Also be careful as "jump off X edge" and "jump across X edge" are not the same things. The difference is pretty self explanatory but I've seen at least one player fubar it without even realizing there was a difference.
For completeness sake:
"jump off X edge" jumps from the roof down onto whatever is below in the direction of that edge. Usually the street below.
"jump across X edge" is when you want to try and jump across whatever is below you in the direction of that edge and over to the roof on the other side. Usually for jumping across the street below while jumping from one rooftop to another.