Bottle Rockets can cause fires, generally though they are competent for very short periods of time and have to involve only lighter materials (like wood shakes or grasses). I can tell you though that I have seen numerous cases of mortars that don't reach proper height explode and the materials land on roofs while still glowing hot (embers). If they were fired from the rooftop, then the launching platforms are good potential source also (post-launch fires).

Again I don't know the specifics of your roofing.