3D model: Okkake-daisen-tsugi by thejoinery on Sketchfab
Assembly insights
Okkake Daisen Tsugi combines a dadoed rabbet with a stepped scarf, creating a joint that cannot be slipped together sideways. The piece with the inverted T-tenon must be inserted lengthwise, then locked with draw pins (komisen) or keys (shachi). The joint became feasible during the Edo period as carpenter's tools improved, and it remains a hallmark of structural joinery for traditional buildings.
Historical context
Okkake Daisen Tsugi (送掛大栓継ぎ) is one of the most secure lengthening joints in Japanese carpentry. The housed rabbet prevents lateral separation while the stepped scarf distributes shear across multiple shoulders. Edo-period advances in saw and chisel quality made this geometry practical at scale. It appears in foundation footings, wall plates, eave purlins, and sliding door tracks where failure is not an option.
Construction steps
- Establish centerlines on both timbers; lay out the dadoed rabbet and stepped scarf with a story stick.
- Cut the dado and rabbet on one member; shape the inverted T-tenon on the mating piece.
- Saw and chisel the stepped scarf profile on both halves to match.
- Insert the T-tenon piece lengthwise into the dadoed member; the joint cannot assemble sideways.
- Lock with draw pins (komisen) or keys (shachi); test-fit under load before final installation.
Where to see this joint in Japan
- Historic shrines and temples — Japan — wall plates and structural framing
- Restored farmhouses — Gifu and Nagano — foundation and eave work
Variations
Key type and shoulder count vary with timber dimensions. Some versions use a single large shachi key; others rely on multiple komisen pins. Related joints include Daiwa Tsugi (scalloped scarf) and Kanawa Tsugi (metal-ring scarf).
Applications
- Foundation footings
- Wall plates
- Eave purlins
- Sliding door tracks
Tools required
- Ryoba saw
- Nomi chisels
- Kanna plane
- Marking gauge
- Wood mallet