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.