Door knockers are not always bad, however you should never make a decision at the door. Get the information from the individual and do your research on the company. Check reviews, websites, facebook and other social media. Was the individual wearing a company brand logo? Were they in a company truck? Frequently, a third party sales individual will go door to door to get roofing contracts and then sell them to other roofing companies. This could cause an issue down the road in the event of mishaps or warranty issues because your contract is with the third party. Do your research before making any decisions.