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Main room of an ancient Roman house

Cavaedium or atrium are Latin names for the principal room of an ancient Roman house, which usually had a central opening in the roof (compluvium) and a rainwater pool (impluvium) beneath it. The cavaedium passively collected, filtered, stored, and cooled rainwater. It also daylit, passively cooled and passively ventilated the house. The atrium was the most important room of the ancient Roman house. The main entrance led into it; patrones received their clientes there, and marriages, funerals, and other ceremonies were conducted there. In earlier and more modest homes, the atrium was the common room used for most household activities; in richer homes, it became mainly a reception room, with private life moving deeper into the (larger) house. The atrium was generally the most elaborate room, with the finest finishings, wall paintings, and furnishings. The atrium was entered either through a shop

These terms are thought by many to be synonymous; The tollere liberum (ceremony for a newborn), dedication of the bulla at Liberalia (coming of age, male), and confarreatio marriage were described as conducted in the atrium, in front of the lararium. The wedding couch or bed, the lectus genialis, was placed in the atrium, on the side opposite the door or in one of the alae. The lectus funebris, or funeral couch, was placed in the atrium, and the body of the deceased was laid in state upon it with feet facing the door. It was thus a sort of living room. Spinning and weaving, major household tasks, were traditionally done in the atrium, The homes of the richest Romans became more ostentatious as increasing wealth inequality increased housing inequality, and as it became less socially important to display frugalitas and abundentia, and more important to display tryphé. This patron class use as a reception room made the atrium a semi public space, The pars rusticana was centered around a peristyle; it did not exist in early Roman houses. Greek culture was high status in Roman culture (see Hellenization). The peristyle was borrowed from Greek architecture and became popular, eventually sometimes replacing the atrium. In the countryside the order was sometimes reversed; the pars urbana cortile, which one entered from the main street entrance, was a peristyle. The atrium was then buried in the depths of the house, often near a portico (an outwards looking colonnade on one or more outside walls) with a view of the landscape.

The atrium often contained a small chapel to the ancestral spirits (lararium). The household safe (arca) was also kept in the atrium; it contained family treasures and important documents. A puteal, a low cylindrical covered wellhead through which water could be drawn from the cistern below, was often present. There would also be works of art, especially statuary, which was set beside the impluvium, on tables, in niches, on walls, etc. These pillars support a beam, which supports a wall, which in turn supports the rafters in the middle of the atrium. There is no need for horizontal beams spanning the entire room; the rafters have shorter spans. This allows a larger room to be roofed.

This style has a rectangle of pillars around the roof opening. It is like the tetrastyle, but with more than four pillars. It resembles a peristyle. If the lower layer was sufficiently robust, it could support a second story.

In this style, the roofs, instead of sloping down towards the compluvium, sloped outwards from the compluvium, the gutters being on the outer walls; there was still an opening in the roof, and an impluvium to catch the rain falling through (and presumably fed by the gutters). This species of roof, Vitruvius states, is constantly in want of repair, as the water does not easily run away, owing to the stoppage in the rainwater pipes. This type was rare; only one had been found in Pompeii, as of lc=yes.

The atrium testudinatum was employed when the atrium was small and another floor was built over it. The name comes from the Latin word [[:wiktionary:testudo#Latin, which means a turtle or tortoise, and by transference a covered vault. The Encyclopædia Britannica said that no example of this type had been found at Pompeii, as of lc=yes. While narrow homes had atria that were not flanked by cubiculums and alae, and even atria where the impluvium was set against a wall or in a corner, homes smaller than 5 meters across are generally testudinate. This arrangement would give the sloping rafters an equal span on each side of the atrium walls.