would damage them. Seeping through the bottom of the impluvium, the water passed into cisterns, from which it was drawn for household purposes. The compluvial opening might be shaded by a coloured veil, probably of an open, airy weave. Roof structure Five structural types of cavaedia (or atria) are described by the architect Vitruvius: The atrium tuscanicum has no pillars; beams span the entire room, framing a rectangular hole in the center and supporting the roof (commonest) The atrium corinthium has (more than four) pillars along the edges of the roof opening (newly fashionable in Vitruvius' time) The atrium tetrastylum has four pillars at the corners of the roof opening (not common) The atrium displuviatum has outwards-sloping roofs that do not collect water, like most modern roofs (rare) The atrium testudinatum was fully roofed-over, with another floor on top instead of an opening to the sky (very rare) The full original description, in his sixth book on engineering, is fairly succinct: Tuscan cavaedium The Tuscan atrium seems to be the most common type in Pompeii. 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