The post-tensioned concrete roof of the Ruth Fisher School near Tonopah, Arizona, is not finished with any roofing material. The concrete was simply left as cast. A conventional 4000-psi mix was used, but to keep cracks from forming and to keep cracks that do form watertight, the amount of post-tensioning force was increased. Instead of being stressed to the normal 140 to 150 psi, the school roof was post-tensioned to 280 to 300 psi.
To produce an R-value of 24, the 7-inch-thick concrete roof was cast in place over a 3-inch-thick layer of expanded polystyrene. The polystyrene was prefabricated in panels with 5/8-inch-thick gypsum wallboard glued to the underside. The panels were set on 3/4-inch-thick plywood forms with the polystyrene face up. Concrete was placed directly on the polystyrene. The cement paste permeated into the top surface of the polystyrene, bonding the concrete and polystyrene together. Mechanical fasteners were not used. Reinforcement, fire sprinkling pipes and post-tensioning and electrical conduit were supported by rebar chairs with square metal bases that prevented the chairs from sinking into the polystyrene.
All finishing was done with a wood float; a steel trowel was not used, because the hard burnished surface that steel troweling produces accelerates drying. After four days of curing, when the concrete had reached 60 percent of its ultimate strength, the concrete was post-tensioned. A concrete roof was used on the Ruth Fisher School because of concrete's temperature moderating properties. According to the architect, the concrete and polystyrene provide a 12-hour thermal lag between outside and inside temperatures. In addition to the energy savings, the architect believes his clients save on roofing maintenance and periodic reroofing costs. Also, he believes the post-tensioned roof is an ideal platform for solar collectors no roofing material to be punctured by collector supports or damaged by foot traffic.