The metal roof on my fairly new home (4 years old) is leaking. The leaks are occuring beside two large dormers. The roof was installed by a building contractor, not a roofing contractor, although he assured me he had installed metal roofs in the past with no problems. The roof itself is constructed with 16" I-Joists 12" of insulation, he then strapped the roof with 1X4, then applied the metal roofing with roofing screws. The valleys have sheet metal extending 15" up either side, and he used flashing where the dormer walls met the roof. My concern is that he did not sheath the roof or even use tar paper under the metal roofing. When I questioned him on this he told me that with the 12" insulation and 4" airspace I would not have any problems. Is roofing with metal, with no sheathing or tar paper a common or approved method . I tried to find installation methods on the net. with no success. Any help would be appreciated. Thanks.