Kanawa-tsugi: the long splice

A beam is never long enough when it needs to be, and old-growth timber is never as long as the old forest was. Japanese carpenters answered this with the splice joints, the tsugi family, and the most admired of them is kanawa-tsugi, the mortised scarf joint: two beam ends cut into interlocking stepped faces that slide together, lock, and look from the side like a straight grain line crossing the beam.

What the joint does

A plain scarf cut at a long angle would work if it could be clamped forever. The kanawa-tsugi replaces the clamp with geometry: each beam end carries a mortise and a tenon cut into the slanted face, so the two halves key into each other and cannot pull apart along the grain. Under bending, the long sloping faces share the load; under tension, the hidden tenon resists. A wedge or pin through the joint keeps it seated.

Why it is cut like a puzzle

The joint is genuinely difficult. Each face must be true on two axes at once, and the mortise-and-tenon inside the scarf must meet its counterpart within a fraction of a millimetre for the joint to close fully. It is usually cut on the ground, test-assembled, marked, adjusted, then taken apart and cut again. The payoff is a joint that needs no plate, no bolts and no glue to stay closed, and that can be opened again by driving it in reverse, which is exactly what makes it repairable.

When it is the right answer

The file's rule: use a splice like this when the joint must stay invisible and the beam is worth the labour, a visible beam in a room, a repair inside a historic frame, a sill that must carry on past a doorway. For hidden framing in new work, a bolted or plated splice is faster and the engineering is easier to check. The kanawa-tsugi is the choice when the joint itself is the evidence of care.

Inspection points

A sound splice shows a hairline seam with no daylight through the stepped faces, a wedge that is tight and proud on the driven side, and no gaps opening under load. A gap that grows with the seasons is telling you the faces never met fully, or that the wood is moving around a joint that was cut for a different moisture content.

The sawyer's mark

On old beams the joint sometimes still carries the pencil of the man who cut it: layout lines in sumi ink, faint but legible, marking the stepped faces before they were sawn. Those lines are part of the evidence of the craft, and restorers leave them in place; a splice cleaned up to look new loses the proof of how it was made, and the proof is the joint.

The card, filed

Kanawa-tsugi is the splice for a beam that must stay beautiful while it works: a puzzle cut in the grain that holds without hardware and opens for repair. It costs a day of careful cutting and gives back a century of honesty.