The Romans found and developed a way to use internal arches ("a curved structure that forms the upper edge of an open space, or the space between a bridge's supports" (Arch)) and vaults ("an arch-shaped structure, usually of masonry, used as the ceiling of a room or other enclosed space, as the roof of a building, or as the support for a ceiling or roof" (Arch and Vault)) to provide roofs for structures of increasing size and complexity
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The arched structure is one reason that the Roman's bridges were advanced for their time. The Romans were advanced in their ways of making a permanent bridge. First, they made a boat bridge that consisted of boats lain side by side. From the boat bridge, work began on the permanent bridge. It was to be made of wood and supported on stone towers called piers, which were to stand in the river. A Cofferdam was a double skinned box made of planks with the space between the inner and outer planks being filled with clay. This box was floated into position and then weighted down until it sank. The water was pumped out with buckets or a device called an Archimedean screw or tympanum The Cofferdams were built, so the laborers could erect the piers without going under water. First the piles of the Cofferdam, a piece of wood sharpened like a pencil, were driven into the riverbed, and then pumped. When the pumping was finished, each pier stood on a foundation of tar covered piles and was constructed of carefully cut stones on the inside. The mortar used between the stones contained pozzolana, a volcanic ash. When the piers reached a height of 30 feet above the river, wooden arches were hoisted into its place between them. A wooden road was nailed to the arches and covered with a layer of earth. The finished road stood over sixty feet above the river. For major or busy bridges sometimes stone went in place of wood. The semicircular arch was usually used for bridges.
