A garage door is a large door on a garage that opens either manually or by an electric motor (a garage door opener). Garage doors are frequently large enough to accommodate automobiles and other vehicles. Small garage doors may be made in a single panel that tilts up and back across the garage ceiling. Larger doors are usually made in several jointed panels that roll up on tracks across the garage ceiling, or into a roll above the doorway. The operating mechanism is spring-loaded or counterbalanced to offset the weight of the door and reduce human or motor effort required to operate the door. Less commonly, some garage doors slide or swing horizontally. Doors are made of wood, metal, or fiberglass, and may be insulated to prevent heat loss. Warehouses, bus garages and locomotive sheds have larger versions.
Single panel doors are constructed from one monolithic panel. From the closed position a single panel door swings up and overhead with a hinge on each side (known as jamb type hardware) to the fully open position. A disadvantage of monolithic panel doors is that the swing up arc of the door occurs partially outside the garage. This means a vehicle must stop and park a distance in front of the door to avoid being hit by the garage door when it is opened.
Sectional doors are usually constructed of three to eight panels and slide up and overhead. Sectional doors occupy exactly the same amount of internal garage space as a monolithic door. Sectional doors have two advantages over single panel monolithic doors:
1) Sectional doors do not require any space outside the garage to open. A vehicle may park very close to the garage before opening the door. 2) Each panel of a sectional door has its own connection to the door track. This increases reliability and robustness compared to monolithic doors, which have only a few track connections for the whole panel.
Roller doors are usually constructed of corrugated steel. They evolved from cover window and door coverings. Other materials can be used (e.g.; transparent corrugated fibreglass) where strong impact resistance is not required. Corrugations give the door strength against impacts. A typical single car garage roller door has a preloaded spring inside the rolling mechanism. The spring reduces the effort required to open the door.
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