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.