A rod that is on top of a doorway, even if it is one hundred cubits high, spreads tu-mah no matter how narrow it is, these are the words of R’ Yehoshua. R’ Yochanan ben Nuri says: Do not treat this any stricter than a ledge.

A ziz-type ledge encircles the entire house and overlaps three fingerbreadths of the doorway. If tumah is inside the house, utensils underneath [the ledge] become tamei. [If] tumah is underneath [the ledge], R’ Eliezer rules the house tamei, and R’ Yehoshua rules the house tahor. The same applies to a courtyard surrounded by an awning.