Ohalot, Chapter Fifteen, Mishnah One
The first two sections of today’s mishnah are found word for word in 11:3. I have copied my explanation from there.
1) A thick woolen jacket or a thick wooden block does not bring uncleanness until they are one handbreadth high off the ground.
2) If [garments] are folded one above the other they do not bring uncleanness until the uppermost is one handbreadth high off the ground.
3) Tablets of wood [placed] one above the other do not bring uncleanness unless the uppermost is one handbreadth high off the ground;
4) But if they were of marble, the uncleanness cleaves upwards and downwards.
Section one: Despite the fact that these items are one handbreadth thick, they are not considered to be an ohel such that there is an ohel directly beneath the split in the roof of the portico. For something to be an ohel it must also have an empty space.
Section two: If there are a few garments folded one on another, they bring uncleanness when placed underneath the split if the bottom part of the upper garment is at least one handbreadth off the ground. In other words, the entire upper garment must be one handbreadth above the ground in order for it to form an ohel over the other garments. We might have thought that since the presence of the lower garments does not prevent an ohel from being formed, then the lower folds of the upper garment also wouldn’t prevent an ohel from being formed. The mishnah teaches that there is a difference between the lower and upper garments. The lower garments are in the ohel and the upper garment forms the ohel.
Section three: When it comes to pieces of wood, the upper piece is considered an ohel if it is at least a handbreadth over the ground.
Section four: The slabs of marble placed one on top of another are treated as if they were all one tablet. Therefore, no matter how high the upper tablet is, it does not form an ohel. In other words, marble is treated as one piece (like the coats in section one) whereas the pieces of wood are considered separate.