Reference for Menachot 66:10

הכי קאמר אע"פ שיש לה פצימין פטורה שאין עשויין אלא לחיזוק לתקרה אמר אביי חזינא להו לאיספלידי דבי מר דאית להו פצימי ולית להו מזוזתא קסבר לחיזוק תקרה הוא דעבידי

is exempt from mezuzah, since it has no door-posts. It follows, however, that if it had door-posts it would require a mezuzah, but surely [the posts] were made only as supports for the ceiling! - He meant to say this: even though it has door-posts it is exempt, for they were made only as supports for the ceiling.

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