Types of Construction and Material Combustibility
It is important to understand how a building will perform in a fire. Minimum construction requirements are established to help maintain structural integrity for the time needed for evacuation or relocation to a safe location in the building. The combustibility of a material gives an indication of how quickly a fire will grow. Both of these aspects are essential to fire and life safety.
NFPA 220, Standard on Types of Building Construction, defines types of building construction based on the combustibility and the fire resistance rating of a building's structural elements. When we talk about fire resistance rating, we mean the time, in minutes or hours, that materials or assemblies have withstood a fire exposure as determined by specific tests.
NFPA 101 requires certain occupancies to meet minimum construction requirements, which can be found in section 1, subsection 6 of any of the occupancy chapter (XX.1.6). NFPA 101 isn’t the only code that specifies minimum construction types, other codes, such as a building code will also specify minimum construction types. Often times the type of construction that the building is permitted to be made out of correlates to how many stories the building will have and whether or not the building will have sprinklers installed.
NFPA Construction Types
NFPA 220 breaks down building construction into five different types which relate to the material, each one of these types is numbered one through five (in roman numerals). When codes and standards refer to the type of construction required or permitted there are three numbers in parenthesis that follow the type of construction. These numbers indicate the fire resistance rating in hours of different structural elements that are required. The image below gives an example of how you might see this rating in a document and explains the different types as well as the following numbers.
- Type I: Noncombustible (or limited-combustible) construction with a high level of fire resistance, typically concrete construction.
- Type II: Noncombustible (or limited-combustible) construction with a lower level of fire resistance than Type I, typically this is steel construction with or without fireproofing.
- Type III: Exterior walls and structural elements are noncombustible or limited-combustible materials, and interior structural elements, walls, arches, floors, and roofs are wood that is smaller than what is required for Type IV construction. This is usually called ordinary construction and an example of this is a mixed masonry/wood building.
- Type IV: Fire walls, exterior walls, and interior bearing walls are approved noncombustible or limited-combustible materials. Other interior structural elements, arches, floors, and roofs are solid or laminated wood or cross-laminated timber. There are certain dimensional requirements:
- Columns – 8in (205mm) x 8in (205mm) if supporting floor, 6in (150mm) x 8in (205mm) if supporting roof
- Beams – 6in (150mm) x 10in (255mm) if supporting floor, 4in x 6in (150mm) if supporting roof
- Arches – Varies 8in (205mm) x 8in (205mm) to 4in (100mm) x 6in (150mm)
- Floors – 3in (75mm) or 4in (100mm) thick
- Type V: Structural elements, walls, arches, floors, and roofs are wood or other approved material. Most residential construction is Type V.
- First Digit (X00): Exterior bearing walls
- Second Digit (0X0): Columns, beams, girders, trusses and arches, supporting bearing walls, columns or loads from more than one floor.
- Third Digit (00X): Floor construction
Outside of the construction type and fire resistance rating of the structural elements there are also different designations for what is considered a combustible material, limited combustible material and noncombustible material.
Materials that pass the criteria in ASTM E136 when tested in accordance with either ASTM E136 or ASTM E2652 are considered noncombustible. Also, any inherently noncombustible materials can be considered noncombustible without having to be tested. Although the standard doesn’t explicitly say exactly what is inherently noncombustible the associated annex material goes on to suggest that it consists of materials such as concrete, masonry, glass and steel.
Limited Combustible Material
Material that is considered limited combustible needs to meet certain criteria.
- It needs to be able to produce a heat value less than 3,500 BTU/lb when tested in accordance with NFPA 259. (For context paper has a heat value of approximately 7,000 BTU/lb, wood is about 10,000 BTU/lb while most plastics are in the 15,000 to 22,000 BTU/lb range)
- Tested in accordance with ASTM E2965 at an incident heat flux of 75kW/m2 for 20 minutes and meet the following conditions.
a. Peak heat release rate doesn’t exceed 150kW/m2 for more than 10 seconds
b. Total heat released is less than 8MJ/m2
- Either one of the following
a. Material has a noncombustible base with a surface that doesn’t have a flame spread index greater than 50 when tested in accordance with ASTM E84. The surface ontop of the noncombustible base can’t be thicker than 1/8th inch (3.2mm)
b. Flame spread index is less than 25 when tested with ASTM E84 or UL 723, even if the material is cut.
An example of a limited combustible material is gypsum wallboard.
Defining combustible materials is done so by process of elimination. If the materials don’t meet the definition of limited-combustible or noncombustible then it is a combustible material. A common example of a combustible material is untreated wood.
Ensuring a building remains structurally sound and that materials react to fire predictably is important to overall life safety. Understanding and complying with construction type requirements is the first step in creating a safe built environment. We gave some common examples of each type of construction, what are some other examples? Let me know in the comments below.