Rabbi Yishmael says that a house must have four stones in order to be susceptible to nega impurity; Rabbi Akiva says eight. Rabbi Yishmael used to say that a nega doesn’t cause impurity until the size of two split beans appears on two stones or one stone; Rabbi Akiva says it must appear on two stones but not on one stone. Rabbi Eliezer the son of Rabbi Shimon says that the size of two split beans must appear on two stones, on two walls in a corner; it must be two split beans long by one split bean wide.
In order to be susceptible to nega impurity, there must be enough wood to put under the lintel; Rabbi Yehuda says enough to make the block behind the lintel. There must be enough earth to fill the space between two beams. The walls of a cattle stall or of a partition are not susceptible to nega impurity. Houses in Jerusalem and outside of Israel are not susceptible to nega impurity.