 Hypocausts were used for heating hot baths (thermae), houses and other buildings, whether public or private.The floor was raised above the ground by pillars, called pilae stacks, with a layer of tiles, then a layer of concrete, then another of tiles on top; and spaces were left inside the walls so that hot air and smoke from the furnace would pass through these enclosed areas and out of flues in the roof, thereby heating but not polluting the interior of the room.In Sicily truss roofs presumably appeared as early as 550 BC.Their potential was fully realized in the Roman period, which saw trussed roofs over 30 wide spanning the rectangular spaces of monumental public buildings such as temples, basilicas, and later churches.