so no keying, no real roof ?

How about using a backdrop (out of focus to imitate the depth of field) and a windblower ?

I don't know what it is that you are trying to shoot, but in the past I have shot from below ground in a pit or from a very low angle with the subject on a ladder. Do you need to show the rooftop? It's not clear what you are after... It could be more a matter of careful editing than just shooting.

When you shoot some thing on the ground. Use a white or green back ground. Then when you edit. On one of the tracks you enter it as a second picture on the time line that follows the picture you want. Depending on the type of editing program you have. That's how I would tackle your problem.