5.3 Types of Building Construction & 3-Digit Hourly Ratings

5.3 Types of Building Construction & 3-Digit Hourly Ratings

The fundamental mission of structural fire protection is to prevent building collapse during a fire, thereby preserving life safety for evacuating occupants and providing structural survivability for interior firefighting operations. While architects often view structural framing through the prism of structural loads, seismic resistance, and spatial aesthetics, the fire plan examiner evaluates structural framing through thermal degradation, combustibility, and hourly fire endurance.

In the NFPA regulatory framework, structural fire performance is standardized in NFPA 220 (Standard on Types of Building Construction). Under NFPA 1030 JPR 8.3.4 and JPR 8.3.4, a fire plan examiner must verify the proposed construction type, analyze the hourly fire-resistance ratings of all structural members, and identify noncombustible, combustible, and fire-retardant-treated wood (FRTW) elements.

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The Five Fundamental Construction Types

NFPA 220 divides all buildings into five basic construction types designated by Roman numerals Type I through Type V:

Type I: Fire-Resistive (Type I-442 and Type I-332)

In Type I construction, all structural elements—including exterior bearing walls, interior bearing walls, columns, beams, girders, floor assemblies, and roof structures—are constructed entirely of noncombustible or limited-combustible materials possessing high fire-resistance ratings. Structural frames typically consist of reinforced cast-in-place concrete, post-tensioned concrete, or structural steel protected by spray-applied fire-resistive materials (SFRM), intumescent coatings, or concrete encasement.

• Type I-442: Demands 4-hour exterior bearing walls, 4-hour structural framing, and 2-hour floor assemblies. Used for high-rise towers, large institutional hospitals, and mega-structures.
• Type I-332: Demands 3-hour exterior bearing walls, 3-hour structural framing, and 2-hour floor assemblies.

Type II: Noncombustible (Type II-222, Type II-111, and Type II-000)

In Type II construction, all structural elements are composed of noncombustible or limited-combustible materials, exactly like Type I, but the required fire-resistance ratings are significantly lower or non-existent.

• Type II-222: Demands 2-hour exterior bearing walls, 2-hour structural framing, and 2-hour floors (equivalent in fire resistance to Type I-332 for floor slabs).
• Type II-111: Demands 1-hour exterior bearing walls, 1-hour structural framing, and 1-hour floors. Often achieved using gypsum board membrane protection or light SFRM on steel beams and columns.
• Type II-000 (Unprotected Noncombustible): Structural elements have zero fire-resistance rating. While structural members will not fuel the fire (noncombustible), bare steel joists, beams, and columns are directly exposed to thermal energy.

Type III: Exterior Protected / Ordinary (Type III-211 and Type III-200)

In Type III construction, the exterior bearing walls must be noncombustible or limited-combustible materials (such as concrete, brick, or concrete masonry units [CMU]), while the interior structural elements, floors, and roofs are permitted to be any material permitted by code, typically combustible wood framing or unprotected steel.

• Type III-211: 2-hour exterior bearing walls with 1-hour protected interior framing and 1-hour floors.
• Type III-200: 2-hour exterior bearing walls with completely unprotected combustible interior framing and floors.

Type IV: Heavy Timber (Type IV-2HH)

Type IV construction—historically termed "Mill Construction" and now designated as Type IV-2HH—features exterior bearing walls constructed of noncombustible materials providing a minimum 2-hour fire-resistance rating. The interior structural framing consists of massive solid-sawn or glued-laminated timber elements without concealed spaces.

• Columns must be at least 8 in. nominal in width and depth when supporting floor loads, and at least 6 in. nominal when supporting roof loads.
• Floor beams and girders must be at least 6 in. nominal in width and 10 in. nominal in depth.
• Floors must consist of solid splined or laminated wood planks at least 3 in. nominal in thickness, covered with 1-in. nominal wood flooring or approved noncombustible topping.
• Roof decks must be at least 2 in. nominal in thickness.

The Science of Heavy Timber Fire Endurance: Heavy timber does not rely on applied gypsum boards or chemical coatings. Instead, it relies on the physics of char formation. When massive timber is exposed to fire, the outer wood chars at a highly predictable rate of approximately 1.5 inches per hour (0.6 mm/min). This carbonized char layer acts as an effective thermal insulator, retarding heat transfer into the inner unburned wood core, which maintains full structural tensile and compressive strength for extended durations.

Type V: Wood Frame (Type V-111 and Type V-000)

In Type V construction, structural elements, exterior walls, and interior framing are permitted to be constructed of any material permitted by code, typically lightweight wood dimensional lumber, wood I-joists, and oriented strand board (OSB).

• Type V-111 (Protected Wood Frame): 1-hour exterior bearing walls, 1-hour structural framing, and 1-hour floor assemblies (typically achieved via 5/8-inch Type X gypsum board membranes).
• Type V-000 (Unprotected Wood Frame): Zero fire-resistance rating. Common in single-family homes and small commercial structures. Highly vulnerable to rapid burn-through and collapse when lightweight engineered wood trusses are exposed to fire.

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Decoding the 3-Digit Hourly Rating System

In NFPA 220, construction types are designated by Roman numerals followed by three Arabic numerals in parentheses (e.g., Type I (442), Type II (111), or Type V (000)). Plan examiners must decode these digits in their strict standardized sequence:

1. First Digit: Hourly fire-resistance rating of exterior bearing walls.
2. Second Digit: Hourly fire-resistance rating of columns, beams, girders, trusses, and arches supporting bearing walls, columns, or more than one floor (the primary structural frame).
3. Third Digit: Hourly fire-resistance rating of floor construction and secondary floor members.

What about the Roof? Roof construction ratings are not embedded in the 3-digit sequence; they are published in an adjacent column in NFPA 220 Table 4.1.1. In Type I-442, the roof requires a 2-hour rating; in Type I-332, a 1.5-hour rating; in Type II-111, a 1-hour rating; and in Type II-000, zero rating.

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Material Classifications and FRTW Allowances

Plan examiners must distinguish between three statutory material definitions in NFPA 220:

• Noncombustible Material: A material that, in the form in which it is used and under the conditions anticipated, will not ignite, burn, support combustion, or release flammable vapors when subjected to fire or heat. Compliance is verified by passing ASTM E136 (Standard Test Method for Behavior of Materials in a Vertical Tube Furnace at 750°C).
• Limited-Combustible Material: A material not complying with noncombustible definitions that has a potential heat value not exceeding 3,500 BTU/lb (8,141 kJ/kg) per NFPA 259, and possesses a flame spread index not exceeding 25 per ASTM E84 / UL 723.
• Fire-Retardant-Treated Wood (FRTW): Wood products pressure-impregnated with fire-retardant chemicals that reduce combustibility. The product must be tested under ASTM E84 / UL 723 extended for a 30-minute period, showing a flame spread index $\le 25$ with no evidence of significant progressive combustion.

Where is FRTW Allowed in Noncombustible Construction?

Because noncombustible construction types (Types I, II, III, and IV) strictly limit combustible elements, the code provides specific exceptions where FRTW may be substituted for noncombustible materials:

1. Exterior Nonbearing Walls: Permitted in Type III and Type IV construction where the fire separation distance is sufficient.
2. Interior Nonbearing Partitions: Permitted in Type I and Type II buildings for interior nonbearing demising partitions requiring a 1-hour or 2-hour rating.
3. Roof Assemblies: Permitted in Type I and Type II buildings where the roof assembly is located more than 20 feet above the floor level immediately below.