PURPOSE: To produce units efficiently by dividing a grider upper-wall section into three upper-wall component parts, and by preassembling every upper-wall component part by each upper walls component, in a previous process for assembling a roof unit.

CONSTITUTION: With the respective sections of three upper-wall component parts 3a, 3b, 3c assembled previously in a previous process for assmebling a roof unit, a girder upper-wall section 3 is formed by using joint metal fittings and the like. After that, a side upper-wall section 2 assembled previously in the previous process in the same manner is fixed on the girder upper-wall section 3 by nailing and after that, the tip of the side upper-wall section 2 and the tip of structure joint material are connected to each other with a truss, and with various members, a roof surface 1 is formed and after that, plaster boards are fixed on the external side surfaces of the grider upper-wall section 3 and the side upper-wall section 2 by nailing.