Greek architecture is trabeated: horizontal members carried on vertical supports. Stone is strong in compression and weak in tension, so a stone beam can only span a short distance, which is why Greek interiors are small and full of columns.
The arch converts load into compression along its curve, and a vault is an arch extended. Roman builders combined the arch with concrete and could then roof large spaces without internal supports — the barrel vault, the groin vault formed where two barrels cross, and the dome.
Every vault requires centring: a temporary timber formwork carrying the structure until it can stand on its own. That centring is invisible archaeologically but is the practical constraint on what could be built, and the timber for it was a major cost.
What the evidence supports
Arcuated construction is the structural distinction between Greek and Roman building.
Visible across the two traditions and reflected in interior scale: Greek interiors are constrained by beam spans, Roman ones are not.
Timber centring was required and has left no direct trace.
Its necessity follows from the structure. Impressions of formwork boards survive in some concrete surfaces, which is the closest evidence.
The groin vault allowed openings in the side walls.
Because thrust concentrates at the corners rather than along the whole side. This is what makes the great bath halls possible.
Examples
The evidence level on each answers how we know what the building looked like, not whether it existed.
- Evidence level: Documented
Rome · c. 126 CE
The Pantheon dome
Coffered, oculus open to the sky, and unreinforced. Its span was not exceeded for well over a millennium. Standing and still roofed, because the building became a church in the seventh century.
- Evidence level: Documented
Rome · Begun c. 308 CE
The vaults of the Basilica of Maxentius
Coffered groin vaults; the surviving aisle bays show the scale the technique reached. Three bays survive standing.
Primary sources
Vitruvius, On Architecture 5.10, 6.8
Vaulting in baths and the construction of ceilings.
How to read the evidence labels
- Evidence level: Documented
- Supported by a named primary text or by excavated material evidence.
- Evidence level: Probable
- Supported by strong inference that most specialists accept.
- Evidence level: Disputed
- Substantial specialist disagreement exists; competing readings are stated.
- Evidence level: Literary
- Belongs to poetic or mythological construction rather than to history.
- Evidence level: Mythological
- Belongs to shared religious or mythic tradition rather than to any single text.
- Evidence level: Unknown
- The available evidence is insufficient to decide.
These labels describe the status of the evidence, not our confidence in a conclusion. A claim marked Literary is not a claim we doubt; it is a claim about what kind of thing a poem is.