In urban and suburban areas, it is quite common to have two buildings directly adjacent to each other that share a wall. In different regions of the country, this wall is commonly referred to as a "party wall," an "area separation wall," or a "fire wall." However, though the term "firewall" is arguably more all-encompassing and self-evident, in the context of single-family residential construction, the term "area separation wall" best describes the fire-resistance rated wall between townhouses, and is the term used by the gypsum industry for this type of building system.
An area separation wall used to separate townhouse units is almost always required by model building codes to withstand exposure to fire for two hours. It must also be designed so that in the event of a fire, it will permit the collapse of the burning building while leaving the adjacent building protected and intact. To further prevent the spread of fire from one building to the next, model codes typically require that an area separation wall rests on the foundation and extends 30 inches beyond the roof as a parapet.