Design for Repair and Reuse, and Digital Fabrication of Traditional Japanese Timber Joinery: a Daimochi Tsugi case study

Construction, renovation and demolition activities generate 30-40% of global solid waste. Yet, buildings are rarely designed for repair and reuse. Traditional Japanese timber joinery offers a historically grounded counter-model. Daimochi tsugi is a shear-resistant load-bearing scarf joint used in the roof structures of temples, shrines and wagoya buildings since the Kamakura period (1185-1333). This work takes it as a case study to investigate how its geometric design embeds logic for repair and reuse, including structural continuity when a supporting column is temporarily removed. Experimental testing further reveals a gravity-locking behaviour: the roof load actively closes the joint, allowing it to recover and sustain additional load even after partial failure. To further understand how it can be fabricated in current practice, the joint is digitally modelled, analysed using finite element methods and fabricated at 1:1 scale by both traditional handcraft and by CNC milling. Structural testing shows handcrafted joints sustain ∼40% higher peak load than CNC counterparts, due to fabrication precision and contact quality. The study demonstrates how vernacular construction logic can inform contemporary design-for-deconstruction principles and circular construction practice, and shows how traditional joinery can be translated into machine-readable geometry for digital fabrication.