Ohalos, Chapter 6, Mishnah 5. The mishnayos in this part of the chapter treat tumah that is lodged neither in the room below nor in the chamber above, but concealed within the thickness of the structure that divides them: a ground floor dwelling with an aliyoh built over it. The present Mishnah takes up tumah resting at the very bottom of that divider, in the hollow among the beams that stand along the tops of the house's walls, and it defines when such tumah carries the din of a stopped-up grave, when it pierces upward alone, and when the room beneath is tamei in every case.
The words of the Mishnah
The Mishnah's language: "tumah bein hakoros" - tumah situated among the beams; "ve'sachtehah keklipas hashum" - with a plaster covering underneath it no thicker than the skin of a garlic. "Im yesh sham posei'ach tefach" - if that location holds an open handbreadth - "hakol tamei" - all of it contracts tumah. "Ve'im ein sham posei'ach tefach" - but where no such open handbreadth exists - "ro'in es hatumah" - we look upon the tumah - "ke'ilu hu atum" - as though it were stopped up. "Hoyso nirehs bisoch habayis" - and if the tumah can be seen from within the room below - "bein kach u'vein kach habayis tamei" - the room is tamei in either event. Let us now take these clauses one at a time.
How a floor was built
To follow this Mishnah one has to picture how a second story was constructed. The ground floor room was enclosed by its walls, the kosel habayis. When a person wanted to add an aliyoh above, he set a series of short upright beams along the top of those walls, running around the perimeter. Those uprights are the "koros" of our Mishnah. Across the tops of the uprights he laid long beams running from one side to the other, and those crossbeams formed the actual base of the upper floor. Over the crossbeams he spread a layer of plaster, and that plaster surface, the mezivah, was the floor that the people upstairs walked on. Underneath the whole arrangement, beneath the uprights and sometimes rising a bit into the gaps between them, there was a second layer of plaster, and this bottom layer was extremely thin. That thin layer served as the ceiling of the room below.
Imagine standing in the aliyoh and drilling a hole downward. First the drill passes through the plaster of the upper floor. Next it passes through the crossbeams that span from side to side. Then it breaks into an open space, the space in which the short upright beams stand. Finally it meets the thin plaster ceiling of the house below. A plumber working from the other direction, drilling upward from the ground floor to run a pipe, meets the same layers in reverse: the thin plaster ceiling, then the hollow space with its upright beams, then the crossbeams, then the plaster of the upper floor.
Which space are we discussing?
The Mishnah before ours dealt with tumah located in the mezivah, meaning tumah among the crossbeams that span the room or in the plaster spread over them. Here we descend one level, to the hollow underneath those crossbeams, the area occupied by the short uprights resting along the tops of the house's walls, closed off from below by the thin coat of plaster.
The Bartenura teaches that this Mishnah is speaking according to the opinion of Rebbi Yehuda. Rebbi Yehuda maintains that the entire mezivah is attributed to the aliyoh above. It is only because of his position that our case is a chiddush at all, as we will explain at the end.
The first case: an open tefach
"Im yesh sham posei'ach tefach, hakol tamei." The halacha now turns on measuring the place in which the tumah lies. Should that pocket amount to a cubic tefach, a handbreadth in length, in width and in height, it takes on the status of a kever sasum, a stopped-up grave. A stopped-up grave is itself tamei, and it transmits tumah in both directions, downward no less than upward. The consequence is that the dwelling underneath is tamei and the aliyoh overhead is tamei too.
A question presents itself. In the earlier Mishnah, which dealt with tumah situated higher up in the mezivah, nothing was said about an open handbreadth. Why not? In truth the identical measurement is required there as well. It went unmentioned because such a case hardly occurs: within the crossbeams themselves, or in the plaster coating them, one would not expect a cavity a handbreadth in each dimension. In the region our Mishnah describes, by contrast, below the mezivah and above the thin ceiling, where the uprights stand with spaces between them, a cavity of that size is quite ordinary, and so the Mishnah states the requirement outright.
The second case: less than a tefach
"Ve'im ein sham posei'ach tefach" - where the place holding the tumah lacks a handbreadth in each of its three dimensions, it does not receive the din of a kever. Its status is rather tumah retzutzah, tumah pressed in with nowhere to expand. Tumah of this sort bores upward in a direct line: it penetrates the crossbeams and the plaster resting on them and makes the aliyoh tamei, yet it does not descend, so the room beneath stays tahor. When the Mishnah directs that we view the tumah "ke'ilu hu atum," as though stopped up, this is precisely the intent: blocked below, while above it the way lies open.
The third case: the tumah is visible from the house
"Hoyso nirehs bisoch habayis, bein kach u'vein kach habayis tamei." Here the Mishnah speaks of a case in which the thin plaster ceiling is missing, so that a person standing in the room below can look up between the uprights and see the tumah itself. Since there is no covering separating the tumah from the house, the measurement of a tefach is no longer relevant. Whether or not the pocket holds a square tefach, the house below is tamei. The Tosfos Yom Tov notes that in this situation the aliyoh above remains tahor.
Why this is a chiddush only according to Rebbi Yehuda
We said at the outset, in the name of the Bartenura, that our Mishnah follows Rebbi Yehuda. Let us see what would be the halacha according to the Chachamim. The Chachamim hold, as we learn from the previous Mishnah, that tumah found in the lower portion of the crossbeams and the plaster above them belongs automatically to the house below. If that is so where the tumah lies relatively high up, then certainly when it lies lower still, in the hollow among the upright beams, it would belong to the house below, and there would be nothing new to teach.
Rebbi Yehuda, though, maintains that whatever is found within the crossbeams and their plaster, even in their lower reaches, is annexed to the story above. On his view a genuine question emerges: how is tumah treated when it lies beneath all of that, in the gap among the short beams standing along the tops of the house's walls? Our Mishnah supplies the answer. Where plaster closes the space from underneath, even a coat as flimsy as garlic skin, and the cavity measures a handbreadth in every direction, the place is a kever, and both what lies under it and what lies over it are tamei. Where no such handbreadth exists, it is no kever, and the tumah drives straight upward, making the aliyoh tamei while leaving the lower room tahor. All of this holds only so long as the tumah is hidden from the room below. Should that thin coat beneath the uprights be lacking, so that the tumah is visible from inside, then regardless of the measurement the ground floor dwelling is tamei.