To provide a roof structure and a construction method thereof preventing the lowering of working efficiency while displaying excellent vapor permeability and waterproof property and specially watertight sealing performance around a nail shaft without changing any driving tool such as a needle used for fixing a roof underlayment, a hammer tacker or the like.

The roof structure includes a roof board, the roof underlayment and a roofer successively laid on the roof board, the roof underlayment has 0.3 of sliding friction coefficient with the roof board and the needle holding strength of 49.0 N (5.0 kgf), a plurality of roof underlayments are laid on the roof board in the direction toward a ridge from the edge of eaves of a roof, the ends at one side of the roof underlayments parallel to the direction toward the ridge from the edge of eaves of the roof are overlapped each other at a predetermined width so as to make the roof underlayment on the side of the edge of eaves as a lower layer, and the lower layer of the overlap is fixed to the roof board at intervals less than 200 mm with the needle.