Four of the rectangular hyperbolic paraboloidal surfaces may be arranged so that the outer edge of the shell is level and the low point is at the center where it is supported by a column. It is necessary to provide drainage for rain at the low point through a pipe in the column. A row of these units may be placed side by side and tilted so that a clerestory is formed between the rows, or a skylight may be provided by leaving a space between each unit. Individual glass blocks are sometimes placed in the shell to provide lighting. These shells may be diamond shaped in plan rather than rectangular.