If the building has levels of roof such as the large orange roofed two storey buildings can be accesed by placing a slide in the ramps section to get onto the first part of it.
Skate 3 trick onto roof.
The easiest way is to open up the object dropper by holding l1 and pressing circle.
You just need to practice i.
For skate 3 on the xbox 360 a gamefaqs message board topic titled pool of tricks handplant the roof.
In one of the own the lot challenges in the suburbs there is a challenge called pool og tricks where you skate the pool and have to handplant the roof.
Just pump in the bowl pumping is easy do the tutorial when you have enough speed go up the ramp that has the roof and push the left stick forward as you ollie off.