Why? Roofing contractors are their customers. They know who pays their bills on time, what companies are prepared for deliveries, if there are problems on their jobs. The distributors know if the company is run like a business. That is important.
The best people to verify the quality of the installation is the people they have installed a roof for. Ask the contractor if they have a job in progress you can drive by. See how the jobsite looks, ask the homeowners if this has been a good experience so far. Call for a referral on a job a few years old, did they have to call the contractor back?
It is important for safety on your job and the safety of the employees. Contractors that don't pay insurance can offer pricing way lower. Why should you care? If they get hurt on your job you may be liable.
The lowest estimate may be a great deal BUT do your homework. If the contractor makes no profit how can he survive to fix your roof later if there is a problem? Perhaps he is not carrying insurance. Perhaps he measured incorrectly. A price too low can mean trouble.
Not all states have contractor's licenses and not all cities have building permits but it is always safe to check and make sure the contractor is licensed and your job is permitted when required. If you get caught without a permit the building inspector could make you tear-off your roof.
When you hire the undocumented, the unlicensed and the uninsured you may be saving money now but you pay for it in other ways. Higher taxes and insurance. Do the right thing. They also are unfair competition for licensed contractors who operate with bonds, insurance and other responsible business practices. You will have no recourse if the installation is inferior and the roof fails. Know what you are buying and who you are buying it from.
Make sure your expectations are put in writing and make sure you understand your contract. Be wary of large down payments and don't let payments get ahead of the work. You are making a substantial risk with your money and your new roof.
This kind of project deserves a quality job. Your roof protects the structure of your home, all of your possessions and your family. You could get kitchen cabinets on sale but there should be no cutting corners in roofing.
Value is good, Cheap is bad.
A few final things to remember:
There can always be hidden surprises when you tear the old roof off. Wood and termite damage will add extra cost to your roof.
You always need to allow extra material for cutting, waste and doubling up on materials around critical areas.
Proper Ventilation for your roof may be extra. Your contractor will need to evaluate each job based on insulation and your roofs ability to ventilate your house. Chances are good that you will have a good system in place. Improper ventilation could result in failure of your roof systems, such as causing wood rot or early deterioration of the roofing materials.
Before you begin we recommend you visit a terrific website called AskARoofer.com there you have an opportunity to ask a few questions. Also visit the National Roofing Contractors Association's (NRCA) Everybody Needs a Roof website.
Good luck with your project!