Tomobe – Shikii Replacement

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One of the changes we decided to make during our ongoing renovation was to replace some of the tatami mat floors with wood. As much as I like tatami, ours were quite old and definitely in need of repair, which can be pretty costly. I’m also a major allergy sufferer and after living in the house for the past year, I’m pretty sure the old tatami are contributing to my daily sneezing and runny nose. So concurrent with the wall work that I covered over the past few post, I also worked on the floor to prepare for laying down some solid wood cedar flooring.

Here’s the floor structure with the tatami and subfloor boards pulled up. The tightly spaced members are smaller floor joist called neda (根太). The neda rest on top of a larger floor joist running down the middle of the space and then terminate on a brace (nuki, 貫).

Here’s a closer look at another area showing the neda resting on a nuki. I found it pretty cool to see how the neda were put together from small poles of varying size. The upper face of each neda was hewn flat with an adze and then the height was equalized by using a combination of wedging and notching.

For the most part the original neda were totally fine, but their height was set such that the subfloor and tatami mats would make the final floor surface nearly flush with the bottom door tracks. The cedar floor boards are 30 mm thick which is about half the thickness of the tatami, so to maintain the original floor height I needed to raise the level of the neda. Neda for tatami are also spaced a bit wider than those used with a wood floor, so with the spacing and height issues in mind I decided to replace the neda with new material. So the old ones came out.

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Something else I needed to address before diving into the floor work was to replace the lower door tracks, called shikii (敷居), which were seriously worn. The floor boards will ultimately butt right up next to the shikii, so the shikii replacement needed to happen first.

The shikii are joined to the posts on either end using simple joinery, making them removeable and repairable. In the photo below you can see a shikii on the right that is joined to the post with a removeable spline that bridges matching grooves in both the post and shikii. On the front face of the post an adjacent shikii has already been removed showing a shallow mortise where the joining shikii mated with a stub tenon. Below that is a shallow groove cut for the old sub floor to nest into the post.

The joinery on the ends of the shikii solidy fix each end, but with those joints alone there’s nothing preventing the middle from flexing up and down. The builders of our house used a pretty cool connection to deal with that problem. In the photo below you can see a small vertical piece which joins the shikii using a sliding dovetail joint and extends down to the cross brace where it was joined with a peg. The peg seems to have dissappeared long ago but you can still see the hole.

Here’s a similar area with the peg still installed.

Here’s a look at the sliding dovetail end of the vertical tie piece.

It’s a pretty cool bit of joinery if you ask me, and I saw no reason to change it so I decided to do the same thing when replacing these pieces.

Removing the old shikii was a fairly straightforward process. Removall involves taking out the spline on one end, then lightly spreading the posts to allow the shikii to pivot upwards with the stub tenon end of being the pivot point, and the splined end being the free end. (There are pictures of installation below which detail the process in reverse).

With the old shikkii out it was interesting to look at the joinery and material a bit closer. The shikii below is red pine and the underside has a natural edge with remnants of the bark still on it.

Here’s a closer look at the corner of the shikii where the shoulder was cut to match a chamfer on the post. Nice attention to detail.

This is the location on the post where the above end of the shikii once sat. On the left corner of the post is the chamfer where that little extension was cut on the shoulder.

The opposite end of the shikii looks like this, with grooves designed to mate with splines. There’s also another sweet little mitered extention on the far right.

Here’s the mating location for that end of the shikii. I removed the horizontal spline, but the vertical spline is still attached to the post. The stub tenon on the opposite end, and the combination of splines on this end make for a pretty ingenious system allowing the shikii to be solidly fixed yet also completely removeable/repairable.

Here’s a closer look at the dovetail joint for the vertical tie piece.

The bottom of the shikii has a step cut to account for a slight difference in the height of the adjoining subfloor, and that step passes right through the location of the dovetail joint. So the shoulders of the dovetail were cut to account for the step. Pretty cool!

Now on to making the new shikii.

For material I used some nice clean knot free hinoki cypress. After milling the stock to size I used my groover to cut the grooves for the sliding doors. I forgot to take pictures while grooving out the shikii, but the piece below illustrates the overall process. (This is a special door track for a pocket door.)

After cutting the grooves I milled and glued in some hard maple inserts into the bottom of each groove. The grooves were initially cut deeper to account for the 3mm thick maple strips. Hardwood inserts help the door tracks to last much longer since they can withstand a lot more wear and tear from the sliding doors. The track below is nearly 4m long, and I used up pretty much every clamp I had to glue in the maple inserts.

The final step before cutting and fitting the tracks was to hand plane the visible faces of the shikii.

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Fitting shikii is a pretty cool process, which requires a bit of scribing using a Japanese square (sashigane, 差し金). Since the shikii needs to fit tightly between two posts each end has to be cut to seamlessly mate with it’s respective post, and the length needs to be dead on. Scribing is necessary because almost all posts will take on some movement over time, especially in the form of a slight amount of twisting. That means each end of the shikii needs a cut that is slightly off of square by a certain amount.

To get started, first you set the uncut shikii against both posts so that the ends are extending beyond each post. A crucial step is to ensure that the shikii and posts are parallel to one another, and it’s not a bad idea to clamp the shikii in place to prevent it from moving during scribing. Next holding the long edge of the square up to the edge of the shikii slide the square up against the post to determine which corner of the post it touches first. In the photo below you can see the far corner of the post is touching the square while the near corner has a gap. Holding the square in this way you can accurately translate the point where the square is touching the post onto the shikii, by marking off the square. The key is to translate the location on the post to the correct side of the shikii. In other words if the square touches the far side of the post, mark on the far side of the shikii, and vice versa.

Here’s a close-up of my mark after sliding the square out of the way.

The next step involves sliding the square so that the short end sits flush against the post. I intentionally hold the long edge of the square on the surface of the shikii and place a couple of marks along the length of the long edge. These marks will translate the post face, no matter what angle it’s sitting at, onto the shikii.

The opposite side is done exactly the same. First I checked which corner of the post was touching, and this time it was the near corner. You can see the post is out of square only slightly, but it’s enough to warrant going through this process.

Since the square was touching the near side of the post, I placed a light mark off the square on the near side of the shikii. (Ignore the pencil mark on the right).

Then once again I held the short edge of the square flush to the post and made a couple of marks down the long length of the square on the surface of the shikii. These marks are effectively giving you the angle of the post face.

Here’s the first mark….

… and farther down the second.

The farther apart the marks the more accurate your reading will be. And the purpose of only placing two marks versus drawing a long line is to avoid marking the surface of the finish planed shikii as much as possible. Afterwards I’ll erase the marks.

Unfortunately I didn’t take pictures of the next step but hopefully a quick description will suffice. Start by aligning the long edge of the square up to the marks that translated the angle of the post face. Holding the long edge along those marks, slide the square up to the mark that indicated the nearst corner of the post. Aligning all those marks should reproduce an accurate angle of cut along the short edge of the square. If you mark and cut carefully you should end up with a perfect match between the shikii and post.

Here’s the grooved end of the new shikii after cutting to my scribe line. Like the original I added the small mitered corner to fit the chamfer on the post. The other end of the shikii was cut similarly but with a stub tenon.

Now for installation. I used a small jack and piece of wood to lightly flex the posts open, just enough to allow the new shikii to fit into place. Then I inserted the stub tenon of the shikii into it’s mortise and began to slide the grooved end down into position.

A little more pressure on the jack and I could slide the shikii down further. This is a tricky process. You don’t want to spread the posts so much that they actually move position, instead you’re just kind of introducing a bit of bow into them. But you also need to flex them enough that the shikii can slide in without marring the surface of the post.

Here’s the underside of the grooved end just as it’s starting to mate with the vertical spline.

Stepping back here’s what the setup looks like. The left side is the stub tenon end, with the grooved end on the right. At this point the grooved end has to drop down just a bit more to fully seat into postition.

I should have taken a better close up of the final fit, but in the photo below the shikii has closed up nicely to the post, and you can just make out where the little mitered corners wraps around to mate with the post chamfer.

A few days later I did the same thing on an even longer shikii directly adjacent to the last one. The span between posts here was approx. 3.6m so I needed a longer shikii and longer jack setup. The right side of the shikii with the stub tenon is locked in place and I’m pivoting the whole thing down on the left, with just a bit more to go.

This long shikii engaged with a couple of tenons coming out of a short post in the middle of the span from below. A detail I copied from the original.

There’s lots more of this work in the future. All the door tracks in our house are pretty worn out after 120 years of use.

Thanks for reading.