A key difference between a single- and a multifamily dwelling is the requirement for an occupancy fire barrier, which is needed to prevent relatively small kitchen or cigarette fires from spreading from apartment to apartment. There are three basic fire barriers you’ll deal with in small multifamily construction.
The most common is the one-hour fire barrier, which resembles the required fire barrier separating an attached garage from the dwelling portion of a house. You can achieve this separation easily by laminating both sides of the occupancy wall with 5/8-inch type X gypsum wallboard from the foundation right up to your roof sheathing, or by creating a one-hour envelope with 5/8-inch type X drywall on the occupancy wall and ceiling. This one-hour occupancy barrier is typical for two-to-four-dwelling buildings built on a single lot with one owner.
Condominiums and apartments with three or more stories or with five or more units typically require at least a two-hour fire barrier. It’s hard to become creative with firewalls, and they are costly to build, but many different approved assemblies already exist.
When there is separate ownership on separate lots, such as with a townhouse or a duplex, you will have to provide a two-hour fire barrier and a structural occupancy separation. The latter consists of two unconnected structural support walls along the common boundary, so that — theoretically — if one half of the building collapsed, the other half would remain intact. This wall assembly typically consists of two one-hour walls built side by side without plumbing or ductwork, with a 1-inch space in between. Furthermore, under certain circumstances, you are required to build a parapet or install fire-resistant roof sheathing extending 5 feet on either side of the property line. These requirements make townhouse construction inherently expensive.
If you build with common corridors, interior stairwells, and other enclosed common areas, such as a laundry room, you will have to address more issues of fire separation. Most of these are greatly simplified with the addition of a fire sprinkler system.
