Cavaedium

Tablelegs, puteal, and opus sectile impluvium, House of Publius Servilius Casca Longus (close-up)

Roman townhouses rarely had windows, as they often had very little exterior wall. Where present, windows were placed above eye-level, and they were small[4] and contained clathri, window lattices.[4] The compluvium provided most or all of the light to the atrium, its alae, and the adjacent cubiculums. The doorways of the cubicli were usually arranged to maximize the light, and when present, the alae were often needed to let light into the rooms flanking the tablinum.

Traditionally, the atrium collected rainwater. Most atria had compluvium roofs, which sloped inwards towards the hole in the center of the roof; these shed rain water into the impluvium (pool) underneath (the impluvium was usually the same size and shape as the compluvium hole). The water in the impluvium then slowly seeped through the porous bottom of the impluvium into a water storage cistern below. Water for household use could be drawn up in buckets via the puteal (a lidded cylinder set over a hole in the top of the cistern as a wellhead).

In dry weather, water drawn from the cistern could be thrown into the impluvium to evaporatively cool the cistern and the house, and drive a draft. In wet weather, impluvium overflow would generally run out the front door into the street, which was much lower than the interior floor.

The atrium might also contain a fountain, which piped in drinking water from an aqueduct (riverwater and well water was preferred for drinking).[5] Reliable piped water made the rain-collecting function dispensable, and Late-Empire domi often replace atria with peristyle gardens, which could also be made larger.

After a birth, a bed for Juno and a table for Hercules were set up in the atrium.[6] 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.[4][7] 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.[7]

The view over the impluvium, through the tablinum, and into the peristyle; the pillars, wallpaintings, and stonework were designed make this view impressive. House of Menander (view of the same room as the lead image of this article, but in the opposite direction).

The cavaedium was a communal space. Varro says it is "left open to the common use of all".[5] Vitruvius describes it as a room which "any of the people have a perfect right to enter, even without an invitation".[8] It was thus a sort of living-room. Spinning and weaving, major household tasks, were traditionally done in the atrium,[4] as were other domestic occupations.[5] Toys, flutes, writing materials, cutlery, and crockery have all been found in atria.[2]

Historically, the atrium was probably used for cooking, with the compluvium serving as a smoke-hole. This probably continued in poorer households, but richer ones developed a separate kitchen. Similarly, historically the household slept in the cubicles opening off the atrium, but as townhouses became deeper family tended to live deeper in the house, and these became guest rooms (unless the house has dedicated guest rooms, a hospitium).[4] Slaves and servants might sleep in the entrance to their master's cubiculum.

Richer Romans received visitors in the mornings,[4] as their clientes gathered in the atrium for the salutatio. The patron often greeted them from the tablinum.[9] The atrium was generally the most elaborate room,[5] with the finest artworks,[9] wall paintings, imported marble wall-linings, and marble or mosaic floors (mosaics being even more expensive than imported marble slabs).[5] 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,[4] and it came to be known as the pars urbanan, the city part of the house. The more private space behind the tablinum was the pars rusticana, the rural part.[10] 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.[2] As a result, the names for the pars urbanan are in Latin, while the names for the pars rusticana are Greek loan-words.[11] 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.[2][12]

The Tuscan atrium seems to be the most common type in Pompeii.[14] The Tuscan type has the advantage that the walls and pillars are very well-protected from the elements. For a smaller cavaedium, it is a simple, light structure. In a larger cavaedium, though, it requires very massive timber beams.

The Tuscan structure requires two wooden beams that span the entire atrium, running horizontally from wall to wall (see image). Since the atrium is usually longer than it is wide (Vitruvius advises width:length ratios of 3:5, 2:3, or 1:√2), the beams usually run across the width of the atrium. The ends of the beams are typically encastrated (set in a socket in the wall). The beams must be thick enough not to bend excessively under the weight of the roof. As the atrium gets wider, the span gets larger, until it is impractical or unaffordable to get beams which are big enough. The Tuscan style was therefore not used on very large atria. Since a large atrium was a status symbol, the more complex structures used for large atria were sometimes used in small atria which could have been covered with a simpler Tuscan roof.

The main horizontal beams form two sides of the roof opening. The other two sides are formed by shorter horizontal cross-beams, laid across (or jointed into) the main beams. The four valley rafters slope down from the corners of the walls to the corners of the central roof opening. The upper ends of the valley rafters rest on the wall, and the bottom ends on the horizontal beams. The smaller rafters rest their upper ends on the wall, and their lower ends mostly on the beams and cross-beams; the short rafters in the mitered corners of the roof rest on the valley rafters.

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  Tuscan atrium. Impluvium is mossy; behind, the main door is flanked by two shops. House of Sallust, Pompeii (floorplan).
• 
  Roof framing of the atrium of the small House of the Bronze Herm, Herculaneum (floor plan). The four large diagonal valley rafters and the smaller rafters all slope down from the top of the walls to the horizontal beams.
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  Atrium tuscanicum, no pillars. View towards the tablinum. Note the cartibulum (stone table) with unusually simple legs, the impluvium (dry), and the circular (covered) hole giving access to the cistern, just to the right of the impluvium. House of Marcus Lucretius Fronto, Pompeii (another view)

In the atrium tetrastylum additional support was required in consequence of the dimensions of the hall; this was given by columns placed at the four angles of the impluvium.[14] This style is not common in Pompeii.[15] 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.

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  Atrium tetrastylum, four pillars. Villa San Marco, Stabiae. In this image, the atrium is roofed by a white tarp, laced down at the eaves and supported by scaffolding.
• 
  Same, but with a reconstructed roof. The lack of plaster shows how four horizontal beams, running between the four columns, support a short masonry wall (opus incertum), which in turn supports the rafters
• 
  The figure is leaning against the cartibulum, the lararium is against the wall behind her, and the statue to her right is a small fountain cascading into a shallow trough and thence into the impluvium.
• 
  Atrium tetrastylum (not a very accurate reconstruction)

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  Atrium corinthium, multiple pillars. Small atrium of the House of Menander, Pompeii. Note how the diagonal valley rafters, at the mitered corners, are set lower than the smaller orthogonal rafters. This atrium is smaller than the clear-span Tuscan main atrium in the same house (floorplan model).
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  Pompejanum reconstruction (glass roof is an unrealistic modern addition), loosely based on the square atrium in the House of Castor and Pollux, Pompeii (floorplans)
• 
  House of the Corinthian Atrium, in Herculaneum. The six pillars are stuccoed tufa, repaired with brick. This atrium is halfway to being a peristyle; planters flank a grassy area. The central marble fountain was fed by an aqueduct, making the original purpose of the atrium, a structure for gathering rainwater, superfluous. The original well remains, beside the nearest pillar (floorplan).
• 
  16-column peristyle atrium of the Domvs Romana, in Malta (1st century BC). Note puteal and overflow hole of impluvium.
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  Atrium with puteal, House of the Lake, Roman-era Delos. Delos was short on freshwater and atria there generally collected it.
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  Two stories of Corinthian atria/peristyles. House of the Hermes, Roman-era Delos

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.[14]

This type was rare; only one had been found in Pompeii, as of 1911.[14]

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  Impluviate atrium
• 
  Displuviate atrium (rare)
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  Functioning compluvium roof in the 1865 Lavoir (public laundry-house), Baigneux-les-Juifs, France
• 
  Similar Lavoir from outside.

The atrium testudinatum was employed when the atrium was small and another floor was built over it.[14] The name comes from the Latin word testudo, 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 1911.[14] 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.[2]