3D model: Hira-isuka-tsugi by thejoinery on Sketchfab
Assembly insights
Kone Tsugi teaches the fundamentals of lengthening joints. The interlocking shoulders provide mechanical strength while the angled cut increases gluing surface area. This joint bridges the gap between simple edge joints and complex temple-grade tsugite techniques. It is often a student's first mechanically locked scarf.
Historical context
Kone Tsugi (腰根継ぎ, root-waist splice) features interlocking shoulders that prevent the two halves from sliding apart—a step beyond glue-only Kiritsuke Tsugi. The name refers to the waist-like narrowing at the joint center. Carpenters use it for beam extensions in house framing and as preparation for advanced scarfs like Kanawa Tsugi and Okkake Daisen Tsugi.
Construction steps
- Mark the scarf angle and shoulder positions on both timbers using a marking gauge.
- Saw the angled scarf faces, then chisel the interlocking shoulders on each member.
- Cut the mating half to match; test-fit until shoulders seat without gaps.
- Apply adhesive if used; assemble and clamp along the full length.
- Optional: add a draw pin through the shoulders for structural applications.
Where to see this joint in Japan
- Traditional house framing — Rural Japan — beam extensions and repairs
- Woodworking schools — Japan — taught as an intermediate tsugite exercise
Variations
Shoulder count and scarf angle vary with timber size. Some versions add a through-key (shachi). For temple work, craftsmen advance to Kanawa Tsugi or Daiwa Tsugi when loads demand greater shear resistance.
Applications
- Beam extensions
- Long spans
- Repair work
Tools required
- Ryoba saw
- Chisels
- Kanna plane
- Marking gauge