The Metrodome's air-supported roof was designed by the inventor of air-supported structures, David H. Geiger, through his New York-based Geiger Berger Associates, and manufactured and installed by Birdair Structures. An air-supported structure supported by positive air pressure, it required 250,000 ft³/min (120 m³/s) of air to keep it inflated. The air pressure was supplied by twenty fans. The roof was made of two layers: the outer layers were Teflon-coated fiberglass and the inner was a proprietary acoustical fabric. By design, the dead air space between the layers insulated the roof; in winter, warm air was blown into space between layers to help melt snow that had accumulated on top. At the time it was built, the of fabric made the roof the largest expanse ever done in that manner. The outside Teflon membrane was of an inch thick and the inner liner of woven fiberglass was of an inch thick. The entire roof weighed roughly . It reached , or about 16 stories, at its highest point.