Hurricane coverings, commonly known as shutters, are used in hurricane mitigation to protect houses and other structures from damage caused by storms. Hurricane shutters are used to prevent windows from being broken by flying objects during a storm. Although the negative pressure caused by high-speed wind flowing over a building roof can cause the roof to fail with the building envelope intact, broken windows allow the air pressure to rise inside a building, creating an even greater pressure difference and increasing the likelihood of roof failure. Shutters are frequently constructed from steel or aluminium, but homeowners sometimes use the low-cost alternative of plywood. The shutters are affixed to the outside of the building with screws, hurricane clips, or a track system. Advanced shutters may be motorized, and they may fold away when not in use. Types of hurricane coverings Three major types of shutters are known alternatively as panel systems. They are usually made of plywood, metal (aluminium or steel), or polycarbonate plastic. Lexan panels are transparent, and may be flat or have been heat-formed into a corrugated shape to further increase impact resistance. Panels must be attached to the structure via screws in a direct mount or on tracks. When not in use, storm panels are stored in an easy to reach location. Newly constructed homes sometimes have an assigned area for storage of storm panels. Another type of shutter is an accordion shutter which are made from interlocking vertical blades which slide into place horizontally. Another type is the roll-up or rolling shutter, consisting of a series of slats that form a curtain with both sides of the curtain being inserted into guide rails. The curtain is then rolled onto an axle which is covered by a housing. Rolling shutters can be operated manually by gear, by pull strap or can be motorized independently or in conjunction with a manual override. Motorized shutters can be operated by either a switch or remote and can be controlled either individually or in groups. On loss of power caused by hurricanes, motorized shutters must be operated manually, requiring either access to the motor (which can be difficult) or a pre-installed manual override. Most often, a manual override is operated using a gear. When deployed, all hurricane shutters require special techniques for firefighters to gain access to the interior of a structure. Other types are the Bahamas, Colonials, and Tapco Hurricane Screens.