The Mishnah (12:6) discusses a case of a beam that is bridging two walls and there is a source of tumah (ha'met) beneath it. The Mishnah teaches that if the width of the beam is a tephach then the tumah would spread beneath the beam, making everything under it tameh. The law presented in this Mishnah, as stated, should not be surprising, given that we learnt that in order for an object to act as an ohel to spread tumah it must be a tephach wide.
The Tosfot Yom Tov however comments that the novelty of the Mishnah is that even if the beam is not a tephach wide along it fulls length, it would still make anything underneath the entire beam tameh. This is the position of the Rambam (Tumat Ha'Met 12:7). The Kesef Mishnah citing the Ri Korkus explains, that the Mishnah's language appears to suggest that this is indeed the case. The Mishnah writes that "it spreads the tumah underneath all of it". The addition of the words "underneath all of it" appears to suggests that this would include even those parts that are not a tephach wide.
The position of the Rambam is actually the subject of debate. Recall that in order for flesh from a corpse to be a source of tumah under an ohel, it must be the volume of a kezayit. We will learn later in the masechet (15:10) about the case where there is a half kezayit of tumah in one house and another half in another. The Mishnah teaches that if a person puts one hand in each house, if the hands are each a tephach wide, it if considered is if there is a full kezayit under one ohel and the contents of both houses would become tameh. The mefarshim ask that granted that the hands might be a tephach wide, the arms will likely not. How then can the person act as a ohel to combine the two houses? According to the Rambam above, this is not a problem. Just like the beam can spread the tumah even under the areas that are less than a tephach wide, the tumah would also spread under the parts of the arm that are less than a tephach wide.
The Rosh (15:10) however understands that that Mishnah must be understood as referring to a case involving a fat person whose arms are a tephach wide. (Note that the word yad used in the Mishnah can either refer to a hand or arm.) If however part of the arm was less than a tephach wide then the tumah would not spread beyond that point. The Rosh brings a proof from another Mishnah (15:2) that discusses a case of two square tiles, raised from the ground a tephach, and touching only at the corners. The Mishnah teaches that if the tumat ha'met is underneath one, then the keilim underneath the other are tahor. We find therefore, that the tumah cannot spread past a point that is less than a tephach wide.
The Chazon Nachum however explains that the Rambam would maintain that the case of the tiles is different. That case involves two distinct items. In order that the tumah can spread from one ohel to the next, they must be in contact by a tephach. Our case however involves a single item -- the beam. Consequently, since tumah spread underneath it, it is not important if it narrows at parts.
The Tifferet Yisrael (Boaz 12:7) explains that a proof from the Rambam's position comes from an earlier Mishnah (7:2)1. Recall that we learnt that the area under the slanted walls of a tent is considered part of the ohel. Consequently, if there is a source of tumah in the ohel, even keilim that are under that slanted part that is only the width of finger in height would become tameh. We find therefore once tumah is inside an ohel, tumah spread underneath the entire ohel, including those parts that on their own would not qualify as an ohel.
The Tifferet Yisrael continues by suggesting a different answer to the Chazon Nachum above. He explains that the difference between the cases of the tent and beam as compared to the tiles, is that in the former cases the items were intended to be used as a covering -- to act as an ohel. It is only in those cases that the tumah spreads underneath all of it. By contrast, in the case of the tiles which were never intend to be used as coverings, the tumah's spread is limited to only the spaces that fits the technical requirements of an ohel.2
1 See also the Mishnah Achrona.
2 The Tifferet Yisrael does continue with an additional answer similar to the Chazon Nachum. See inside.
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