Near the end of 70b, the Gemoro discusses a Chover's roof adjacent to an Am Ho'oretz, and whether the Am Ho'oretz would have an excuse to be leaning over the Chover's roof. There are two cases - where the Chover's roof is higher and where it is level.
Can you clarify these two cases, and why each is different to the chotzer - this may depend on the translation of the excuse given (measuring / testing / stretching etc)?
For example, why could an Am Ho'oretz not be stretching in the chotzer? Is he more likely to stretch if the Chover's roof is higher than if it is level?
Is it easier to tell where a boundary is in a chotzer than on a roof - in the chotzer you can see where each house is - on a roof you probably can't see demarcations underneath?
Meir Eliezer Bergman
To go through the entire Gemara according to all opinions is beyond the scope of this forum. However, here is a brief explanation.
According to Rebbi Yochanan, the Am ha'Aretz has an excuse to stick his hand above the wall when his courtyard is lower in order to test the fitness of the wall (according to one opinion). Therefore, in this case the Taharos in the courtyard of the Chaver above are not presumed to be Tahor. However, if their courtyards are level, the Taharos are presumed Tahor because he does not have this excuse. Rav holds that whenever an Am ha'Aretz can reach one's yard, we suspect that he does. He therefore holds that as long as he can get to the yard, the Taharos in the courtyard of the Chaver are not presumed Tahor.
Since it is easier to see boundaries on a flat yard, there is no need to test or measure the boundaries as much. This is especially so regarding a flat yard where each person can ask the other to merely help him measure. However, when there is some type of wall upon which the neighbor's roof is situated, if it is a matter of testing the strength of the wall, this may not be done so well by the upper neighbor (as he will merely be looking at the floor of his roof). This could be why Rebbi Yochanan understands that the lower neighbor has such a good excuse.
All the best,