If the roof is made with strength to withstand the weight of water.. roof is in fact the best place for a shallow fish pond, because the heat from house that escaped from insulation will goes to the pond water.. hence energy is put to good use.. lol..
Haha! why not for fish ? if they are the same material.. should be same.. or maybe the roofing liner is of more toxic material.. we should use pond lining for water proofing instead.. less toxic.. (since fishes are not dead using it) lol