According to Varro (Ling. 5,161) and Vitruvius (6,3,1f.) the customary formation of the roof opening of all types of the atrium in the Roman house. The funnel-shaped roof surfaces of the compluvium, which slope inward, conduct rainwater into the impluvium, a basin at the atrium's centre. In the older displuvium the roof surfaces slanted outwards.
E. M. Evans, The Atrium Complex in the Houses of Pompeii, 1980
R. Förtsch, Arch. Komm. zu den Villenbriefen des jüngeren Plinius, 1993, 30-31.