Reference for Eduyot 4:13

A portico which has been split [into two]: If there is uncleanness on the one side, vessels on the other side remain clean. If a person placed his leg or a reed above [the split], he has combined [with the roof to bring the] uncleanness. If he placed the reed on the ground, it does not form a passage for the uncleanness until it is one handbreadth off the ground.