Though not a Roman invention¹, the arch was used extensively by Roman engineers in many types of buildings including bridges, gates and aqueducts.
Elaborate arches could be extended in a tunnel-like fashion to form a barrel vault, with two barrel vaults overlapping at 90 degrees forming a cross-shaped 'groin vault'. Taken to extremes, a groin vault develops into a dome. The Flavian Amphitheatre in Rome, more popularly known as the Coliseum, was formed from a series of pillars and arches.

A Roman arch² was built of stone with a concrete core for durability. The shape of the arch naturally causes the weight of the structure to compress the arch and transferring the forces to the columns or piers supporting the arch. This means that the construction was always in a state of compression, and the forces created by the weight of the structure are equal on each side cancel themselves out³. This has meant that many of these bridges had a very long useful life⁴.

The method of construction was simple, the masons cut large wedge shaped stone blocks equal in size and shape. They were tapered so when laid out they formed half circle. When the arch was constructed the last stone⁵, at the top of the arch, was fitted and locked the others together. This meant that the arch could distribute the weight evenly throughout forming a very stable structure.
The most common forms of the use of the arch are the bridge and aqueduct.

A Roman Arch Used In Bridge Building

Sooner or later all roads may need a bridge, they were built to cross valleys or rivers and to avoid long detours. The key to the Roman bridge is the arch makeing the construction of complex and large bridges possible.

The Stages Of Building A Roman Bridge

There are five main stages to the building a standard Roman Bridge:

Stage 1-

Surveying the river banks for a suitable site for the crossing point⁶. Teams were sent out to plan the route of a proposed road and often altered the course to use a suitable river crossing point. This job was often given to the military engineers.

Stage 2-

Digging a foundation, the foundation for a single arch bridge were cut into the banks at each side of the river. The foundations were built of stone and with rubble and concrete infill. The foundations were important as they prevented the arch of the bridge from spreading, and preventing collapse. Multi arch bridges are more upright than standard arches, with longer straight piers supporting a semicircular arch. The piers are often braced with a stretch of masonry known as a travertine rib or missionary bands⁷ which helps protect against sideways movement caused by pressure from the arches above.

Stage 3-

The stone arch was constructed over a timber frame that was the intended shape of the arch. The inside of the arch was stone lined as were the outer walls.

Note, the maximum height of an arch was 21 meters, to avoid collapse caused by twisting forces, the maximum span was approximately 34 meters. If extra height was required it was gained by placing second set of smaller arches on top of the first, in some cases a third row of yet smaller arches. The maximum recorded height was 51 meters in three rows of arches⁸.

Stage 4-

When the arch was finished the stonework of the rest of the structure it was supporting was completed. The method used in laying the stonework known as stretcher and header⁹, resulting in incredibly strong structures which could be further strengthened with stone joints and metalwork. The size of the stone blocks was reduced as the bridge got higher. The core of the arch and the road bed was often of rubble and concrete infill.

Stage 5 -

The parapets were built of stone and the road surface was laid. All single arched bridges were slightly higher in the middle than at the edges to shed rainwater and aid drainage.

Multi Arch Bridges

Starting with the same initial stages, the only difference was the construction of evenly spaced piers on the riverbed. This was achieved by sinking two rings of wooden piles into the riverbed to form enclosed areas. The space between the piles was packed with clay forming a simple coffer dam, the water and mud bailed out allowing the piers to be constructed on dry firm ground. These were filled with large stones rubble and concrete to form small islands. These were the pier foundations. Construction was then completed as previously described in stages 3 and 4.

The parapets were built of stone and the road surface was laid. All multi arched bridges had the road slightly higher in the middle than at the edges, this was to shed rain water and aid drainage. The parapets were also given drainage points. Also most multi arched bridges slope slightly to allow drainage.

Roman aqueducts

An aqueduct is in its simple form is a bridge to move water to supply a town or settlement. This was built in the same way as a bridge over a river. The main difference between an aqueduct and a bridge is that the level of the water channel was precisely engineered to provide a slight slope or fall to allow the water to flow. The most striking forms are those that consist of three or four rows of arches, built one on top of the other to achieve height to span a valley.

Roman Concrete

Roman concrete, which was made with slaked lime, sand and volcanic ash. This was forgotten and it was not until the 1800's that anybody thought of a recipe that was better and Portland cement was invented. Roman concrete was the first that could set under water in much the same way as modern concrete


The trick to making good concrete is to get the silicon in sand to chemically bind to the calcium from the limestone (calcium carbonate). Portland cement uses fine china clay. The Romans used volcanic ash which is, essentially, fine, pure crystallised sand (silicon dioxide). The reasons modern cement is better than roman cement is not because it is stronger or longer lasting (it can be but many commercial varieties aren't) but because the clay can be pre-mixed with the treated lime mixture (slaked lime) and then ground into a powder (clinker) and stored in rough conditions in paper bags. Roman cement had to be mixed on site and slaked lime is very dangerous when wet and is not easy to store. To keep it dry they had to use sealed amphora.

Roman Bridges

• The Oldest Pons Aemilius in Rome built in 62 BC.
• The Longest Trajan's Aemilius (Trajan's Bridge) over the River Danube. This was the longest recorded bridge for over 1000 years.
• The Largest Arch Pont romain de Vaison-la-Romaine (The Roman Bridge) over the River Ouvèze in France.

Roman Aqueducts

• Pont du Gard, France 19 BC.
• Fréjus, France 51 BC.
• Mainz, Germany 72 BC.