Design & Tools

Heights and Areas

The allowable height and area of a building is determined by construction type, the level of passive fire resistance provided to the structure and the intended occupancy. Table 503 of the 2012 International Building Code (IBC) describes the relationship between these factors and the allowable height and area of a structure. A building with ample fire access around the perimeter and equipped with a NFPA 13-compliant sprinkler system can expand on these allowances.

Types of Construction:

  • Types I and II construction require building elements constructed of non-combustible materials. However, wood is allowed per 2012 IBC 603.1. 
  • Type III construction requires exterior walls to be noncombustible material or fire retardant treated (FRT) wood having a minimum 2-hour fire-resistance rating; roof, floors and partition walls are allowed to be wood.
  • Type IV construction is a special type of construction, heavy timber, in which wall, roof and floor elements have prescriptive size requirements and there is a limitation for concealed spaces. The exterior walls can be either non-combustible material or FRT wood similar to Type III.
  • Type V construction allows the entire structure to be of wood.

Levels of fire resistance:

  • “A” indicates that the structure has minimum 1-hour fire-resistance rating requirements for structural elements. Because of these requirements, protected structures are permitted to have an increased number of stories, greater overall building height and larger building footprint.
  • “B” indicates that the structure requires little to no fire-resistance ratings for structural elements (e.g., none in Type V construction and only for exterior load-bearing walls in Type III). This type of construction can be an advantage because designers can often incorporate exposed light-framing members.