To provide a roof structure of a house and a construction method of a roof, wherein the roof structure can easily be constructed, prevents generation of dew condensation while keeping its heat insulation performance, and retaining strength of the roof structure.

Each roof rafter 10 consists of a rectangular part 11 whose cross section is a rectangle shape, and a flange part 15 which is provided below the rectangular part 11 and is wider than the rectangular part 11. A heat insulating member 20 consists of a fitting part 25 incorporated between the adjacent roof rafters 10, and protrusions 27 provided at the upper portions on both sides of the fitting part 25. The fitting part 25 is formed such that both the side surfaces thereof abut on the corresponding side surfaces 12 of the rectangular part 11 of the respective roof rafter 10 when they are incorporated. The protrusion 27 includes an extension sheet 28a which continues to an upper sheet 21 of the heat insulating member 20, and an extension heat insulation layer 28b which is bonded to the lower surface of the extension sheet 28a and is made of the same material as a heat insulation layer 23 of the fitting part 25. Further, the protrusion 27 is formed so as to rotate around a boundary part B of the upper sheet 21 as a rotational center and to abut on the upper sheet 21 when it is incorporated.