1. Ensure that the roofing contractor provides a current copy of his public and employers liability insurance. Do not take his word that he is insured. Insist that he provides you with an up to date copy of his public and employers liability insurance certificate and ring the broker or the insurance company to ensure that it is a legitimate insurance policy.
2. Check if the roofing contractor is a member of the C.I.F or National Guild of Mastercraftsmen. This will generally ensure that they have been vetted by a legitimate organisation in terms of the quality of their workmanship etc.
3. Insist upon getting a list of recommendations of clients that the contractor has done work for in the area and ring one or two recommendations to ascertain information about the roofing contractor.
4. Ensure that the roofing contractor has a fixed business premise with a landline. This is necessary to ensure that the contractor is a legitimate business and is also registered for Vat.
Generally speaking in my experience if the above-mentioned criteria are followed it will normally sort out the competent roofing contractors from the contractors that are less than competent.