Ch.4 - Construction Types and Building Sizes

Delia Tacheny
INT 324 - Building Codes
Reading: The Codes Guidebook for Interiors

Keywords:

• building elements
• construction types
• Noncombustible 
• Limited Combustible
• Combustible 
• building sizes

In the similar way that buildings are classified according to occupancy, buildings are classified according to construction; in fact, occupancy classification and construction types are related, as I'll mention later. 

Every building is made of "building elements" which may be structural or not -- elements such as foundations, bearing walls, roofs, ceilings/ floors, columns, and beams. Additional non-structural building elements may include partitions, sheathing, doors, windows, etc.; various finishes and materials play a role in building construction, which are more within the realm of interior design. Nevertheless, all of these building elements determine the construction type. So what exactly IS a construction type?

Both the ICC and NFPA refer to the general construction types as Type I, Type II, Type III, Type IV, and Type V (though some requirements within these types may vary between codes). The purpose of construction types primarily relates to fire-rating. The construction types dictate material requirements... so before we get into discussing each type... let's discuss different types of materials:

By code, building materials are categorized into noncombustible, limited combustible, and combustible -- according to their resistance to burning. Defining factors include how easily the materials ignite, how long they burn once ignited, how quickly the flames spread, and how much heat the material generates. 

Noncombustible - materials that will not ignite, burn, support combustion, or release flammable vapors when subject to fire or heat (Determined either inherently or by passing ASTM E136, Examples include steel, concrete, masonry, and glass)

Combustible - materials that will ignite and continue to burn (Example: wood, though fire-treated wood does exist).

Limited Combustible - certain types of fire-resistant materials... [they] do not meet the requirements of noncombustible material because they do have some capacity to burn (must pass NFPA 259, Examples include treated materials, gypsum board, etc.)

Now that we understand the different types of materials, this will help us classify the different construction types:

Type I: Type I buildings are the highest level of fire-rating usually constructed out of all noncombustible materials -- typically supported by steel or concrete. High-rises, and large buildings with high occupancies are usually within this category. 

Type II: (Similar to Type I with some varying restrictions)

Type III: This type uses a mix of combustible and non-combustible materials -- typically the exterior utilizes noncombustible materials while the interior incorporates wood structural elements. Type III buildings may be small/medium office or retail spaces, as well as urban buildings, where spread of fire from building to building is more of a concern. 

Type IV: This type of construction is often referred to as mass/ heavy timber where the structural elements are made of wood that has a minimum cross section and density that provides more strength and fire resistance than the typical wood construction. This system is popularly seen in historic buildings and is also referred to as mill construction. 

Type V: This is the most combustible construction type in which buildings are constructed of completely wood elements. Examples include residential homes, small offices, and convenience stores.

Construction types can also be broken up into A (protected) or B (unprotected). Simply put, "protected" simply implies more fire-resistant measure. If a building has a mixed construction type system, the type is usually classified to the least resistant material. Construction types play in to building sizes and allowances. Varying types have restrictions on the amount of square footage and number of stories a building is allowed to have. However, these allowances also depend on factors such as if there is a sprinkler system or not. All of this information is better explained and laid out in graphical tables which can easily be looked up (similar to the one shown below).

Occupancy comes into play as well, because obviously buildings with more people, should probably be safer and more-fire restrictive. A hospital should not be a Type V construction type, in the same way that a single-family household does not need to be constructed of steel and concrete. It's always important to consider both occupancy as well as sustainability (which may also affect material choice). 

Construction type is honestly fairly simple to understand, but EXTREMELY important to be aware of in construction, architecture, and interior design. Hopefully this blog laid out the basics in a helpful way!

Source(s):

Kennon, Katherine E., and Sharon Koomen Harmon. “Ch.4: Construction Types and Building Sizes.” The Codes Guidebook for Interiors, John Wiley & Sons, Inc., Hoboken, NJ, 2022.