Joint No 1: Sumitome hozo sashi

This is joint number one in Sebastian’s PROJECT MAYHEM. Sumitome hozo sashi is the connection made between the corners of the sill plates of a timber frame.

I just finished! Rushed to the computer! Here is my best effort to cut this joint. From layout to assembly it took five hours. My stock was nicely squared 3×3″. I wouldn’t want to have cut this on anything smaller, and you’ll see why when it comes time to make the haunched tenon.


The tenon I left a little long, partly as a result of layout, and partly because it can then be sawn flush after wedges are driven in. I didn’t make the cuts for the tenon wedges because I’d like to be able to take this apart.


I used my sashigane for almost all of the marking and measuring, including the miter angle. You could even mark the tapered tongue with a direct 1:12 or 1:16 ratio using the sashigane, maintaining the overall proportion of the joint regardless of the size of the stock. I found a really very fine tipped marker and it worked very nicely, always providing a consistent line width.


Here is the layout for the mortised sill plate.


Layout for the tenon. There’s actually one line missing where I extended the tapered tongue cut line on to the end grain.


Layout for the dovetailed tenon post.


Because these are timber framing joints one of the things I decided to try as part of the exercise was keeping the timber horizontal on the bench. No vertical clamping in a bench vise.  If this were an actual 20′ sill plate you’d have no choice but to make your cuts like this.


My paring slick. The chisel for this was a Harbor Freight buy, literally worth less than a dollar a chisel. Turned the handle from an old broom handle on the lathe. The chisel is only hardened at the tip! CHEAP! But it works, you know?


This is one of two Japanese chisels I own. The other is my 6mm mortise chisel. It really helps to have good steel when making wide paring cuts on end grain. The quality of the cut surface is a direct reflection of the steel.


The post tenon, cut and chamfered.


Now for the mortised sill plate. I started by chopping the through mortise from both sides. If this were a real timber frame I’d be hogging out the mortise waste with a brace and bit. I didn’t have the right size bit for this tenon though, so it was chopped with a 3/4″ bench chisel. The haunches will be cut later with a saw.


The dovetailed mortise for the post was cut to a depth such that 1/4″ of wood was left between the two mortises. Sokozarai-nomi for cleaning the bottom to depth (a bottom so clean you can show it to you grandma). I also made a small depth gauge for this.


Then I cut and pared the vertical haunch, as well as the tapered tongue and miter. All my cuts were made with a 210mm ryoba saw. If I was cutting 6×6 or 8×8 beams I think a 270mm ryoba would be quite perfect. I then added the layout for the haunch depth. It is marked out to be 1/16″ deeper than the haunch on the tenon, so marking on the angled tongue won’t make a huge difference.


The sides of the horizontal haunch could then be sawn with the ryoba. This is one of those places where I’m glad I’m not using small timber.


The finished mortise.


The sill plate with the tenon was more challenging. I cut it in a bunch of spots, starting with the tapered tongue and the inside edge of the tenon, as well as one of the tenon shoulder lines that form the haunch.


I don’t have a frame saw so I just chopped down from both sides to remove the waste.


With the internal surfaces pared flat and clean I extended the tenon cheek lines on the back surface with a marking gauge. I tried to mark over the knife lines with pen, but there’s very little room to get in there.


BAM! Here is why this joint is easier in big timber. Now my saw has enough room to finish the tenon cheek and shoulder cuts.


The layout could then be finished and the tenon chamfered.


The rest of the joint was mortised in. Seriously, I’m glad to have this chisel.


The finished tenon with two dimensional haunches.


This could be a beautiful corner to a timber frame work shop!


And if you want me to pick next week’s joint, how about this nifty scarf joint? It uses two wedges, one from either side, as well as pegs to lock together.

9 thoughts on “Joint No 1: Sumitome hozo sashi”

    1. Whoops….a question, sort of.

      When I did my layout, I cheated and made the end grain covering part/tongue thing, I did a straight tongue, no taper. I think that the tapered joint would be better, because it would be more likely to draw the joint together as the wedge is hammered in. In my version, the wedge actually caused the joint to separate, and also split the tenon ( mostly because I cut my joint so poorly that there was ROOM to split. Not a problem with yours.).

      Do you have any thoughts on the overall design?

      1. With the straight sides to the tongue it would be a very all or nothing thing, the fit that is. The taper seems more forgiving to me, though it could certainly still split if it was too tight. Also, it doesn’t come together until the joint is driven all the way home, so assembly would be smoother if the timber was really large.

        Also, if I was cutting the tenon for wedges I would cut for two wedges straight down the tenon. Getting the taper on the wedges you use correct is tricky, I’ve split half of the tenons I’ve tried to wedge. The needed taper for the wedges is almost nothing, and the bottom of the wedge can not be wider than the saw kerf. It helps to have a saw that makes a reasonably large kerf too, or else you’re cutting these thin little wedges that snap in half while you’re pounding them in.

        Do you have visions of a timber frame shop dancing in your head yet?

        1. Well……I would be ecstatic, to have anything right now, lashed bamboo and a tin roof would be great. Hell, even PVC and plastic would be fine, haha.

          It’s wet here!

    1. Yup, miter was directly from the saw. I still had to pare a bit to get it flat, but sawing directly to the line is the goal because it is always faster.

      These exercises should be both quite challenging and developmental in progressive difficulty. Its hard to know what to pick, and seriously, if you want to do a different one I’m all for it, but I added a photo to the post at the end of a fun scarf joint I’ve been wanting to try for ages. What do you think?

  1. Great job, Im considering this joint for our timber frame houses front porch sills…the normal pegged though mortise and tenon corner joint on Western timber frames is lacking…if exposed it checks horribly and in short order looks like ass…All of the main sill corner will be covered with sheathing so I don’t know if Ill go this extreme with them, but the porch sills will be exposed and this would be nice, or at least some kind of mitered connection.

    Any thoughts? It will be in White oak, so not as behaved as Pine…Kinda wish there were some dumbed down version of this joint, maybe I can make one….

    1. White oak sounds lovely! As far as my thoughts on this joint, I’d say go ahead and give it a shot, its not as hard as it might look. Interestingly enough this is one of the simpler variations of this joint, there’s others with more haunching on the tenon and mitered tenon shoulders. The whole point, given the small tenon size is to limit axial twisting forces, wouldn’t want that groundsill moving around on you. That said, it becomes a lot less of a concern if your groundsill is bolted to a foundation, but fun none the less.

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