In last week’s post on ishiba-date (石場建て), the practice of using foundation stones (礎石, soseki) under timber posts, I noted that these stones are often river stones, used in their natural state without any working or dressing. If this is the case, then a problem immediately becomes apparent: how are the timber post and foundation stone to be accurately mated? Ideally, the full cross-section of the post end must bear fully on the foundation stone, for several reasons: to spread the load transmitted by the post to the stone to the maximum extent, to avoid any stress concentrations and possible localised crushing of the timber; to give the post maximum ‘grip’ on the stone to prevent any sideways movement; and to eliminate any gaps or depressions where water could enter, remain, and eventually rot out the post base. Given that timber is easier to work than stone, it makes sense to have the timber conform to the stone and not the other way around. Additionally, the work can be done by a carpenter, who is already required to build the house, whereas working stone requires a mason, i.e. bringing in an extra trade.

The task of matching the timber post end to the surface of the foundation stone might sound simple, but giving it some further thought makes it clear that it isn’t so straightforward. The post will sit on a convex section of the stone, so the post end must be made concave, and not in one but in two dimensions, i.e. a compound curve must be formed into the post end. The means by which this is achieved, with only a compass and templates, is quite ingenious.

First (B-1), a centre point is marked on the stone, two axis lines are drawn (Japanese carpenters traditionally use India ink for marking, not chalk or pencil) through this point at right angles to one another, and the ends of the axes are marked with the four cardinal points of north, south, east and west (1).

Then a template made of a thin veneer of hinoki (Japanese cypress, Chamaecyparis obtusa) is placed on the stone to align with one of the axis lines (2) and an inked compass is used to transfer the profile of the stone along this axis to the template (3).

The ‘transfer template’ is cut along the marked line to create a concave cutout (5). This concave template is then placed against another template (6) and the cut line is traced onto this ‘final template’, which is then cut (7), giving a convex cutout that describes the profile required through the centre of the post end, along one axis.

The whole process is then repeated for the other axis, giving two ‘final’ templates.

Next (B-2), the square post, with the centreline of each of its sides marked, is placed onto the foundation stone so that these centreline markings align with the two axis lines on the stone (1). A compass is used to mark the profile of the foundation stone at each face of the post onto these faces (2). The end of the post is cut square close to the profile lines (3), and a chisel is then used to remove the final material from the four faces up to the profile lines (4).

Then the first concave centreline template is placed against the post end and material is carefully chiselled out from the ‘interior’ of the post end until its profile along the relevant axis matches that of the template (5). The procedure is then repeated for the other axis using the other template (6).

Picture (7) shows a dowel inserted into holes drilled into the centrepoints of the foundation stone and post end. This is to accurately locate the post on the stone and to ensures that the post doesn’t shift off centre during construction; it likely wouldn’t be of much structural use in an earthquake.

The same procedure is used for round posts, except that four axis lines and four final templates are required instead of just two (B-3).