A modular concrete building is made of a plurality of precast concrete panels which may be assembled to provide a building. The panel sections include lower frame panels or support members, wall panels, floor panels, and roof panels. The building may be assembled by securing the floor panels to the support members, securing the wall panels to the floor panels, and securing the roof panels to the wall panels and to adjacent roof panels. The roof panels are made of precast concrete and include stiffening ribs.