Tomobe – Ne-tsugi

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….continuing on from round one of jacking up the house….

…what followed was more demo to open up the walls for some ne-tsugi (post repair using slicing joints) as well as some more foundation work.

Demo was also necessary because the earthen plaster walls had separated internally as a result of the sill beam rotting out. As the sill beams rotted and the posts began to sink lower and lower, the vertical nuki (bracing inside the wall) also bottomed out and began to flex under pressure. That flex caused the inside and outside layers of mud to separate. The result was a pretty floppy feeling wall. In the photo below Eric, a very generous helper, is removing some plywood panels which the previous owner had applied to the earthen wall as a method to shore things up… or so I suspect.

Underneath the thin plywood panels was a layer of black decorative plaster that was applied to the walls during an even earlier repair job. (It’s really interesting to dig into an 115 year old house. There literally are layers upon layers of work to sort through and puzzle together).

Below you can see the chigai-dana (違い棚) with the black layer of plaster removed, exposing the original naka-nuri (中塗り), or middle layer of plaster.

I’m far from an expert, but a simplistic explanation of typical Japanese earthen plaster walls involves three layers, starting with the rough layer, ara-kabe (荒壁); the middle layer, naka-nuri (中塗り); and a finish layer, shikkui (漆喰). In between all of that is a bamboo lattice work that serves as a structure for the initial ara-kabe layer to bind to. From there both sides of the wall are built out to whichever desired finish is required.

In the photo below you can see one of the walls we demoed as the bamboo lattice started to appear. The traces of white that you see are the last remnants of the shikkui lime plaster. One of the amazing features of Japanese plaster is that each layer is fairly distiguishable from one another.

Zooming in you can see some of the straw and palm fibers used as binders in the middle, naka-nuri, layer of plaster.

If you look closely at the photo below you can see all the different levels of Japanese earthen plaster. You’ve got the fully exposed bamboo lattice on the far left; the lower half of the middle wall section shows the rough ara-kabe, while the upper portion shows the naka-nuri; and on the right wall section you can again see ara-kabe below, with shikkui lime plaster above.

Here’s a closer look at the bamboo lattice, ara-kabe, and naka-nuri layers.

Eventually Eric and I removed all the earthen plaster, saving the naka-nuri and ara-kabe, since those layers can simply be rehydrated and reapplied (how amazing is that!). The raw ingredients are simply clay, sand and straw.

The chigai-dana (違い棚) was particularly interesting to see with the walls opened up.

With the earthen plaster demo complete, we finally started to prepare for some post repair, starting with a corner post which was the most badly damaged of the bunch. If you look closely you can see the overall setup which involved two jacks with temporary posts used to jack up and support the load of the building while we proceeded to work on the post.

Before actually cutting and splicing the post Eric and I first addressed some issues with the foundation stones (soseki 礎石) which had seriously disintigrated over time. The main foundation stones for our house are a type of sandstone. On the north side of the house where water and frost issues were more extreme, the stones literally crumbled over time, and in some cases were eroded down to fragments. I’m not entirely sure why the builders of the house chose sandstone. It’s something I’ve been researching and have yet to come up with an answer. In my own experience working with Somakosha, and seeing other old building foundation stones, granite seems to be the go to choice, which is dramatically different when it comes to erosion.

In any case, the stones on the north side of the house were pretty rough, so I decided to completely replace them. Replacement started with removing the bad stones and digging out a fairly deep hole….

…followed by a layer of larger stones and rubble, compacted with some smaller gravel.

Eric and I pounded and compacted several layers of gravel and stones to build up a solid base for a new foundation stone.

And eventually we got the new stone in. It doesn’t look like much in the photo below, but it’s a nice beefy chunk of granite, which will hopefully prove to be a much more durable and long lived soseki.

Here’s the next post down the line with another new granite stone, and compacted rock and gravel foundation.

And another. These stones were all just laying about on our property. Some of the square ones I cut to size with an angle grinder but the round ones were more or less ready to go. This type of foundation work is pretty new territory for me, but I did my best to research traditional methods, and adapted from there.

Now onto the main event, ne-tsugi, aka cutting and splicing the rotted posts. The first post I tackled was the corner post. The rot and termite damage was primarily isolated to the lower end, so I picked a location between the nuki roughly mid way up and layed out a kanawa-tsugi joint. With jacks and temp posts in place to support the building I began by cutting off the lower end of the post.

Cutting a kanawa-tsugi on a floating post is always a bit tricky. Your really have to contort yourself to get into a position where you can cleanly cut to the lines.

Here you can see a little more clearly the end grain of the post when I was working on trimming the stub tenon portion of the joint. I used a bit of water to soften the grain which made paring the old brittle sugi much easier and made for a cleaner result. It was nice to see how solid the wood was after cutting off the rotted portion.

Eventually I ended up with this. I used a combination of power and hand tools. Mainly a trimmer router and circular saw to rough things out, and hand planes and chisels to bring everything down to the line.

Below is the opposite half of the joint on a clean piece of hinoki which I cut in the shop. The trickiest part about kanawa joints is getting the layout dead on. Since both halves of the joint are layed out independently, accurate layout is a must. These aren’t like dovetails or similar joints where you can scribe one piece to the next.

Here’s a close up of the joint after a test fit. There were a couple areas where the stub tenons were bottoming out preventing the shoulders from fully closing, so I disassembled things and made some adjustments with a chisel.

Below is another look from later on once the shoulders started to come together better. You can also see that I left the new hinoki portion slightly oversized from the old sugi post. It’s pretty tricky to match new material to old, since even a post milled square will take on a different shape over time as the wood moves. So one way of dealing with that is to intentionally leave the new material slightly oversize.

Below you can see the first post repair complete, and I’ve started on the second kanawa-tsugi.

And here are the two post splices complete. I took out some of the bamboo lattice to give more access for doing the kanawa-tsugi joints, but soon replaced that in prep for reapplying mud to the walls. More on that next time.

Thanks for reading.