It's not quite that simple. Flat plate collectors are roof components, and they have all the problems of roofing. Making them leakproof despite big thermal stresses takes some work. Brazed copper plumbing works. Plastics, not as well.[1]
You usually want a water tank to buffer the thing, so you can get hot water at night. Temperature will be all over the place without that. Giant hot water tanks in attics need extra structural support, which was a problem in Florida at one time.
There are some fancier setups that will work in cold weather. Some systems use antifreeze in the panels, and a heat exchanger.
[1] http://www.alternative-energy-tutorials.com/solar-hot-water/...