Does a datacenter really needs a roof?
Take a look at this picture:

Is there any chance it looks like a datacenter to you? :)
Well, believe it or not, but it is. it’s actually the 4th generation of Microsoft datacenter design. A couple of very interesting posts has been published by the design team, I highly recommend you check them out:
Our Vision for Generation 4 Modular Data Centers - One way of Getting it just right . . .
Microsoft's Generation 4 Data Center Vision - the Architects' Perspective