[NH_Bulldog]

Just a thought; you mention that the current 200w panel has 12-gauge wiring and that the wiring from the roof is 10-gauge? If there is one 10-gauge wire from the roof, you cannot just add another panel and run two panels through that single 10-gauge wire, you will need to add a second 10-gauge wire or upgrade the existing 10-gauge to 8-gauge to accommodate the increased power. Like Stircrazy mentioned, a 40A controller (or larger) would also be needed.

For example;
14 AWG for 15A
12 AWG for 20A
10 AWG for 30A
8 AWG for 40A
6AWG for 55A

As for roof space, that is a question only you can answer. Between roof vents, AC units, attic vents, skylight(s) and tank vents, there may not be as much free space up there as you might think. It may require spreading the panels out across the roof as opposed to locating them in one central area. I would get the panel and mount measurements and cut a carboard template the same size and then head up to the roof and see what your layout might be and whether it is feasible for your situation.