In the desert north of Tucson, HK Associates has built a 220-square-metre house around a single idea: a roof shaped like a wing. Named "white wing house", Casa Ala Blanca turns an inverted butterfly roof into shade, a 3,700-square-foot rainwater catchment and a mount for solar panels, all at once. The result is a net-zero home that opens north to the Tortolita Mountains while shielding itself from the western sun, its everyday needs met by a form that does several jobs at the same time.