Try reading from the links above and see if that helps any. Also, try asking your search question differently. Instead of asking "how much water" will the roof hold, try a search for "snow weight on roof" or "how much snow weight will roof hold" and see if a Google search shows anything.
Most people would not be putting water up on a roof and so the question will not be asked much online. But, people do have snow on a roof and so the snow weight question will be easier to search.
That sounds to me like too much water up top. Without doing the math, and since I have not seen the roof, then of course, I do not know. But, just off the top, it sounds like too much. Another option would be to place some water up on top, but have other containers on the ground and connected? Would the upper tanks give you the pressure needed but the lower tanks expand the stored amounts?