There is no pattern in when a single radioactive atom decays (I don't know the word), most people are convinced it's truly nondeterministic as far as I know.
http://invisiblethreads.com/potd/collections/galleries.php?cc=1 dunno if posted before or not. just a bunch of images of urban decay in and around ontario it seems.
Or use them to make the silhouette more interesting. Try to communicate decay/wear and tear in the geometry as well as texture if this piece is supposed to be run down.
I would follow the reference more. It helps with the natural decay of that environment. This is in reference to the moisture/buildup/discoloration on that prop. Yours looks more random.