The history of Roman architecture taught at university is the history of monuments: the Pantheon, the Colosseum, the Forums, the baths. It is the history of the 1% — of imperial buildings and temples. What is missing is the history of the 99%: where the Romans lived. The answer is: in the insulae. Multi-story rental buildings, densely inhabited, technically precarious, lacking running water and heating on the upper floors, with shops and taverns on the ground floor. They are the urban norm of the Roman empire — and they have almost completely disappeared.
The problem is simple: insulae were built poorly because the Roman construction market rewarded speed and density, not quality. The owner rented each floor to different families, with rents decreasing the higher you went (the ground floor was the most expensive — fewer stairs, access to the shops; the sixth floor was the cheapest — the hottest in summer, the coldest in winter, without water, with the greatest risk in case of fire). The law set height limits — Augustus at 21 meters, Trajan at 18 — but they were systematically violated.
How many there were, and who owned them
How many insulae were there in Rome? The most frequently cited sources are the Regionary Catalogues, 4th-century AD lists describing the city's fourteen regions and counting their buildings: in total they record about 46,600 insulae against fewer than 1,800 domus, the single-family houses of the rich. The figure should be treated with caution, because scholars still debate whether "insula" there means an entire building or a single rented dwelling. But the ratio is telling all the same: for every family living in a house of its own, dozens lived in rented rooms, often just one or two.
Insulae were an investment, and their owners often belonged to the elite. Cicero owned shops and rented buildings at Puteoli, and in a letter to his friend Atticus he relates, with bitter irony, that two of his buildings have collapsed and the rest are full of cracks, so that not only the tenants but even the mice have moved out. Marcus Licinius Crassus, one of the richest men of the late Republic, according to Plutarch built his property fortune in an even more unscrupulous way: when a fire broke out he arrived with a team of trained slaves, bought the burning building and its neighbours at knock-down prices from their desperate owners, and only then had the fire put out. They are anecdotes, but they depict well a market in which rental income mattered more than the safety of the people living there.
Ostia: the preserved insulae
Ostia Antica, the port of Rome, is the place where Roman insulae are best preserved. Not because Ostia was different from Rome — but because it was gradually abandoned in the Middle Ages and not overwritten by later construction. The Caseggiati of Ostia show varied typologies: insulae with an internal courtyard, insulae as a compact block, mixed insulae with shops on the ground floor. The standard structure uses fired brick (opus testaceum) for the load-bearing walls, wooden floors, and a tile roof.
The Caseggiato di Diana in Ostia (2nd century AD) is one of the best-preserved examples: four floors, with an internal courtyard equipped with a water cistern, balconies projecting over the front, shops on the ground floor. On the first floor were the largest and most expensive apartments, with windows onto the courtyard and frescoes on the walls. On the upper floors the apartments became increasingly small and spartan.
Building technique explains both the durability of the better insulae and the fragility of the others. The lower floors were of Roman concrete faced with brick, solid and hardly flammable. On the upper floors, to save weight and money, opus craticium was often used: a timber frame filled with rubble and mortar, quick and cheap to build. Vitruvius, in the second book of his treatise, condemns it without mincing words: he wishes it had never been invented, because in a fire it burns like a torch and damp rots the timber and cracks the plaster. The floors were of timber beams, and balconies projected over the street on brackets. Windows rarely had glass, which was expensive: they were closed with wooden shutters, curtains or cloth panels, with the result that in winter one had to choose between cold and darkness.
Water was a privilege of the ground floor. The aqueducts brought water to public fountains and, for a fee, to those who obtained a concession for a private connection with lead pipes; but the pressure was not enough to carry it to the upper floors. Those who lived high up had to climb with buckets from the nearest fountain, and likewise come down with their waste. Private latrines were rare: people used the public latrines, the foricae, with several seats side by side, or chamber pots to be emptied, and ancient sources also tell of people who emptied them straight out of the window.
Living in an insula
The liveliest witness to this life is the poet Juvenal, who in his third satire, at the beginning of the 2nd century AD, has a friend leaving Rome list everything that makes the city unliveable: houses that collapse, fires that spread from the lower floors to those sleeping under the roof, noise that makes sleep impossible. The noise had a precise cause. A law from Caesar's time banned, with some exceptions, the movement of carts in the city during the day, to free the streets from congestion; as a result goods were transported at night, and heavy carts rolling beneath the windows accompanied the sleep of those living along the main streets. It is one of the first documented examples of an urban rule designed to solve one problem that creates another.
Not all insulae were hovels. At Ostia there are also good-quality housing complexes, such as the so-called Garden Houses, built in Hadrian's time around a large shared inner garden, with spacious, bright apartments, or the House of the Painted Vaults, with decorated ceilings. On the ground floor of the main streets there were shops, workshops and places selling ready-made food, like the thermopolium on the Via di Diana with its masonry counter. The insula was thus a mixed building, in which housing, work and commerce coexisted, and in which quality depended on the location, the floor and the tenant's purse.
Urban risk
Rome burned often. The Great Fire of 64 AD, under Nero, struck ten of the city's fourteen districts: according to Tacitus three were razed to the ground and seven badly damaged. But it was not an exceptional event: ancient sources record large and small fires with regular frequency. The structure of the insulae explains why: wood in the floors, braziers for heating and cooking, shops with flammable materials on the ground floor, extreme population density, narrow streets that hindered firefighters' access. The Roman insula is a firetrap.
Nero, after the fire of 64, rebuilt Rome with stricter regulations: wider streets, mandatory porticoes in front of buildings, stone walls instead of wood, limited heights. It is one of the first documented fire-safety building regulations in urban history. And here one must be precise: the building code is an ancient invention, not a modern one. The problem of urban building safety was already being addressed with construction regulations in the Roman empire — with partial results, as later fires demonstrate.
Even before Nero, Augustus had tackled the problem of fire through organisation. In 6 AD he established the corps of vigiles, a combination of fire brigade and night police, organised in seven cohorts each responsible for two of the city's fourteen regions, with barracks and guard posts spread across the districts. The vigiles had buckets, hand pumps, ladders, wet blankets and axes for pulling down structures to create firebreaks. At Ostia the barracks of the vigiles still survive, a large building around a courtyard. It is proof that the Romans knew the risk well: the problem was not ignorance, but the difficulty of imposing rules on a building market that profited from breaking them.
The persistence of the type
But let's return to architecture. The Roman insula has an extraordinary typological persistence. The multi-story rental building with shops on the ground floor and apartments above is still the dominant building type in European historic centers — and in many cities around the world. The form has not changed in 2000 years. What has changed is the technology: elevators instead of wooden stairs, running water on every floor, central heating, fire-safety regulations. The spatial structure — shops below, dwellings above, internal courtyard or street facade — is the same as the insula of Ostia.
The social hierarchy by floor also survived for a long time. In the Parisian apartment blocks built during Haussmann's works in the second half of the 19th century, the ground floor housed shops, the second floor, the so-called noble floor with its continuous balcony, was the most prestigious, and going up the apartments became more modest, all the way to the little servants' rooms under the roof. It is exactly the scale of values of the insula at Ostia. What overturned it was not a social reform but a technology: the lift. Once climbing seven floors no longer takes effort, the top floor, bright, quiet and with the best view, becomes the most sought after, and in contemporary cities the penthouse is worth more than the first floor. The form of the building remained the same for two thousand years; an electric motor was enough to change its social meaning.
Where to see an insula today
In Rome the most accessible example is the Insula dell'Ara Coeli, at the foot of the staircase leading up to the church of Santa Maria in Aracoeli, next to the Capitoline. Discovered in the 1930s during the demolitions carried out to open the great streets of the centre, it preserves parts of four storeys from the 2nd century AD: at the lowest level the shops can be recognised, above them the apartments, and on the façade the remains of a small medieval church built inside the ancient building, with a bell tower, can still be seen embedded in the wall. People passing by car or bus in front of the Vittoriano see it without noticing. To really understand the building type, however, you need a day at Ostia Antica, half an hour by train from Rome: there you can walk inside the insulae, climb the masonry stairs that led to the upper floors and follow whole streets in which shops, courtyards and apartment blocks follow one another as in a modern neighbourhood.