An adaptation in lightweight concrete of the slab roof of coral used in the Bermuda Islands for protection against the hot tropical sun is becoming popular in Florida and other southern states. The lightweight version is made of vermiculite insulating concrete, cast in place monolithically in a series of offsets, or steps that slope from ridge to eaves. Since the insulation is on the outside of the construction, heat is blocked before it can enter the building thus reducing the cost of air conditioning. In addition, such roofs are highly wind-resistant surviving through storms as strong as hurricane Donna. The vermiculite topping also protects the life of asphalt and tar waterproofing materials by shielding them from the sun's heat and actinic rays.