PURPOSE: To moderate the temp. rise of an attic in the summer by providing an upper roof and lower roof with a confined space using a heat insulating film which is stretched over the undersurfaces of rafts, and allowing these spaces to be open to outdoors directly.

CONSTITUTION: At a ventilation gallery 18 in the middle folding part of a mansard roof 2, a vent forming piece 18c is provided between a lower roof mounting piece 18a and an upper roof mounting piece 18b. A heat insulating film 14 is furnished on the rear surfaces of an upper and a lower roof 3, 4. The ventilation of the attic is made from a ridge ventilation gallery 17 via ventilating spaces 15, 16 partitioned by the film 14, wherein the confined ventilating space 15 on the back of the upper roof 3 is open directly outdoors, so that the air even in the back-roof space 10A of the lower roof 4 is circulated via the space 15 in good performance in association with enhancement of the ventilating flow.