Why Hasn’t the Pantheon’s Dome Collapsed?: How the Romans Engineered the Dome to Last 19 Centuries and Counting

In Rome, one does­n’t have to look ter­ri­bly hard to find ancient build­ings. But even in the Eter­nal City, not all ancient build­ings have come down to us in equal­ly good shape, and prac­ti­cal­ly none of them have held up as well as the Pan­theon. Once a Roman tem­ple and now a Catholic church (as well as a for­mi­da­ble tourist attrac­tion), it gives its vis­i­tors the clear­est and most direct sense pos­si­ble of the majesty of antiq­ui­ty. But how has it man­aged to remain intact for nine­teen cen­turies and count­ing when so much else in ancient Rome’s built envi­ron­ment has been lost? Ancient-his­to­ry Youtu­ber Gar­rett Ryan explains that in the video above.

“Any answer has to begin with con­crete,” Ryan says, the Roman vari­ety of which “cured incred­i­bly hard, even under­wa­ter. Sea water, in fact, made it stronger.” Its strength “enabled the cre­ation of vaults and domes that rev­o­lu­tion­ized archi­tec­ture,” not least the still-sub­lime dome of the Pan­theon itself.

Anoth­er impor­tant fac­tor is the Roman bricks, “more like thick tiles than mod­ern rec­tan­gu­lar bricks,” used to con­struct the arch­es in its walls. These “helped to direct the gar­gan­tu­an weight of the rotun­da toward the mason­ry ‘piers’ between the recess­es. And since the arch­es, made almost entire­ly of brick, set much more quick­ly than the con­crete fill in which they were embed­ded, they stiff­ened the struc­ture as it rose.”

This has­n’t kept the Pan­theon’s floor from sink­ing, cracks from open­ing in its walls, but such com­par­a­tive­ly minor defects could hard­ly dis­tract from the spec­ta­cle of the dome (a feat not equaled until Fil­ip­po Brunelleschi came along about 1300 years lat­er). “The archi­tect of the Pan­theon man­aged hor­i­zon­tal thrust — that is, pre­vent­ed the dome from spread­ing or push­ing out the build­ing beneath it – by mak­ing the wall of the rotun­da extreme­ly thick and embed­ding the low­er third of the dome in their mass.” Even the ocu­lus at the very top strength­ens it, “both by obvi­at­ing the need for a struc­tural­ly dan­ger­ous crown and through its mason­ry rim, which func­tioned like the key­stone of an arch.” We may no longer pay trib­ute to the gods or emper­ors to whom it was first ded­i­cat­ed, but as an object of archi­tec­tur­al wor­ship, the Pan­theon will sure­ly out­last many gen­er­a­tions to come.

Relat­ed con­tent:

The Beau­ty & Inge­nu­ity of the Pan­theon, Ancient Rome’s Best-Pre­served Mon­u­ment: An Intro­duc­tion

How to Make Roman Con­crete, One of Human Civilization’s Longest-Last­ing Build­ing Mate­ri­als

A Street Musi­cian Plays Pink Floyd’s “Time” in Front of the 1,900-Year-Old Pan­theon in Rome

How the World’s Biggest Dome Was Built: The Sto­ry of Fil­ip­po Brunelleschi and the Duo­mo in Flo­rence

The Mys­tery Final­ly Solved: Why Has Roman Con­crete Been So Durable?

Build­ing The Colos­se­um: The Icon of Rome

Based in Seoul, Col­in Marshall writes and broad­casts on cities, lan­guage, and cul­ture. His projects include the Sub­stack newslet­ter Books on Cities, the book The State­less City: a Walk through 21st-Cen­tu­ry Los Ange­les and the video series The City in Cin­e­ma. Fol­low him on Twit­ter at @colinmarshall or on Face­book.

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  • Tom Simmons says:

    I’m lov­ing the series of Roman
    Con­struc­tion …from roads to build­ing using the spe­cial­ized con­crete meth­ods.
    I love these arti­cles

  • Sebastiaan says:

    Nice arti­cle Col­in!
    But what about Soltaniyeh in Iran, why is that schol­ars seemed to for­get Per­sian archi­tec­ture. Brune­leschi prob­a­bly got his inspi­ra­tion from the East. Nev­er­the­less Domes are majes­tic.

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