This is a little off the wall but it looks just like a freshly tarred roof. Do some of the roof with a grimmy black and a dusting of gray chalk. Do the rest of the area with a thick coat of Black Liquid Electrical Tape so that it looks like the roof is being freshly tarred. What really makes the sceen is a set of "Woodland Sceenics Roofing Figures". They come with a tar pot and some guys spreading tar with rakes. Looks cool.