PEX tubing would be freeze resistant (it can stretch a little), but without it being in a frame it's still going to loose heat pretty fast and without a storage tank you are only going to have water when the sun is shining, that is going to be your biggest issue, roofs tend to be strong enough tho that if you really do live in an area that is warm all winter you could do a roof top storage tank and thermo-siphon so you don't need a pump, you would need a tank that could handle the pressure tho but then you just put it in a super insulated box on your roof and you have solar heated hot water 24 hours a day.
a system like this would not however work well in an area that had freezing temps as you could not be assured that the tank would never freeze, but if you got the rare frost at night the thermal mass of the tank in the super insulated box would even keep the water hot for a few days.
PEX, at least the stuff that NIBCO makes is UV resistant for short term exposure and with luck the black pant on it would keep it protected from UV longer.
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