Concrete dome roofs for large water reservoirs have several advantages over flat roofs. Dome roofs require no columns inside the structure and no built-up or membrane roofing material which demands regular maintenance. Little reinforcing steel is needed for prestressed concrete domes, and, in fact, little concrete is needed to carry out the spatial design. Prefabricated concrete elements temporarily supported on steel trusses were used in Nigeria to facilitate construction of water reservoirs 130 to 170 feet in diameter.