According to the National Association of the Remodeling Industry (NARI), warm weather often ushers in a wave of home improvement [1] scams.
As welcome as a swarm of termites, these home improvement con artists show up at your door offering an array of services from repaving your driveway to roof repair, and usually demand a hefty down payment up front. They might offer a great deal, based on “using materials left over from a previous job,” but homeowners should remember that a great deal isn't always what it seems.
Frequently, these fly-by-night home improvement scammers drive vehicles with out-of-state license plates, or set up temporary offices from which they can move quickly once authorities start looking for them.
Before writing that check, and especially before allowing any unknown individual into your home, NARI suggests that you:

Be skeptical of home improvement scams if a contractor:
NARI is a professional association whose members voluntarily subscribe to a strict code of ethics. Consumers can avoid home improvement scams by calling the NARI National hotline at 800-611-NARI and request a free copy of NARI's brochure, How to Select a Remodeling Professional, or visit www.RemodelToday.com [2] and click on the homeowner's guide for more information.
Links:
[1] http://www.moneypit.com
[2] http://www.moneypit.com/www.remodeltoday.com/