[sblock=DM Commentary]Hmmm, okay my interpretation of things.
Technically you only need to roll acrobatics if moving.
However, fighting requires movement, even firing a crossbow and reloading. To be simple, probably include spell casting too. So I am asking for Acrobatics Checks for two reasons:
1. No where to brace yourself on the roof
2. You will need to be perched on the edge to even see down to the alley floor, a precarious position requiring some attention at all times
DC already includes modifiers for slope and surface.
Failure means you slip and fall prone on the roof. Let's say, Reflex save DC12 to not fall off the roof when adjacent to the edge. Treated like 30ft drop since you will likely have downward momentum before reaching mid-air to add another 5ft onto the 25ft height. Makes for round number for calculating damage.
As for wisdom of the plan, that is up to you to determine.

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