I have no experience though I would worry about contamination from paint, rust and metal.
Maybe there is a way to make a catchment system on top of the roof with safer materials?
Invasive plants are Earth's way of insisting we notice her medicines. Stephen Herrod Buhner
Everyone learns what works by learning what doesn't work. Stephen Herrod Buhner
Anne is correct.
the only way is to create a new surface for collection.
I would build a new structure and use it for collection and shade, but if money is a real issue, some form of plastic sheet suspended to drain to a single point may work.