HYPOCAUST, in architecture, an open space below a floor to allow the passage of hot air and smoke, in order to heat the room above. This type of heating was developed to a high degree by the Romans who used it, not only in the warm and hot rooms of the baths (q.v.), but also almost universally in private houses in the northern provinces. Many examples of such hypocausts exist in villa and house foundations in Roman centres in Ger many and England. Although the usual custom was to lead the smoke from a hypocaust into a single vertical flue through which it escaped into the open air, where greater warmth was desired, several flues would lead up from the hypocaust in the side walls of the room ; at times, these wall flues consisted of hollow oblong tiles, set close together, entirely around the room. The usual construction of a hypocaust consisted of a layer of tiles, 2 ft. square, laid continuously in a bed of concrete for the bottom surface, piers approximately 8 in. square and about 2 ft. apart as the supports, and a floor above of concrete or of large square tiles supporting a bed of concrete, on which the finished floor of marble, tesserae or mosaic was laid.