This week we extend our examination of posts (hashira 柱) in minka to consider them within the context of the whole building, in both structural and planning aspects. This will require going into the subject of the roof structure (koya 小屋) of minka, which means getting ahead of ourselves somewhat, as roofs will be treated separately in more detail in future entries.
In old minka, it was the rule that posts were always placed directly under each end of every koya-bari ( 小屋梁), the main transverse roof beams that carry the load of the roof structure.
The type of roof structure (koya 小屋) shown here, known as Wa-goya, ‘Japanese roof framing’, is still in common use. Three transverse roof beams (koya-bari 小屋梁) can be seen. Note that there is no post under the end of the middle koya-bari, indicating this to be a modern example of the Wa-goya. Diagonal bracing, in both vertical and horizontal planes, is also something not typically seen in traditional minka roof framing.
In addition to these posts, most minka, other than the smallest and simplest, also made use of internal posts providing intermediate support to the roof beams. These are typically placed at a spacing of around one ken (間) or six Japanese feet (shaku 尺, 303mm), standardised in modern times at 1,818mm. These internal posts are usually positioned at the locations of internal partitions, but free-standing posts can also be seen, in the earthen-floored doma and niwa utility areas of many minka. In old minka, the regularity of the post spacing was often strictly maintained, and this emphasis on structure could often be at the expense of practicality and function, resulting in posts that were ‘stranded’ outside of partition lines and in the middle of spaces. While these posts are often ‘in the way’ in practical terms, their existence indicates that they were either considered advisable from a structural standpoint, or unavoidable from an economic one, if roof beams capable of spanning greater distances were not available or affordable.
The roofs of many minka display a two-part structure, with an ‘upper roof’ (jо̄-ya 上屋) and ‘lower roof’ (ge-ya 下屋). The jо̄-ya is the main, inner structure, consisting of the sasu-gumi (叉首組) framing: the principal purlins (moya 母屋) and principal rafters (sasu 叉首) that support them. These members are carried on tall internal posts called jо̄-ya bashira (上屋柱). The ge-ya bashira (下屋柱) are the shorter posts that ring the perimeter of the building in the plane of the external wall, and carry the load of the common rafters (taruki 垂木) and the thick layer of thatch, but not the sasu, which terminate at the ends of the koya-bari at the top of the jо̄-ya bashira.
In many minka, the ge-ya bashira are relatively slender and placed at a pitch of half a ken to one ken, while the stouter jо̄-ya bashira are spaced at one ken.
The two rings of jо̄-ya and ge-ya posts are typically around half a ken or 909mm apart, forming a kind of perimeter colonnade which presents various design possibilities that were exploited in later periods, by using the space to form raised verandahs (engawa 縁側), built-in closets (oshi-ire 押入), alcoves, etc.
If the eaves are particularly deep, the ends of the rafters might be supported with a further ring of free-standing external posts, as seen on the left-hand side of the section below.
Post layout plan of the Furui residence showing the inner ring of jо̄-ya posts at one ken spacing, the outer ring of ge-ya posts at both half ken and one ken spacing, posts at internal partitions, and one free-standing post in the centre of the niwa (にわ).
Photograph of the Furui residence showing the free-standing post in the middle of the niwa.
Transverse section through a minka showing the inner taller jо̄-ya posts supporting the sasu-gumi (note the sasu with pointed ends terminating at the junction between the koya-gumi and jо̄-ya bashira) and the outer shorter ge-ya posts at the external walls supporting only the rafters and roof covering.