As we’ve seen the Roman Arch moved things along quite a bit architecturally. But with a little innovation they increased the utility of the masonry ten fold. The arch as a single entity is good for a doorway, window or portico. But if you wanted a larger covered space you need additional support.
The solution came in the form of barrel vaults and groin vaults. Barrel vaults are essentially a long extension of a single arch. They form a long tunneling effect. Barrel vaults were utilized to build the large warehouses through out the city in ancient time, in order to store the vast shipments of goods coming into the city. Some of the most visible examples of barrel vaults are the ruins of the Basilica of Maxentius. To see them in person, you realize what a large open space they really are. Seen from both the Forum and Via dei Fori Imperiali, it’s an imposing structure.
Groin vaults are when two barrel vaults intersect at ninety degree angles. With the combined strength of four posts or piers, groin vaults require less buttressing and there for are an even sturdier structure. The groin vault heralded many of Rome’s greatest building projects. Today, one of the most important places you can see groin vaults is at the Colosseum. Forming a continuous corridor around the arena, the groin vaults allowed the high ceilings and monumental stadium.