Compluvium!
OMG, I was reading my Roman Culture and Society sourcebook today and one of the assigned readings was on the design of Roman city houses. Apparently, there's a thing called the compluvium, which is "a quadrangular opening in the roof over the atrium. Rainwater flowed through this opening into a small pool (called the impluvium) below." And I just thought that was so cool. Note to self, if I were ever to design my own house I'd get a compluvium and a impluvium.