To prevent the interior of a building from being exposed to wind and rain when a roof is incomplete and construct a strong roof support structure.

To remodel the roof part of an existing building, the roof part on the support base 20 of the building is first dismantled for removal and then a lower roofing envelope plate 18 bent to form a roof surface for blocking wind and rain and projections 22 for increasing its strength is laid over the support base 20 so that the projections 22 constitute a sheathing roof board supporting base. Next, a sheathing roof board 26 is secured on the supporting base to construct the roof support structure formed with a plurality of hollow reinforcing spaces 23. A roofing material 38 is laid over the sheathing roof board 26 to construct the roof surface.