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.