spear point stop-splayed cogged lap joint with folding wedges

The undersquinted cogged lap joint with folding wedges is a longitudinal I-joint where two members interlock through a hook-shaped lap connection and are secured against displacement by driven wedges. The joint combines form-fitting hook surfaces with wedge reinforcement, creating a tension-resistant connection that prevents longitudinal displacement and transmits both compressive and tensile forces.
This joint belongs to the lap joint family while simultaneously exhibiting characteristics of notched joints. It is related to the simple straight cogged lap joint and the angled cogged lap joint. In European carpentry this connection is known as a wedged cogged lap joint, while in Japanese timber construction similar principles can be found in the wari-kusabi connection. The characteristic geometry with undersquinted cuts distinguishes it from other cogged lap forms and enables reliable force transmission through wedge reinforcement.
The geometry consists of two hook-shaped interlocking lap surfaces, each cutting approximately one-third to half of the member thickness. The hook surfaces run obliquely to the member axis, achieving improved interlocking. After assembly, slots are sawed or chiseled through both members, into which wooden wedges are driven obliquely. These folding wedges spread the connection and press the hook surfaces firmly together, creating a form-fitting and force-fitting bond.
The undersquinted cogged lap joint with wedges offers high tensile strength through the hook-shaped interlocking of the lap surfaces. The wedge reinforcement prevents the members from pulling apart and allows for re-tensioning in case of shrinkage. The connection can be executed without metallic fasteners and enables the extension of members when timber length is limited. However, the notching significantly reduces the load-bearing capacity of both members. The connection is labor-intensive to manufacture and requires precise fitting accuracy. Improper wedge setting can lead to splitting of the members.
Traditionally, this joint is used in structural carpentry for beams, purlins, and rails in timber-frame construction. It is employed when members need to be extended and high tensile forces must be transferred. In wall, floor, and roof structures it is frequently encountered in historical buildings. The joint is also suitable for repairing damaged beams, as it allows the replacement of partial sections without metallic fasteners. In furniture making it is used for solid frame constructions.
Fabrication is carried out by precisely marking the hook-shaped lap surfaces with their undersquinted cuts, accurately sawing the hook contours, and cleanly chiseling out the material. The two members are assembled and subsequently the wedge slots are sawed. The wooden wedges are fitted precisely and driven in obliquely, additionally tensioning the connection. The fitting accuracy of the hook surfaces and correct alignment of the wedges are crucial for the structural integrity and durability of the connection.