Römerbad Broschüre

THE UNDERFLOOR HEATING-SYSTEM (HYPOCAUSTUM) – MODERN DOMESTIC TECHNOLOGY FOR MORE COMFORT 23
The wood ignited in the furnace-room (praefurnium) created hot smoke gases, which flowed through a flue 21 into the
hollow spaces beneath the floors of the hot (caldarium) and the warm bath (tepidarium). The hot air was thereby conveyed
upwards through hollow tiles (tubuli) or flat tiles with spacers fixed to the inside of the walls and thence escaped through the
chimney-pot into the open air. The whole heating-system lay invisible to the bathers beneath the floors and behind the
plastered surfaces of the walls. Neither smoke nor gases reached the bathing rooms.
“The floors of the hot baths are to be made as follows. First, the bottom is paved with tiles of a foot and a half (1 Roman
foot = 29·6 cm) inclining towards the furnace, so that if a ball be thrown into it, it will not remain therein, but roll back to the
mouth of the furnace; thus the flame will better spread out under the floor. Upon this, piers of eight inch bricks are raised,
at such a distance from each other, that tiles of two feet may form their covering. The piers are to be two feet in height, and
are to be laid in clay mixed with hair, on which the above-mentioned two feet tiles are placed, which carry the pavement”
Vitruvius, de architectura V, 10, 2, translation J. Gwilt [1826].
The hollow space of the hot bath consisted of a lower floor of large, flat tiles 21, while that of the neighbouring lower floor of
the warm bath was made of a coarse concrete (opus signinum). Tiled piers were made mostly from square or sometimes round
tiles. Set at regular intervals these carried the large floor-tiles above (suspensurae). In the case of the bath-house in Bonn the
recommendation by Vitruvius just cited was only partly followed. Instead of the roughly 0·6 m high piers, the interior space
measured some 1,2 m, i.e. the piers were built to double the normal height. This was presumably to optimise the effectiveness
of the heating 22. //