw rope was sometimes used to join members where the loads were not great, but this was not common as rice was not cultivated in the region and so any rice straw used had to be bought from flatland regions.

JAPANESE MINKA XXVIII - ROOF FRAMING 5: GASSHO-ZUKURI FRAMING 1

In this and the next few instalments of this series on roof framing, we will consider gasshо̄-zukuri (合掌造り, lit. ‘praying hands construction’) roof framing, a style of minka found primarily in the basin of the Shо̄ River (Shо̄-gawa 庄川), which flows northwards from the northern part of Gifu Prefecture and though western Toyama prefecture, emptying into Toyama Bay at Takaoka City.

The famed gasshо̄-zukuri minka of Shirakawa-go, Gifu Prefecture.

In principle, gasshо̄-zukuri framing is not very different from the most general type of sasu-gumi (叉首組) construction discussed in previous posts. But as the huge, steeply pitched gasshо̄ roof spaces consist of three, four or even five ‘attic’ levels that are used for raising silkworms, and as sericulture requires good light and ventilation, these roofs must be gabled, with multiple large openings in the gable walls. This is in contrast to the hipped roofs that result from the tripod configuration of typical sasu-gumi framing.

In the region in which gasshо̄-zukuri minka are found, the floor above ground level is known as shita-ni-kai (したニ階, lit. ‘lower second floor’), the floor above that is called the ue-ni-kai (うえ二階, lit. ‘upper second floor’), and the floor above that is the sora-ni-kai (そら二階, lit. ‘sky second floor’). These upper storeys are floored with reeds or timber slats, again to maximise ventilation for the silkworms and their only food, mulberry leaves. It is said that if everything is quiet, the sound of silkworms munching their mulberry leaves on the attic floors above can be heard by the inhabitants on the ground floor.

An upper floor of a gasshо̄-zukuri minka with timber slat flooring.

Diorama showing suspended reed mats holding silkworms and mulberry leaves.

As discussed in a previous post, the lower roof beam framing in gasshо̄-zukuri makes use of bent ‘hockey stick’ beams known as chо̄na-bari (ちょうな梁 or 手斧梁), to account for the height difference between the taller inner jо̄-ya (上屋) or central shu-ya (主屋) posts and the shorter, outer ge-ya (下屋) posts. There is another layer of beams called narashi (ならし) or gasshо̄-bari (合掌梁) over these chо̄na-bari, making the beam framing a two tiered construction.

Cutaway model of a gasshо̄-zukuri minka, clearly showing the chо̄na-bari and three attic floors.

The principal rafters (sasu 叉首) in gasshо̄-zukuri are themselves called gasshо̄, and are particularly magnificent timbers, sometimes 30cm or more square or round; this is necessary both so that they don’t deflect over their long length and for them to be able to carry the large loads from multiple attic floors. To these gasshо̄ are attached intermediate horizontal transverse members called kainagi (かいなぎ) or mizu-bari (水梁, lit. ‘water beam’), which act as both ties to prevent the gasshо̄ spreading or sagging and as beams to support the attic floors.

The uppermost attic floor of a gasshо̄-zukuri minka, partly floored with reed. Note the massive sasu.

It is because of these special characteristics and due to its grand scale that gasshо̄-zukuri framing is classified separately to standard sasu-gumi construction. Recall that in sasu-gumi construction, the principal rafters running from the end of the ridgepole down to the short or ‘gable’ side of the roof are known as oi-sasu (追い叉首, lit. ‘following sasu’; these rafters, together with the corner sasu that make up half of the pyramid structure that gives these roofs their strength against lateral forces, form the end plane of the hipped roof. In some gasshо̄-zukuri, these members may survive somewhat atavistically, concealed within the gabled roof space, as if someone had taken an existing hipped roof and built a gabled roof over top of it, which may be the case. Instead of forming the end plane of a hipped roof as in the sasu-gumi, they serve as diagonal bracing members (sujikai 筋違). What would in a hipped roof be the corner sasu (sumi-sasu 隅叉首, lit. ‘corner principal rafter’) are known in a gabled roof as sumi-gasshо̄ (隅合掌, lit. ‘corner gasshо̄’) or о̄-hagai (大はがい). These members together preserve the triangulating function of the tripod or pyramid framing in the sasu-gumi, and prevent the gasshо̄-zukuri roof from toppling under longitudinal wind or earthquake loads. This method of bracing is only seen in gasshо̄-zukuri and not in other forms of Japanese minka.

Illustration of the peculiar ‘hipped roof within a gable roof’ construction of some gasshо̄-zukuri minka.

VERNACUAR PICTURES 16

No time to write or translate this week, so instead a picture of easily the narrowest street (lane? squeeze?) I’ve ever seen, in Kyoto, Japan. It’s probably only 15cm from gutter to gutter and less than a metre between property boundaries. Obviously from pre-automobile times, it certainly wouldn’t be allowed if proposed today!

JAPANESE MINKA XXVII - ROOF FRAMING 4: SASU FRAMING 3

A simple yet stable tripod structure can be created by tying three poles together at their heads and spreading their feet apart in two dimensions, making this structure able to resist forces from all directions. Such structures are employed in the Japanese rice-drying racks (haza 稲架) discussed last week, and are also used in Japan and elsewhere to suspend cooking pots over fires. Perhaps the purest example of the use of this principle in minka roof framing (koya-gumi 小屋組) is in the distinctive thatched-wall minka of the Ainu (アイヌ) people, known as chise (チセ).

The Ainu are an ethnic group culturally, linguistically, and genetically distinct from the Japanese; historically their lands extended from the northern part of Honshū, throughout Hokkaidо̄, and into the Kuril Islands, Sakhalin Island, and the tip of the Kamchatka Peninsula.

Ainu people with chise in the background.

Pre-historical and historical range of the Ainu.

Exterior of a chise and associated outbuildings, showing uniformly thatched roof and walls, giving these dwellings a coat-like appearance.

Entrance of a chise.

On the left, a female outhouse called the menokoru; on the right, a male outhouse, called the asinru.

Interior of a chise.

Interior of a chise.

The roof framing (koya-gumi 小屋組) of chise was erected on the ground, then lifted whole onto the horidate-bashira (堀立て柱), posts sunk directly into the ground. These koya-gumi therefore had to be rigid structures that would not deform. The principal rafters (sasu 叉首) of these structures are called ketsunni (ケツンニ). As can be seen in the diagram below, the tripods at each gable end of the roof were spanned by a ridgepole (kitaiomani キタイオマニ); the structural arrangement is identical to that seen in the haza, with two sasu running inwards to bear on the long wall plates (sobeshini ソべシニ), one on each side, and a single sasu running outwards to land in the centre of the gable wall plate (soetomotsueppu ソエトモツェップ). Corners are formed with corner sasu (sanperappunikani サンペラップ二カニ) that meet the ridgepole slightly inwards of the apex of the tripods, and intermediate long-side sasu called sakarikanii (サカリカニー) are then laid from the ridgepole to the long wall plates.

The roof framing of a chise, with members labelled with their Ainu names: the ridgepole (kitaiomani キタイオマニ), intermediate purlins (sakiriomappu サキリオマップ), principal rafters (sakarikanii サカリカニー), end wall plates (soetomotsueppu ソエトモツェップ), long wall plates (sobeshini ソべシニ), transverse beams (umangi ウマンギ) long side rafters (sakarikanii サカリカニー), corner rafters (sanperappunikani サンペラップ二カニ), and gable framing members (ponrikanii ポンリカニー).

The roof structure of a chise showing the three principal rafters (ketsunni ケツンニ) forming the ‘tripod’, and the long side common rafters (hira-sasu 平叉首, here called chisenkarikani チセンカリカニ).

This triangulated arrangement is not limited to the chise of the Ainu. A variation on the basic tripod structure can be seen in minka in Toyama Prefecture, where the koya-gumi consists of three pyramid structures, each consisting of four poles. The end pyramids consist of two sasu running to the corners of the building (called sumi-sasu 隅叉首, lit. ‘corner sasu’); the other two poles of these pyramids, and the four poles of the middle pyramid, are called diagonal principal rafters (sha-sasu or naname-sasu 斜叉首).

Roof structure of a minka in Toayma prefecture, showing three pyramid arrangement, each pyramid having four poles (poles on the rear side of the roof are truncated for clarity).

Roof structure of a minka in Toyama Prefecture showing the diagonal principal rafters (sha-sasu or naname-sasu 斜叉首).

Another variation can be seen in Ishikawa prefecture, where there are pairs of diagonal principal rafters incorporated into the long sides of the roof that meet the inner poles of the end pyramids at their feet, and the long-side sasu (hira-sasu 平叉首) at their tops, to provide longitudinal bracing (see diagram below).

Minka roof framing from Ishikawa prefecture showing two pairs of diagonal principal rafters (sha-sasu or naname-sasu 斜叉首) between the end pyramids and the long side principal rafters (hira-sasu 平叉首).

Roof framing of a minka in Ishikawa prefecture showing diagonal principal rafters (sha-sasu or naname-sasu 斜叉首) and long-side principal rafters (hira-sasu 平叉首).

In both cases, the roofs are steeply pitched acute triangles, and without such triangulating reinforcement these roofs would be prone to leaning or toppling under the heavy wind and snow loads common in Japan’s northern and coastal regions.