Mikvaos 4:1-2

Mikvaos 4:1

If a person put vessels under a rainspout, they invalidate a mikvah; this is true regardless of whether they’re large or small; this is even true of vessels of dung, stone or clay. It makes no difference whether they were placed there intentionally or were forgotten there. This is the opinion of Beis Shammai; Beis Hillel rule it clean in the case where the vessels were forgotten. Rabbi Meir said that they put it to a vote and Beis Shammai had the majority. Nevertheless, they agree that if one forgot vessels in a courtyard, the mikvah remains clean. Rabbi Yosi said that the dispute remains unresolved.

Mikvaos 4:2

Let’s say that someone put a board under a rain spout. If it had a rim, it invalidates a mikvah but if it doesn’t, then it doesn’t invalidate the mikvah. If he stood the board upright to be rinsed, it doesn’t invalidate the mikvah in either case.