T corner-post joint with open slot mortise and tenon

The Y corner-post joint with open slot mortise and tenon is a T-joint connection used to join posts with sills or wall plates in timber frame and roof construction. In this joint, an open slot is cut at the head of a post, into which a cross member engages with its through tenon. The distinctive feature of this connection is its Y-shaped geometry, where two members of equal dimensions meet at right angles.
This joint belongs to the tenon joint family and shows similarities to the Japanese Wanagi-komi and Sammai-gumi. In Western carpentry, it is also known as an "open slot mortise" and is frequently found at corner posts in sill constructions.
The geometry is characterized by a through tenon that extends from the end of one member through an open slot in the other member. The slot is open and extends downward from the head of the post. Unlike closed mortises, this design allows the tenon to be inserted simply from above. Characteristically, the entire cross-section of the inserted member functions as the tenon, making the connection particularly effective.
The primary advantage lies in the uniform load transfer without significant weakening of the inserted member. The connection is especially resistant to spiral grain twist. The through tenon also enables precise positioning and prevents displacement. The open slot form considerably facilitates assembly, as the members can be joined from above.
Disadvantages arise from the considerable weakening of the slotted post, which loses approximately one-third of its cross-section. With improper execution, there is a risk that the post may split at the slot location. The connection is susceptible to moisture ingress and requires careful constructive wood protection measures.
The joint is traditionally used in timber frame construction for connecting corner posts with groundsills. In roof construction, it finds application in connecting ridge posts with tie beams. Due to its high load-bearing capacity, it is particularly suitable for heavily loaded constructions in wall and roof structures. The connection requires no additional wooden fasteners such as wedges or dowels.
Fabrication involves precise marking and chiseling of the slot in the post as well as exact preparation of the through tenon. Fit accuracy is crucial for load-bearing capacity. For larger member dimensions, an additional central tenon (Wanagi tenon) can be incorporated into the center of the slot, engaging with a corresponding mortise in the inserted member to provide additional stability.