I have hired a roofer to reroof my low slope roof. He recommended self-adhering mod bit. I checked the work they have done so far and I notice that the sheets of roofing material on the surface do not always cover the entire width of the roof. There are two or three places at the edge which are covered with several inches of black tar or some sort of sealant. I plan to ask my roofer to explain. How concerned should I be about the quality of this job?