JAPANESE MINKA XXIV - UDATSU POSTS

An udatsu post (udatsu bashira うだつ柱) is a ‘through-post’ that runs from the ground (or ground sill) all the way up to the apex of the roof, thus transferring the load of the roof directly from the ridge pole to the ground. The word udatsu, or alternatively udachi, is an ancient one. Generally it is written with the character 梲; in manyо̄gana (万葉仮名, the ancient writing system developed from around the 5th century AD to represent native Japanese words phonetically by using Chinese characters) it is written 宇太知. Whether udatsu/udachi is an ancient Japanese word that pre-dated Chinese contact and was only later assigned kanji, or a word of Chinese origin that accompanied the arrival of continental architecture into Japan, is unclear. The etymology points to the word originally referring to a short post (tsuka-bashira 束柱) that stands on a roof beam and runs to the ridge pole (in English known as a king post if in tension or a crown post if in compression) or under-purlin (a queen post).

An ancient haniwa (terracotta figures made from the 3rd - 6th century AD) of a building, with a stout udatsu supporting the ridge.

Many minka, whether with thatched or plank/board clad roofs, used a construction method known as ‘rafter construction’ (taruki kо̄zо̄ 棰構造), where only posts supporting the ridge are used; stout rafters (taruki, 棰 or 垂木) span from the ridge to the wall beams/plates, without intermediate support from under-purlins on queen posts. As a result, in most cases tsuka-bashira/udatsu referred to a ridge-supporting post, and eventually udatsu came to refer to such posts exclusively.

In Japan the grandest udatsu bashira are found in the gable-roofed (kiri-zuma zukuri 切妻造り) minka of the Kо̄shū (甲 州) region, old Kai Province, now Yamanashi Prefecture.

Diagram of a gabled roof. The hafu (破風) part of the gable wall (tsuma 妻) is highlighted in blue.

Often the voluminous roof spaces of these minka are used for sericulture and are divided up into two or three levels. The locals call the tall, large-section posts that run uninterrupted from their foundation stones to the ridge, penetrating each level along the way, udatsu. These posts are used both in the plane of the gable wall and at internal locations; the former are visible from the exterior, and because of their location at the gable end, are called happо̄-udatsu, happо̄ being a common alternate name for hafu.

An interior view of the third storey of a three-storey minka, showing two interior udatsu-bashira running up from the floor below and supporting the ridge pole above.

A gable-roofed building in Yamanashi Prefecture with a slender happо̄-udatsu post running from the ground to the ridge, bisecting the central openings on the upper floors. The beams are tenoned and wedged into the post.

JAPANESE MINKA XXIII - FORKED POSTS

Posts made with forked trunks that divide naturally into two branches at their upper end are called in Japanese mata-ki bashira (股木柱) lit. ‘crotch-tree-post’. Sitting a ridge pole or wall beam/plate in the ‘crotch’ and lashing it to the post is among the simplest and most reliable methods of joining these two members, and requires little to no technical skill to construct, so the technique is often seen in utility structures such as storehouses and charcoal huts where more refined methods are either not required or not achievable. No doubt mata-ki were also prized by the builders of the pit-dwellings of prehistoric Japan, but no physical remains of these timber and thatch dwellings have survived.

Mata-gi bashira supporting the ridge pole on a primitive open-walled structure.

When mata-ki bashira are found in minka, their incorporation into the structure is often more sophisticated, giving the impression that the builders relished the design challenges and aesthetic possibilities these irregular timbers presented. In the doma of the Takano House in Shioyama, Yamagata Prefecture, designated an important cultural property, there is a 60cm diameter chestnut post whose upper end is left in its natural, forked state. The posts passes right through the second storey to bear the floor beams of the the upper second storey; not only the crotch of the post but also the legs support these beams, with the beam supported by the legs running perpendicular to the beam in the crotch. These upper floors, used to dry hay and medicinal herbs, storage, and sericulture, are examples of ‘zara-ban-yuka’, lit. ‘rough board floor’, i.e. thin slat-like floorboards laid with generous gaps between them to facilitate ventilation.

The massive mata-gi bashira in the Takano House

In the Hirakawa house in Maruoka, Fukuoka Prefecture, the mata-ki bashira are used at the four corners of the jou-ya and ge-ya, with each post carrying loads in two directions, from both jōya and geya wall beams. Even if the ‘legs’ of the post are cut off level at the ‘crotch’, the ‘flare’ of the timber remains, and provides a wider bearing surface to accommodate multiple horizontal members.

Mata-gi bashira in the Hirakawa House.

This method avoids the need for complicated joints at the corners, and is sturdy and structurally satisfactory, with the caveat that because a moment (torque) is induced in the eccentrically loaded post there is a tendency over time for the foot of the post to twist. This is an issue where posts are tenoned into ground sills, i.e. rotationally constrained, but was not a concern in minka which employed the horidate-bashira method of setting posts directly into the ground (a method which was still common even in the early 19th century in more isolated rural areas), as the base of the post is free to rotate in its hole.

Another mata-gi bashira in the Hirakawa House. The front ‘leg’ seems to have been cut off at the crotch and supports a large transverse beam. The rear leg remains and supports a smaller beam at a higher level. Note also the large natural concavity on the left-hand side of the post

Mata-gi-bashira are often seen in the minka of the Hokuriku region and vicinity, where they attracted some colourful local names befitting their unusual appearance. More generally they were called dōshiyō-bashira, which might be translated as ‘either way post’. In old Etchū province, modern day Toyama Prefecture, they were called ‘rabbit posts’ usagi-bashira, due to the resemblance of the forked end to a rabbit’s ears; in the northern part of old Hida Province, now northern Gifu Prefecture, they were called ‘peek posts’, by association with the act of ‘crotch peeking’ i.e. bending over at the waist and looking backwards and upside-down through one’s own legs.

JAPANESE MINKA XXII - POST MODULES

One point of difference between modern houses and traditional minka is that in most old minka there is no standard size for posts. Even within a single minka every post is unique in its dimensions, even if only slightly, because each was hand-worked and finished from hand-selected timbers, and posts of appropriate dimensions were selected for every location, depending on the size of the load, the span between partitions, and other considerations. This is in contrast to today’s machine-milled posts, which are all essentially ‘fungible’ and of uniform dimensions, typically 90 x 90 or 120 x 120mm.

Nor was the customary post pitch (spacing) of one ken (間) standard throughout time and place. The modern ken is defined as 1,818mm. In older minka, pitches greater than this were relatively common. In regional terms, the difference can be broadly classified into the jōhō-ryū (上方流) or ‘jōhō style’ (Jōhō is an old name for the western region of Honshū centred around the old capital, Kyōto) which employed a larger ken, and the kantō-ryu (関東流), the ‘Kanto style’ (Kantō being the eastern region of Honshū centred around Tōkyō, formerly Edo), which used a smaller ken.

This regional difference persists in modern construction. Today there are three main module systems still in use in Japan: the kyō-ma 京間 or ‘Kyoto ken’ of 1,910mm plus post width, the chū-kyō-ma 中京間 or ‘reduced Kyoto ken’ of 1,820mm plus post width, and the edo-ma 江戸間 ‘Edo ken’ a.k.a inaka-ma 田舎間 ‘country ken’ of 1,820mm. That the chū-kyō-ma and edo-ma seem at first glance to be identical is explained by the fact that the kyō-ma and chū-kyō-ma are ‘double module’ systems, called in Japanese ūchi-hō-sei (内法制, lit. ‘inner system’) or tatami-wari (畳割り, lit. ‘tatami division’), where the first module is measured from inner post face to inner post face, and the second module is the dimension of the post, while the edo-ma is a ‘single module’ system, called in Japanese shin-shin-sei (心々制, lit. ‘heart-heart system’) or hashira-wari (柱割り, lit. ‘post division’), where the ken is measured from post centre to post centre.

As the name tatami-wari suggests, the use of a double module arises from prioritising the uniformity of tatami mat sizes over that of post spacing, and allows for a standardised tatami mat of 1,910mm x 955mm under the kyō-ma system, or 1,820mm x 910mm in the chū-kyō-ma system. In contrast, in a centre-to-centre single module system such as the edo-ma, tatami mats are sized to fit around the fixed post module. Remember that in minka the floor covering does not go from skirting board to skirting board between walls, or flow uninterrupted from room to room at openings, as it does in a western house with floorboards or carpet; there is either a visible ‘tatami stop’ (tatami-yose, 畳寄せ) at the base of the half-timbered walls, or a sill (shikii 敷居) at openings, marking the threshold between rooms, with shallow tracks cut into it to receive the sliding room partitions. Both tatami-yose and shikii are generally of the same width as the posts, i.e. their vertical faces are flush with the faces of the posts, and their upper horizontal surfaces are at finished floor level, i.e. flush with the surface of the tatami.

A renovated minka showing the tracks cut into the floor beams or bearers (ōbiki) and sills (shikii) to take the sliding room partitions, now mostly removed.

Image showing the shikii threshold between two tatami rooms and the tatami-yose at the base of the half-timbered walls.

As an example, consider a room of one ken square. In the edo-ma system, the post spacing in each direction is 1,820mm centre-to-centre. Assuming a post dimension, and therefore a sill width, of 90mm, this means that in a two-mat room, i.e. a room that is 1 ken square, the tatami mat length must be 1,820 - 90 = 1,730mm to fit between the posts, and the width 1,730/2 = 865mm. If 120mm posts are used, this centre-to-centre dimension remains unchanged at 1,820mm, but the tatami mats will now need to be 1,820 - 120 = 1,700mm in length and 1,700/2 = 750mm in width.

A 1 x 1 ken, two-mat room with 90 x 90mm posts on the edo-ma or inaka-ma module.

In a room that is 2 ken square, the corner post spacing in the edo-ma system is simply 1,820 x 2 = 3,640mm centre-to-centre. In this case, the tatami mat length must be (3,640 - 90)/2 = 1,775mm, and the width 887.5mm.

A 2 x 2 ken, eight-mat room with 90 x 90mm posts on the edo-ma or inaka-ma module.

Using the double module chū-kyō-ma system, the tatami mat dimensions are fixed at 1,820 x 910mm regardless of the size of the room, and the posts and sills are fit around them. So in a 1 x 1 ken, two-mat room, the actual post centre-to-centre dimension is the length of the mat (1,820mm) plus half the post dimension at either side, i.e. 1,820 + (2 x 45) = 1,910mm. If 120mm posts are used, the centre-to-centre dimension will become 1,820 + (2 x 60) = 1,940mm.

A 1 x 1 ken, two-mat room with 90 x 90mm posts on the chū-kyō-ma module.

In a 2 ken square, eight-mat room built on the chū-kyō-ma module, with 90mm posts, the corner post spacing is (2 x 1,820) = 3,640mm between internal post faces and (2 x 1,820) + 90 = 3,730mm centre-to-centre.

A 2 x 2 ken, eight-mat room with 90 x 90mm posts on the chū-kyō-ma module.

There have been other regional variations on the length of the primary construction module that have not survived into modern times, among them the 1,760mm + post width double module kaga-ma (加賀間) of the old Kaga province, modern-day Ishikawa Prefecture, in the Hokuriku region of north-central Japan. In general, the regions where tatami mats earliest appeared (from the Kinki region westwards) used the tatami-wari system; in regions where sericulture was practised and other areas where board floors remained common (from the Chūbu region eastwards), the hashira-wari system often predominated. There are also instances of the two different systems being used within the same minka: the double module system in the raised-floor living area of the dwelling, and the single-module in the earthen-floored doma utility area. In the famous silkworm-rearing village of Shirakawa in old Hida Province (now northern Gifu Prefecture), many of the minka have posts in their tatami-floored spaces with a centre-to-centre pitch of 1,850mm, while the board-floored and doma areas of the dwellings show pitches of 1,970mm and 2,120mm.

JAPANESE MINKA XXI - POST ARRANGEMENT

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.

JAPANESE MINKA XX - POSTS

Defined functionally, a post is a slender, vertical structural member that, in a single storey building under normal conditions, transfers loads from the roof down into the ground via the foundation. The difference between a post and a column is not strictly defined, but ‘post’ is typically used to refer to relatively small-section timber, and sometimes steel, members, and ‘column’ to stone, steel, concrete, and large-section timber members, particularly in a classical context. In Japanese, all of the above are conveniently called hashira (柱).

Timber columns (with entasis) at Hōryū-ji, Nara Prefecture, 8th century.

Timber posts in a modern Japanese post-and-beam house under construction. The vertical members are temporary bracing.

A massive 240mm square hinoki (Japanese cypress) post (or column?) in a new traditional-style house.

Previous entries in this series have considered posts in minka only in relation to foundations, and in particular to the three different ways the load transfer from post to ground is achieved: setting the posts straight into the ground (hori-date 堀立て), setting them on foundation stones (ishiba-date 石場建て), or using a groundsill (土台敷き). This and the next few entries will focus on posts in their own right.

In modern Japan, posts are almost always made from either Japanese cedar (sugi, 杉, Cryptomeria japonica) or Japanese cypress (hinoki, 桧/檜, Chamaecyparis obtusa). Both species produce timber that is straight-grained, strong, and relatively soft and easy to work. In the feudal period, sumptuary laws in many parts of this country restricted the use of these timbers to the upper classes, so commoners made use of a much larger variety of species, including conifers like black pine (kuro-matsu, 黒松, Pinus thunbergii), red pine (aka-matsu, 赤松, Pinus densiflora), and hemlock (tsuga, 栂, Tsuga sieboldii), and hardwoods like the lacquer tree (urushi, 漆, Toxicodendron vernicifluum) and zelkova (keyaki, 欅, Zelkova serrata).

In the centuries before finer woodworking tools had been developed, and even in much later times in isolated mountain villages where these tools were not available, posts in minka were only minimally worked with an adze to give them reasonably flat faces. Because of the difficulty of working some species, and the poor structural characteristics of others, posts were typically far larger in section than is seen today. Sometimes bent and even forked timbers were used, giving these posts the appearance of standing trees; naturally the principle was to keep the orientation the same, i.e. with the crown end up and the root end down.

Walls in post-and-beam structures, including minka, consist of a ‘skeleton’ of loadbearing vertical and horizontal linear elements, with the spaces between them infilled or covered with largely non-structural material such as wattle and daub and timber cladding; this is opposed to ‘planar’ structures such as loadbearing masonry (and arguably also timber stud-wall), where the structural element is the whole monolithic wall itself. The Japanese term jiku-gumi (軸組), literally ‘axial assembly’ but perhaps best translated as ‘wall framing,’ refers to the assembly consisting of all the individual structural elements contained within the vertical plane of the wall: posts, tied together at their bases with ground sills, and at their heads with wall or perimeter beams, and along their length with horizontal penetrating ties (nuki, 貫) at roughly one metre centres, wedged into through-mortises in the posts. Where there are openings, this basic structural assembly is augmented by the use of lintels and sills.

A traditional Japanese ‘half-timbered’ wall under construction, showing posts, ground sills or ground beams, wall beams, three intermediate penetrating ties, and an infill of split bamboo wattle, before the application of the daub.

Close-up view of penetrating ties (nuki) wedged into through-mortises in the posts.

An obvious difference between traditional Japanese and European post-and-beam framing is that Japanese buildings did not employ the principle of the truss or quasi-truss; that is to say, they contained no ‘triangulating’ diagonal members (sujikai, 筋違) in the wall plane to brace the structure against lateral loads from wind and earthquake; instead, these forces were taken up entirely by the closely-fitted timber joints and tightly wedged penetrating ties.

A European half-timbered building, showing diagonal bracing and ‘quasi-truss’ elements.

A Japanese half-timbered temple building with no diagonal bracing elements.

There are obviously limits to what such a structural system can withstand, but up to a point it was very effective in absorbing the energy of lateral loads via the mechanism of local deformation (crushing) of the timber at the joints and tie penetrations; the surviving structure could then simply be re-plumbed and re-trued and the wedges driven further in to remove the deformation-induced play. The effectiveness of this system is heavily dependent on the high level of precision and accuracy in the joinery, which is only made possible by the use of fine saws and chisels; similar resistance to lateral loads could not be expected of earlier or more primitive minka with their crude adze-cut joints.