PURPOSE: To reduce the using quantity of mortar and lower a cost and shorten the time of filling work by recessing longer side joints only at both the end sections of the longer side upper-edges of ALC roof panels, and by forming shallow seal grooves at sections except the longer side joints.

CONSTITUTION: Only at both the end sections of the longer side upper edges of ALC roof panels 1, the longer joints 2 of the length of 540mm and the depth of 30-50mm are recessed, and at sections except the longer side joints 2 of the longer side upper-edges, the seal grooves 3 of the depth of 5-15mm are formed. Then, on the beams 4 of a structural body, a plurality of the ALC roof panels 1 are mounted so that the shorter side joints 5 of their shorter side end sections may be formed, and joint reinforcing-bars 7 inserted through bearing members 6 arranged on the beams 4 are arranged to float in the longer side joints 2. After that, the internal sections of the longer side joints 2 and the shorter side joints 5 are filled up with mortar 8, and the roof panels 1 are fitted on the beams 4, and the seal grooves 3 are filled up with sealing material 9, and air-tightness and water-tightness are kept.