I had a former client of mine who was a roofer and I asked him about repairing a roof job that had been screwed up on our strata. He was too busy to do it but what he told me helped. One is to consider using the services of a roof inspector. They don't actually repair the roof but what they do is outline what needs to be done, recommend companies if need be, and inspect it afterwards. I believe they suggested Cambie Roofing but since this was around 15 years ago my memories not to good about that.
Secondly he said there are members of roofing associations you should consider using, and lastly, and perhaps most importantly I found out that the guarantee product supplies offer is (ie 40 year shingles etc) is only as good as the labour being used. So another way to approach this is when buying shingles or using someone installing shingles, make sure the guarantee comes from the supplier of the shingles and that they'll warranty the work, including labour, because what happens is that usually the shingles themselves aren't defective, what's defective is the installation of them and the manufacturer won't cover this if they didn't approve of who's installing them.