Continuing on with our ground-up (literally) examination of the structural systems of the minka, today I would like to build on the post-on-stone method covered in last week’s post to consider a later development: the dodai (土台), or ground sill.
The ground sill (or sill plate, sole plate or ground plate, as it is variously known in English) is the horizontal timber member that sits between the foundation (be that foundation stones, a stone or brick stem wall, etc.) and the posts, and transfers the load of the latter down into the former.
Image showing dodai resting on foundation stones below and supporting posts above.
While the practice of setting each post on its own foundation stone represented a significant improvement over planting the posts directly into the ground, it also has several disadvantages. For one, variability in the height of foundation stones means that the posts are not aligned at their bases and thus the posts will be of variable lengths; given the irregularity of the bearing surface of the stones, the posts are also difficult to set plumb; additionally, the open-grain post ends, though they aren’t in direct contact with the ground, still tend to draw moisture up from the stones, speeding their decay. By the use of a ground sill set on a line of foundation stones, post lengths can be made uniform, post bases can be simply cut square, and posts can be somewhat offset, i.e. placed at locations not directly over foundation stones, with the dodai acting essentially as a beam. While it is true that the dodai must still be worked somewhat so it sits level on the line of foundation stones, the degree of precision required in this isn’t as great as that needed when shaping posts to sit directly onto the stones.
The dodai also acts to tie all the posts together, thus forming a stronger overall structure. The post-to-dodai joint is formed by cutting a through-tenon known as a naga-hozo (長枘) into the post, and opening a corresponding mortise (hozo-ana, 枘穴) in the dodai; the joint can then be pinned with a timber peg (komi-sen, 込み栓), or, in more recent times, a shorter stub-tenon (tan-hozo, 短枘) and blind mortise joint may be used, reinforced by nailing a t-plate to the outside of the joint.
Three types of post-groundsill joint: on the left, a long nagahozo tenon and through-mortise; in the middle, the same but with the addition of a peg through the tenon; on the right, a stub tenon (tan-hozo) and reinforcing steel t-plate.
Close-up showing the peg (komi-sen) pinning the tenoned post into the mortised dodai.
The gap between the ground and dodai is sometimes filled in with smaller stones known as 差し石 sashi-ishi ‘insert stones’ or 覗き石 nozoki-ishi ‘peep stones’.
Sashi-ishi or nozoki-ishi used to infill the gap between the dodai and the ground.
In more recent eras and in more ‘upmarket’ townhouses and the like, foundation stones were replaced with dressed-stone strip foundations known as nunoishi (布石), which provided a continuous, flat support for the dodai.
A dodai (a) bearing on a continuous nuno-ishi stone dressed stone foundation (b).
In modern construction, the dodai rests on top of a reinforced concrete strip stem-wall which forms part of the foundation; stone, concrete, or plastic risers of 20mm or so are used between the foundation and dodai, both to protect the timber against rising damp, and to provide a ventilation gap to the subfloor space.
A modern dodai bearing on a reinforced concrete foundation, and between them a recent innovation: a continuous perforated synthetic strip serving dual purpose as both a damp-proof course and a ‘vent’ providing the necessary airflow to the subfloor space.
Given the proximity of the dodai to the ground, durable rot- and insect-resistant timbers are preferred, especially the cypress species hinoki (檜, Chamaecyparis obtusa) and hiba (檜葉,Thujopsis dolabrata, also known as asunaro アスナロ), heartwood of sugi (杉, Cryptomeria japonica), or Japanese chestnut kuri (栗, Castanea crenata).
One disadvantage of the dodai is that, when it does eventually rot out, it is more trouble to repair or replace than it is to simply cut the bases off individual posts and replace them, while leaving the rest of the post in place, as is done in the case of posts bearing directly on foundation stones. Perhaps because of this, the individual post-bearing foundation stone system survived in many places long after the advent of the dodai, with the addition of a tie (nuki 貫) threaded through mortises cut into the posts, tying them together and giving much of the structural stability of the dodai system without the dodai itself.
Image showing posts founded on individual foundation stones and structurally tied together by the addition of a nuki run through mortises in the posts. Note that the post in the foreground has had its rotten base cut off and replaced - a relatively simple operation which can be done without disturbing any of the other posts.