The purpose of this paper is to make it clear how workers decide design of joint layout of siding board on the exterior wall of a detached house. Questionnaires in reference of materials, considerations in deciding joint layout and the setting of joint layout in elevation models have been sent to 867 exterior wall workers. Main conclusions are the followings : 1) The more experience the workers have, the more frequently they consider drawings, conditions of construction site, outcome after completion, efficient utilization of siding boards, constructability, etc. together. 2)Theories about design of joint layout don't seem to apply much to the setting of joint layout by workers. 3) The setting of joint layout that workers believe to be efficient in utilizing siding boards, may not increase the ratios of boards' utility necessarily, and the layout believed to decrease the work-time improving constructability in most of the case.