"when the faller falls from it": "from it," and not within it. (How so?) If the public thoroughfare were ten hand-breadths above his roof, (and he fell from there upon it, the owner of the house is) not liable, it being written "from it (the roof), and not within it.
R. Yishmael says: Scripture comes to teach you how he is judged, viz. (Koheleth 9:12) "For a man does not know his time, like fishes that are caught in a flimsy net and like birds that are caught in the snare. Like them men are snared in a time of evil (i.e., when their evil is visited upon them), when it falls upon them suddenly."