משנה על גיטין 163:19

All houses may contract negaim uncleanness, except those of non-Jews. If one bought houses from non-Jews, any it must be inspected as if they had then first appeared. A round house, a triangular house, or a house built on a ship, on a raft or on four beams, is not susceptible to negaim uncleanness. But if it was four-sided, even if it was built on four pillars, it is susceptible to uncleanness.