At some point in every homeowners life they will have to deal with roofing contractors due the the fact that the average roof has a life span of between 20-25 years. The big problem you will confront when dealing with them is that they are roofing experts and you aren't, which puts you at a considerable disadvantage during the negotiating process.
One thing that you can do to help you gain an edge when dealing with roofing contractors however; is to educate yourself on not only roofs in general but also on the process of installing a roof on a building. One good suggestion also is to talk with your friends and coworkers and see if they have any qualified roofing contractors that they can refer you to.
In general, there will be a rate that all roofing is charged by depending on the area that you live in, the economic climate and the type of roof that you have in mind. Then other variables come into play, such as if you are having your old roof torn off or roofed over.
When you are talking to a roofing contractor about your job see if they will talk candidly and openly about such items as dry rot repair and what kind of time frame the contractor is going to complete the job in. If you sense that he is being evasive at all and can't give you precise answers than you may want to consider another contractor.
At the same time homeowners are notorious for being problematic, so you will want to let the contractor know that once an agreement is reached and a schedule is set that your house is his to do the job on and there will be no changes or adjustments in what you agreed on.