[The following items also spread tumah and act as barriers against tumah:] ledges, balconies, dovecotes, overhanging boulders and rocks, windows for light, sharp rocks, and branch-canopies and protrusions that can support light plasterwork; these are the words of R’ Meir. The Sages say: medium plasterwork. What are branch-canopies? A tree that overhangs the ground. And [what are] protrusions? [Hedges] that extend out from a fence.