Ohalot, Chapter Fifteen, Mishnah Four
1) [With regard to] a house, sectioned off by boards or curtains from the sides or from the roof beams:
a) If there is uncleanness in the house, vessels beyond the partition remain clean.
b) If there is uncleanness beyond the partition, vessels in the house become unclean. [With regard to] the vessels beyond the partition:
2) If there is a space of a [cubic] handbreadth there, they become unclean,
a) But if not, they are clean.
Section one: The house has been sectioned into two parts such that what was one room is now two. Alternatively, some sort of loft was added such that the house is now separate from the roof beams. The mishnah now discusses what happens if the source of impurity is found in various parts of the house and its partitions.
If the source of impurity is in the house then the vessels that are in the partitioned off inner room, or in the newly sectioned off upper room, remain pure.
However, if the uncleanness is in the inner room or in the upper chamber, it goes out and defiles the vessels in the house because, as we have learned, impurity goes out of a confined space and defiles things that are in its space.
Section two: As far as the petitioned off areas themselves, if they are at least a cubic handbreadth in dimension, then they are an ohel and they are impure. However, if they are less than a cubic handbreadh, then the impurity within would only go up and down and it wouldn’t defile any vessels found on the sides of such a space.