The Mishnah taught: Nor the roof of the house when it is fenced in, to a height of ten tefachim.

Question: What if there was no actual fence, but instead, the roof was surrounded by the backs of houses or by pillars and beams? (Are these also considered a fence to exclude the roof from the sale?)

Proof (our Mishnah): R. Yehudah says that if the fence has a doorway, even if it is not ten tefachim tall, it is not sold.

Conclusion of proof: R. Yehudah was telling the Chachamim that if there is a doorway (or the backs of houses or pillars and beams) the roof is excluded. This shows that the Chachamim disagree with him.