根継ぎ – Ne-tsugi

Using Japanese Joinery to Repair Rotted Posts

Years ago, when I first visited Japan, my wife and I went to a lot of different temples and shrines. Something that caught my eye was how many structures I saw that had been repaired using traditional joinery. It’s a common site to see, particularly on posts/columns. A skilled carpenter will cut out the rotted area of an old post, and splice in a piece of new material, using nothing but wooden joinery. In Japanese the word for this process is call “ne-tsugi” (根継ぎ) and a simple translation is “splicing the roots”. Below is some really amazing ne-tsugi work at the Edo-Tokyo Open Air Museum.

This past month I spent some time working with Somakosha again, and this time one of our jobs was to repair the structure on a 100+ year old house. For the most part the house was in really good condition for it’s age. However there were a few areas where the posts had rotted out pretty bad. So we spent a week or two doing some ne-tsugi, cutting out the rotted posts and splicing them with new material.

Below is a shot from some ne-tsugi work in progress. Not pictured here is the surrounding support structure that we always use to safely do this kind of work. Basically we use a series of temporary beams, posts, and jacks to lift the structure around the post we are going to repair. With the structure safely supported, we proceed by cutting out the rotted portion of the post, followed by some joinery work. In this can we spliced the posts with kanawa-tsugi joinery.

A kanawa-tsugi works by cutting the same joinery on each half of the joint. Then the two halves are slid together and locked with a wedge. The ends have a short, t-shaped, stub tenon that prevents the two halves of the joint from moving apart when assembled.

Kanawa-tsugi by thejoinery on Sketchfab

The guys at Somakosha like to use a variety of kanawa-tsugi that I haven’t seen very often. The stub tenons, which are usually visible are cut so that they are hidden when the joint is finished. Below you can see the mortise that I cut in this post to receive the tenon from the mating half of the joint. Usually the mortise would extend all the way to the surface.

And here is the stub tenon end of the joint.

The beauty of this type of kanawa-tsugi is that the finished joint has a really clean look. The stub tenons still prevent the two halves from twisting or moving apart. But unlike a standard kanawa-tsugi, the tenons are completely hidden.

Here’s a quick video showing a bit more of the process. We use a combination of hand and power tools to cut these joints. Power tools for quickly cutting out the waste areas, followed by hand tools to refine the fit.

For this site we repaired three posts using the method described above. It’s a satisfying feeling to be able to repair a building using this process. The final result not only restores the strength and integrity of the house, but is a beautiful mending of the past with the present.

Thanks for reading.