..I was assuming that you use the Spell to give you the Aspect, then use a normal Athletics roll (on which you Tag said Aspect) to actually jump..
I think this is where the mechanics of aspects overlap with the narrative. If the building is one story high, it should have a border of about +3. If the PC has an athletics of + 1, then even with the +2 from tagging the aspect there is only a 50% chance to onto the roof. (Ok less than 50% but higher than I think would be likely)
However from a narrative point of view, it is hard to imagine someone with no gravity holding him down could not make it up to the top of the building. The lack of gravity is a game changer and should trump the border difficulty because walls are only difficult to cross because of gravity.
I personally think aspects that nullify the inherent rules of the difficulty should be not be rolled for. On the other hand a completely different challenge could enter the picture. For instance if it was really windy, the PC would have to struggle to not get blown downwind.
JesterOC