The largest free form concrete shell in Australia roofs the residence of architect Gene van Grecken who designed his home to fit into the steep rocks overlooking the Pacific Ocean and Pittwater at Bayview. The structure resembles a mushroom and is reminiscent of the natural cave dwellings of early man. Mr. van Greken chose concrete because he felt that it was best adapted to the unusual design of the house. Floor slabs, curved columns and the shell were all cast of concrete on the site. The roof structure consists of a free form thin shell dome conforming to a series of spherical surfaces tangential at the junction. Free edge stresses are resisted by an anchor ring reinforced and prestressed along the perimeter of the shell. The shell edge has been thickened to accommodate edge disturbances while the remainder of the shell has been designed to resist membrane stresses. Edge movements during prestressing were allowed for by providing bearing on neoprene pads at the columns. The roof is 83 feet in diameter and has a radius rise of 10 feet. It is 1 and one-half inches thick except at the ring anchor and entrance arches were it is thicker.