About 80 percent of residential hurricane wind damage starts with wind entry through the garage door. Studies have shown that hurricane winds exert the greatest pressure and suction at the corners of a structure, which is the location of most garage doors. A garage door is held in place only by the door's tracks. Hurricane winds exert both tremendous pressure and suction on the garage door, causing it to flex inward and outward. Consequently hurricanes generate severe stress not only on the door but on its supporting tracks as well. As the pressure builds, the garage door pushes against and pulls away from the garage door tracks. If either the tracks or door give way, the garage door blows in or is sucked out. This allows the full power of the hurricane force winds to enter the compromised structure and attack the roof and walls. Garage doors can now be constructed, tested, and rated for impact and wind resistance. The marginal cost of a rated garage door is only $200 to $300 over a door without wind-resistant features. A retrofit kit to strengthen an existing two-car garage door will cost about $300. A kit usually consists of a vertical post that is placed between the roof and the concrete floor, although other systems also exist. Single-car openings are more resistant to strong winds than two-car garage doors.