Mishnayos Oholos Perek 12 Mishnah 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.

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

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.

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.