You should always do your research to hire an experienced and reputable roofing and exteriors contractor. These are some of the unscrupulous characters that will try to take advantage of you.
Door to door roofing salesmen.
It is best to avoid door to door roofing salesmen who may tempt you with an offer for a free roof inspection. They are not roofers, they are salesmen. They will always find something wrong with your roof and in some extreme cases, may even damage your roof themselves.