Combustible v Non-combustible Construction

To provide a safe building design, understanding the difference between combustible and non-combustible building materials is crucial.

Combustible Materials and Safe Building Design

Every building must be classified as one of five regulated construction types, and each type of construction deals with, among other things, the allowance for combustible materials, or the requirement to use non-combustible materials. Because one of the primary aspects of safe building design is accounting for how a fire will affect the building, it only makes sense that the code regulates the use of combustible versus non-combustible elements in a building.

The commentary on the building code makes this clear. In the commentary to chapter 6 (under the 2018 International Building Code), we read, “The purpose of classifying buildings or structures by their type of construction is to account for the response or participation that a building’s structure will have in a fire condition originating within the building as a result of its occupancy or fuel load. The code requires every building to be classified as one of five possible types of construction: Type I, II, Ill, IV, or V. Each type of construction denotes the kinds of materials that are permitted to be used [i.e., non-combustible steel, concrete, masonry, combustible (wood, plastic) or heavy timber (HT)], and the minimum fire-resistance ratings that are associated with the structural elements in a building having that classification (i.e., 0, 1, 1-1/2, 2 or 3 hours).” We see then, that in addition to establishing a fire-resistance rating, we also need to ensure the material itself is classified appropriately to its combustibility. As it relates to building safety, non-combustible buildings are safer than combustible buildings.

Types of Construction

Types 1 & 2 construction are classified as non-combustible buildings, where the primary building elements of Table 601 are of non-combustible materials. Types 3 & 4 construction include provisions for both non-combustible and combustible building elements (for example, Type 3 construction requires the exterior walls to be non-combustible, but interior building elements can be combustible), while Type 5 construction can be fully combustible. “Stick frame” construction with wood studs and wood sheathing would be considered Type 5 construction, while a building fully constructed of steel and concrete would likely be either Type 1 or 2. The code limits the size of combustible buildings to be smaller in height and area than non-combustible buildings. This makes sense because safer buildings should be allowed to be bigger than less safe buildings. The building code grants increases in height and area when non-combustible construction types are specified.

Combustible Material Exceptions

There are exceptions to the non-combustibility requirements, found in Section 603 of the IBC. One of these exceptions allows for the use of fire-retardant-treated (FRT) wood in non-combustible buildings, as a part of non-loadbearing partitions, non-rated exterior walls, roof construction, or balconies/porches/decks less than 3 stories above grade. It is important to realize that although the code allows for the selective use of FRT wood in non-combustible buildings, FRT wood is still technically considered combustible, otherwise, it wouldn’t be listed as a “combustible material” allowed in Type 1 and 2 construction.

In Conclusion

Ultimately, as a designer of record, understanding the difference between combustible and non-combustible building materials is crucial for a compliant building design. Designing or specifying combustible materials where not allowed in buildings where increases in height and/or area have been granted due to classifying the building as non-combustible can have a devastating impact on the final construction. Per the code commentary: “The aim is to reduce risk of injury to an acceptable level for building occupants by limiting fire load and fire hazards relating to height, area, and occupancy.” If you are a licensed architect, trusted to safeguard the health, safety, and welfare of the public, you don’t want to be responsible for designing a building with an unacceptable level of safety.