Thanks Roy, Rich ToyBox, Phyllis and Jim.
Your answers and advice were very helpful. I ended up ordering epdm ,
external pump, and a strainer from Darron at Horizon(He had the best
price I found.). The local roofing supply places did not sell epdm for
the most part and the ones that did were cheaper than the local water
garden places, but only beat the net's Pond Guard price by about 10
bucks (for 20'x30'). I guess there are not a lot of flat roofs
here. In any case the price difference was not enough to take what I
know is a small chance that the roofing stuff would be toxic.
I will probably order a skimmer and waterfall filter sometime soon,
unless I turn up a homemade solution that will fit my situation.

Thanks again,

Nort