The first objects of this study is to find out the actual conditions of dimensions of joint layout and sizes of siding boards and to grasp their relations. The second one is to simulate ratios of siding boards utility (RSBU) and investigate influencial factors. The third one is to study on primary idea of designing joint layout for increasing RSBU in a detached house. The considerations and the conclusions; 1. From the result of simulation of 4 cases in RSBU, The main reasons of the low utilization rate of material is due to the use of smaller size's siding boards. 2. To increase the RSBU, Using siding boards of various sizes at a same time on a same house is effective. 3. Sizes of siding boards used on exterior wall, which are smaller than "4Q" (Q=455mm) can not be explained by the factor of dimensions of joint layout.
