Class M buildings are characterized by heavy masonry and timber construction. The walls are generally thick loadbearing brick or nonbearing with open wood columns, trusses or arches supporting the load. Floors are heavy laminated wood or a concrete slab on the ground.
Included in this class are Uniform and Basic Building Code, Type III (noncombustible wall), Standard Code Type V and ISO Classes 2 and 4, and those ISO Class 5 and 6 buildings that have load-bearing walls without interior framing and of low-rise design (3 stories or less). This class is also referred to as Masonry or Unprotected Noncombustible, Joisted or Unprotected Masonry, or Ordinary or Unprotected One-hour, and includes certain Two-hour or heavy timber construction.