The Billingsgate Roman House and Baths consisted of two main buildings. Firstly, there was the domestic house, which we believe had three wings that formed a U-shape around a central courtyard. In the middle of 3rd century AD, a bathhouse was built in that courtyard, which connected with the north wing of the house. We use the term bathhouse in the Roman sense, as there was no actual plunge pool, or what we would consider a bath. There was a water supply at one end of the bathhouse which was just a simple cistern that supplied cold water. It was probably used for people to wash themselves down and maybe cool themselves off from the hot rooms.
The actual bathing was more of a sauna. This kind of bathing was a daily activity for Romans. People would first go to the cold room, or frigidarium, where they would undress, perhaps exercise and prepare themselves for the hotter rooms. After that they would have made their way to the hot rooms - first of all the warm room, the tepidarium, and then to the very hot room, the caldarium. These rooms were heated by underfloor heating, known as the hypocaust. It may have been that water was splashed on the floor, to create some steam but otherwise there was no running water. Once in the hot rooms the Romans would have had oil rubbed into their skin, sat with friends, family or by themselves. Once they had spent a good amount of time there, the oil would be scrapped off their skin with an instrument called a strigil.
In the Billingsgate bathhouse, the hypocaust was built to a fairly standard design, but would have been very effective. A furnace heated the two rooms, which was located immediately next to the caldarium. A slave would have fed this fire all day and the hot air was drawn under the floors into the hypocaust. The floors of the caldarium and tepidarium were suspended on stacks of tiles, known as pilae. These pilae were the usual bricks that Romans used to construct their houses, made of terracotta and produced locally. The stacks were about 3 ft high and held the corners of four larger pilae called bipedales, which measured two square Roman feet. A Roman foot was a little shorter than our 12 inches, measuring about 11.6 inches. On top of these bipedales, the floor surface was laid.
The hot air and smoke circulated in these spaces and was drawn up by box-flues in the walls, or under the floor and into hypocaust of the tepidarium. By the time the air reached this room, it would have cooled down, so the room was warm, not hot. There were further box flues in the walls that ran to roof level, which created the draw for the air-flow and allowed the heat to escape.
The Billingsgate Bath House is a great example of a simple hypocaust system. The remains are well preserved, so you can see how this system worked and how the pilae stacks supported the floor.
To gain more of an insight into Billingsgate Bathhouse and how it was built, see the videos below.