PURPOSE: To reduce the quantity of work at an elevated spot so as to improve construction workability by constituting a roof unit of a roof panel, from the back face side of which the roof face disposed on the upper face of a building structure is supported, and support truss parts integrally jointed in advance.

CONSTITUTION: A longitudinal frame member 9a, a lateral frame member 9b and an upper frame member 9c with a specified gradient are combined in approximately trapezoidal shape to form a support truss part 11. The roof face 6 is supported by the support truss part 11 from the back side of a roof panel 7 forming the roof face 6 and integrally jointed to complete a roof unit 10. The roof unit 10 is placed on the horizontal upper face of a building structure 1 and fixed. A roof can be thereby formed with little elevated spot work.