Why the roof though? We have them up in north texas but we always had ours in a window. I have noticed a lot on roofs "out west" and figured a reason but didn't know why. Also brick the norm around here but block and stucco seems more popular the further west we go.
We put them on the roof to cool the whole house. In a window, parts of the house don't cool as well. And here, we have such a great view out of all of our windows, I'd hate to fill one up with a cooler. Our last house had one in the window though and we never bothered to change it.