Abstract: In a fabricationbased assembly workshop, the material flow network in the facility is obtained by merging the routings of all the parts and subassemblies into a precedence hierarchy known as an operations process chart (OPC) in literature, which inherently possesses a graph structure called a directed intree. This paper describes an algorithmic method for the design of workshop layouts that have such a hierarchical directed graph (digraph) representation of their material flows, including a tree manipulation algorithm and a sequence clustering algorithm to form layout modules. Its application in an industrial case study demonstrates the validation of this method.