If you want to start a roofing company you must realize that running a profitable business is a lot more than knowing how to find a leak or replace a roof. Being a successful roofing business owner involves organization, knowledge, patience, perseverance, people-skills, and a number of other traits. Many business owners fail, not because they weren't skilled in their craft but because they were not skilled n running a business.
The larger you want your roofing company to be, the better you need to be as a business owner, as the responsibility will be greater. It is not wrong to dream big, but if you are a new business owner, it is best to start small and work your way up.