Tractate Oholot - Chapter 10 - Mishnah 5 אהלות פרק י׳ משנה ה׳

In a case where the hatchways do not have an opening of a square handbreadth:If there is uncleanness in the house, what is directly [below] the hatchways remains clean. If there is uncleanness directly [below] the hatchways, the house remains clean. [In the case] where the uncleanness is in the house, if an article whether susceptible to uncleanness or insusceptible to uncleanness was placed either in the upper or the lower [hatchway], nothing becomes unclean except the lower story. [In the case] where the uncleanness is directly [below] the hatchways, if an article susceptible to uncleanness was placed either in the upper or lower [hatchway], everything becomes unclean. If the article is insusceptible to uncleanness, whether [it is placed] in the upper or lower [hatchway], nothing becomes unclean except the lower story.