Let's assume that they had a crane or something to get the gun up on top of that building. It's a stretch, but okay. And then let's say that the roof-beams could actually support the weight. Another stretch, but let's go with it. But if they actually fired that gun, it's more than likely that the whole roof would collapse.
So let's say that the roof didn't collapse after the first shot: then they're trapped up there, having given away their position, with no redeployment route.
Tactically inadvisable gun placement, I'd say.