The Roman Baths weren’t built in a day... so how did they do it?

Whilst preparing for National Science and Engineering Week at the Baths, we looked at how the Romans solved five building problems to achieve their bath-house.

The first problem was the marshy ground around the spring. This land was too soft to support the weight of a building. The solution was to pound many tall wooden logs, called ‘piles’, into the mud. These made a strong base for the massive bathstone walls which they used to form the reservoir around the spring.

The excavation of the Scared Spring in 1979.  The black spots are the tops of the Roman piles

The second problem was how to control the water which rose in the spring. The engineers wanted to use it in the Baths and then let it flow down the main drain. They built a low sluice gate in the wall of the reservoir. When the gate was pushed down, the water level would rise and flow down a box drain into the Great Bath. When the gate was lifted, the water rushed through the gap at the bottom straight into the main drain. The water level became too low to flow into the Great Bath, which could then be emptied and cleaned.

The third problem was how to lift the massive stone blocks used in the buildings. The Roman engineers chose to use a five-piece lewis-bolt. Three specially-shaped pieces of the bolt were inserted into a fan-shaped cavity in the stone, and they and a loop handle were held together by a pin. This handle allowed the stones to be lifted with a hook and pulley system.

The Lewis Bolt activity at the Roman Baths

The fourth problem came from the smoke from the hypocaust fires. The smoke would rise through any cracks in a stone floor and enter the warmed rooms above. The Roman solution was to mix and lay thick concrete floors on top of supporting pillars and tiles. The concrete solved the problem as it was both fireproof and made without cracks or joins.

The fifth problem came with the Roman’s liking for high, wide roofs and openings. These needed to be strong enough to support themselves and any wall built above them. The engineers used a semi-circular arch for these places as they knew that this shape spreads the weight sideways and downwards onto strong points on each side.

The Roman arch over the main drain in the Roman Baths

When you visit the Roman Baths you can see these solutions to their building problems. Keep an engineering eye out outside the Baths for these solutions which have been used in buildings and rivers since the Romans.

Nicola Pullan is a visiting researcher from the University of Sydney.