The World Federation One Stop Fiqh

Ruling 37

If there is an intrinsic impurity on the roof of a building, then as long as it keeps raining on the roof, any water that comes into contact with the impure object and then falls down from the roof or gutter is pure. However, after it stops raining, if the water that falls from the roof or gutter is known to have made contact with the impure object, then that water is impure.