Architecture of the Pantheon: The Dome and Construction Techniques

The Pantheon’s architecture is groundbreaking: Its interior is solely lit by sunlight which enters through the “oculus” in the roof. This hole is 9 meters in diameter. The Oculus also provides a good way in for rain and birds.

For those using Imperial units, the dome of the Pantheon spans 142 feet (43.3m) internally  and rises to a height of 143 feet (43.6m) . To begin to understand how this single span dome was made to stand up without further supports we have to look at a section drawing. We will notice that internally the dome actually begins further down than it does externally. The bottom section of the dome is a massive 23 feet (7m) thick in order to counteract the enormous “hoop” stress caused by the dome’s weight.

The 5 rings of 28 decorative panels, “coffering”, of the dome were originally dressed in gold plated bronze. Their size decreases with height in order to increase the sense of size of the dome. The coffers are actually cavities in the dome’s concrete, aimed at making it as light as possible.

The top of the dome is only some 2 feet (60cm) thick and in order to lighten the weight further the cement in the higher portion seems is filled with porous materials such as empty amphoras and pumice stone. The oculus also provides a good means of removing load although its construction must have presented a problem: this is like an arch without a key stone. Different solutions have been proposed to its construction – the simple one of creating an earth mound and building over it would probably have been excessively difficult to put into practice although larger earth works were certainly undertaken by the military as part of war campaigns. A more probable alternative was to use wooden or even ceramic trusses.

Construction of the Pantheon’s dome made use of a “recent” roman invention after the great fire in Rome in AD64 (for which Nero is so well remembered). Following the fire, Nero had established new fire prevention regulations such as the widening of streets, water supply control and restriction of inflammable building materials. Concrete of a new type was invented much along the lines of modern concrete.

Building materials and a new type of concrete

The secret of the new type of mortar was that it contained a readily available volcanic ash called Pozzolana. The strength and benefits of this new cement came from a chemical process of combination with water whilst previous cements depended on water evaporation which tended to make them weaker and slower to cure. So it can be said that “Nero’s” fire was the starting point for a revolution in the construction of arches and vaults and the Pantheon is its greatest example.

Deeper analysis of the Pantheon’s building materials is a tour-de-force in the building materials of ancient Rome. It is made up of 6 annular sections of which the lowest is the foundation made in cement mixed with travertine stone to a thickness of 4.5m. As the rings reach the oculus at the top the materials are progressively lightened: Travertine is replaced by porous Tufa, brick and finally pumice (volcanic ash). The picture below allows you to see 5 of the layers mentioned (not the foundations of course).

The exterior of the perimeter wall is entirely faced in brickwork with inbuilt arches aimed at channeling the weight of the dome down to the ground. The ancient architect Piranesi suggests that similar weight shedding arches were also introduced into the dome itself.

So there we have it: like many great things the ancient Roman Pantheon is amazingly grand whilst at the same time amazingly simple in conception. This simplicity is a direct product of its inherent harmony which no doubt struck all those who set eyes upon it and made it into the unequalled archetype it has become.

In spite of its apparent simplicity, it is quite likely that the Pantheon reserves yet more secrets and surprises, like all those little rooms and areas hidden in the thick walls, but closed to the public. Or for those in search for ancient astrology: Just take a count at the number of rings around the central Oculus (the sun), there are five orders of coffering in the dome plus the circular perimeter wall = 7. Was this a giant planetarium?

• Architecture of the Pantheon part 1 – General Introduction
• Architecture of the Pantheon part 2 – Description of the Pantheon
• Architecture of the Pantheon part 3 – The Dome and Construction Techniques

The Pantheon: Ancient Roman Pantheon | pantheon | Purpose of the Roman Pantheon | architecture of the pantheon |