Ohalot, Chapter Fourteen, Mishnah Four
1) [In the case of] a projection which goes all round the house, occupying space above the doorway to the extent of three fingerbreadths, if there is uncleanness in the house, vessels beneath [the projection] become unclean.
2) If the uncleanness is beneath [the projection]:
a) Rabbi Eliezer declares the house unclean,
b) But Rabbi Joshua declares it clean.
3) A similar [rule applies] to a courtyard surrounded by a portico.
Section one: In this mishnah the projection surrounds the whole house, and therefore it is not treated like the projection that covers the entrance or a window. At the point of the doorway, it is only three fingerbreadths in width, which is less than the handbreadth (a handbreadth is equivalent to three fingerbreadths) usually required for impurity to enter the house when under a projection.
If the uncleanness is in the house, then any vessels underneath the projection are impure for the way of impurity is to go out.
Section two: If the source of impurity is underneath the projection, Rabbi Eliezer says that even though the projection is not a full handbreadth, the impurity enters the house and defiles anything that is in the house. Since the projection surrounds the whole house, it is looked at as part of the house, even though it is not the usually required width.
Rabbi Joshua says that since the projection is not a handbreadth wide, it does not cause the impurity to enter the house.
Section three: The same rules apply in a case where a portico (a roofed area with pillars) lies outside a courtyard. If the roof of the portico is at least three fingerbreadths, but not a full handbreadth, then impurity in the house will defile vessels under the portico, but if impurity is in the portico, Rabbi Eliezer and Rabbi Joshua debate. If the portico is more than a handbreadth, then all agree that impurity found there will enter the house.