Ohalos, Chapter 15, Mishnah 1. Before turning to the language of the Mishnah, it pays to recall two requirements that have accompanied us throughout this masechta. A covering earns the status of an ohel only when two things are true of it: its area must be no smaller than one square tefach, and it must stand off the floor by a tefach, leaving that much clearance underneath.
Once a covering carries that status, two results follow, one strict and one lenient. The strict side is the spread of tumah. Where a covering fails to qualify, a utensil resting off to the side of the tumah remains untouched. Where the covering does qualify, tumah moves horizontally and fills the entire area under the roof, so that everything in that space contracts tumah. The lenient side is that an ohel functions as a chatzitzah, a barrier: tumah below it cannot climb through it, and a utensil placed on the covering stays tahor. Horizontal travel, yes; upward penetration, no.
The upshot is that whether it is to our advantage for a covering to qualify depends entirely on where the utensil happens to be. A utensil lying next to the meis, under that same covering, fares better if the covering does not qualify, for then no tumah reaches it. A utensil resting above the covering that spans the meis fares better if the covering does qualify, for then the roof will hold the tumah down below.
A Thick Garment and a Thick Beam
The Mishnah's first case is introduced with the words "sagos aveh vechoifes aveh." A "sagos" is a garment of wool; a "choifes" is a beam; and "aveh" carries the sense of thickness (its root, ov, means thick). Of these the Mishnah rules "einan mevi'os es hatumah," they do not convey tumah, meaning that they form no ohel in which tumah travels, "ad sheyeheyu gevohin," unless they stand "min ha'aretz besech tefach," a tefach up from the ground.
The Bartenura points out that this halachah was already taught earlier, in the chapter of HaBayis SheNisdak, in Mishnah Gimmel. The Mishnah repeats it here as an introduction to the cases that follow.
What this ruling comes to resolve is the point from which the tefach is taken: do we measure at the upper face of the thick garment or beam, or below it? Imagine a hefty beam whose top surface stands a full tefach above the floor, while the gap from its lower face down to the floor is smaller than that. The ruling is that the measurement is taken at the underside. The tefach in question must be a tefach of clear, unoccupied air.
Layers Folded One Upon Another
The Mishnah goes on: "kefulin zo al gabay zo." Where a number of garments lie folded in a stack, with a meis below them, the calculation shifts: "einan mevi'os es hatumah," they convey no tumah, "ad sheteheyu ha'elyonah gevohah," until the highest of them is raised "min ha'aretz besech tefach," a tefach up from the ground. Only the uppermost garment enters the measurement. If there is a tefach between the floor and the lower face of that highest garment, the requirement is satisfied, even though the folded layers beneath occupy much of that gap. This still counts as an ohel with its tefach of clearance, and tumah will travel horizontally within it.
Why does this case part company with the first one? A single thick garment, or a single thick beam, cannot have its lower portion written off. We are not free to say that its top serves as the roof while its lower part is merely something situated under that roof, for the beam is one object and the garment is one object. With a stack of separate folded garments the analysis differs: although they lie flush against one another with no air between them, the highest may be designated the covering above and the others treated as items positioned below it. Practically every ohel has something or other stored under it; ohalim are rarely bare. The decisive point is that the lower layers form no single unit with the covering over them. There is a roof, there is content situated below, and the required tefach beneath the roof is undisturbed.
Put differently: the Mishnah teaches that the tefach beneath the roof need not consist of empty air. It needs to belong to something other than the covering itself.
Boards of Wood and Slabs of Marble
The Mishnah now extends the same logic to another scenario: "tavla'os shel eitz," wooden boards, piled "zo al gabay zo," one upon another, with a meis below the pile. Here too the ruling reads "einan mevi'os es hatumah," no tumah is conveyed, "ad sheteheyu ha'elyonah gevohah," until the top board is raised "min ha'aretz besech tefach." We do not demand a tefach between the floor and the underside of the bottommost board; a tefach beneath the highest board suffices. Since the boards are unattached to one another, the top one serves as the covering and those under it count as contents situated below, and that is perfectly acceptable.
The Mishnah then draws a distinction: "ve'im hayu shel shayish," were the slabs made of marble, we rule otherwise. In a pile of marble slabs, even a clear tefach between the floor and the lower face of the uppermost slab is not counted as the airspace of an ohel. No ohel exists here at all, and consequently tumah in that place, in the Mishnah's words "bokaas ve'olah, bokaas veyoredes," pierces straight up and straight down without moving to either side.
Why should marble slabs differ from wooden ones? The Bartenura explains: marble comes out of the ground, and so marble is regarded as ground itself. When there is no hollow space beneath, we view the stack as though the earth itself has simply risen higher, and one does not have an ohel over ground.
One could protest that wood also originates in the ground. The difference is that wood arrives by way of a tree that is felled, peeled and shaped through human labor. Marble, by contrast, is quarried from the earth in precisely the form in which it lay there. Marble is stone, and stone belongs to the earth, so it takes on the earth's status automatically. Accordingly, without a genuine tefach of air below the marble, no ohel is created.
Summary
Reviewing Mishnah Aleph: a single thick garment or a single thick beam becomes an ohel only if there is a tefach from the ground to its underside. Several garments folded one upon another form an ohel as long as there is a tefach measured from the underside of the topmost one; the garments below do not diminish the required tefach, since they are not part of the same item as the roof.
The same applies to boards of wood stacked one upon the other: a tefach measured from the underside of the top board is enough.
Slabs of marble stacked one upon the other are different. Since marble is rock that comes out of the earth, it is treated as earth. Even if the distance from the underside of the top slab to the ground measures a tefach, that space is filled with marble, and it is as though the earth has risen to that height. There is no ohel, and any tumah beneath will not spread to the sides but will pierce upward and downward.