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A parapet wall is a low wall, usually enclosing a roof, or a protective barrier at the edge of a terrace or on the side of a bridge. In modern use, one is constructed to provide a barrier to prevent people or objects from falling from the edge of the structure and to slow the spread of fire; in earlier times, these walls were built to afford some protection for rooftop defenders of a structure, such as a castle or city walls, under attack.
There are several types of parapets. One of the most commonly known types is the embattled parapet, or battlement, which is crenelated — that is, it’s built with alternating high and low portions. Defenders would use the high portions for protection, moving to the low sections just long enough to fire their weapons. A perforated parapet is similar, but instead of being “notched” in appearance, the wall is pierced with various shapes, such as circles, trefoils, or quatrefoils, either for decorative or defensive purposes. Paneled walls have ornamental panels facing outward, often carved stone. Plain parapet walls have none of the decorations or features of the other parapets, but may have copings or even corbels, depending on their construction.
While parapets originated in the need for defensive construction atop structures, their use has continued into the modern day, but they defend more from accidents and fire than from attackers. In the evolution of the modern city, for example, what were once attractive features of free-standing houses developed into hazards in densely populated areas. In the city of London, overhanging eaves on houses are considered a fire hazard, and since 1707, roofs must by law be enclosed within a parapet wall.
Most modern flat-roofed construction, whether residential or otherwise, includes these walls at the edges as a matter of safety, both to prevent people from accidentally falling off the roof and to prevent accumulated debris from falling off and possibly injuring people below. Many modern fire codes also require that firewalls extend above a structure’s roof-line by a minimum of 30 inches (76.2 centimeters), and that the wall created be at least as fire resistant as the portion below the roof. Walls of some sort are also found on most bridges, constructed for safety reasons. Where pedestrian travel is commonplace, some bridges’ parapet walls even have handrails installed.