Roof top area is going to be impossible to get, it isn't one of those things that is collected in any country-wide census. You could come up with some kind of guess-timate like 1 m^2 per person (totally fabricated) but you get the idea.
You might also want to troll around for solar radiation data... estimates, or measures, but you could calculate potentials given latitude, and ignoring atmospheric effects... at least for shortwave radiation. You might want to look for terms like "solar insolation potential" or "latitudinal distribution of solar radiation" etc to get values and or equations.
