Comentario sobre Oholot 12:3

נסר – [a board] which is long and narrow, and when it is placed on the mouth of the oven, it extends outward a handbreadth from the east and a handbreadth from the west but from the south and the north it does not extend outward.

[In a case where] the board is placed over the mouth of an old oven projects from either [end] to the extent of one handbreadth but not from the sides: If there is uncleanness under one end [of the board], vessels [under] the other end remain clean. Rabbi Yose declares them unclean. In this case the board extends over the oven in one direction, for instance, north-south, but not east-west. The first opinion holds that this is not sufficient to convey impurity from one side to the other. The impurity would have to travel through the oven and since the oven cannot become impure from the outside it blocks the impurity from going to the other side. In mishnah one the board hung over the oven from all sides, and therefore the impurity could travel from one side to the other. Rabbi Yose holds that the door does not prevent the impurity from traveling from one side to the other.

טומאה בצד זה – underneath the board in the east or in the west.

A betach does not bring uncleanness. A betach is some sort of projection that juts out from a window both inside and out. Maimonides interprets it as something used to climb on. The mishnah says that it doesn't bring uncleanness from the outside into the house because the wall found in between forms a break.

כלים שבצד השני טהורים – that there Is no room for the defilement to pass, for the board doesn’t have leftover from the north or from the south.

If there was a projection ( in it: Rabbi Eliezer says: it [still] does not bring uncleanness. Rabbi Joshua says: we look at the betach as if it is not there, and the projection above brings uncleanness. In this case there is another type of projection sticking out from the house that is found on top of the betach (in Hebrew this is called a ziz). The impurity is found underneath the betach and the question is does the ziz convey impurity into the house?. Rabbi Eliezer holds that the betach separates the impurity from the ziz and therefore the ziz doesn't convey impurity into the house. Rabbi Joshua holds that we look at the situation as if the betach wasn't there. Therefore, the ziz is found to form an ohel over the impurity. The house to which the ziz is attached will also be impure.

ורבי יוסי מטמא – for he holds that the oven does not form a partition. But the Halakha is not according to Rabbi Yossi.

הבטח (a hollow column-like receptacle of rain water near the house; Maimonides holds that it is a projection in front of the window to stand upon in climbing) – There are books that have the reading of אבטח, and it is a building that is made similar to a bathtub in bathhouses that they place water in them for washing, and a board is placed upon it for the ceiling and board has leftover from two sides. But if there is defilement from this side, it doesn’t go out to the other side because the hollow column-like receptacle of rain water near the house interposes.

היה בו זיז (if it had an attachment – a projection from the door frame serving as a shed over the entrance, or a molding projecting from a window-sill as a bracket) – there was a beam going out above from the hollow column-like receptacle of rain water from the east to the west.

רואין את הבטח כיאלו אינו – for even though the hollow column-like receptacle of rain water near the house interposes with the board, in connection with the attachment/a projection from the door frame serving as a shed over the entrance – it does not interpose. And the Halakha is according to Rabbi Yehoshua.