3.2 Construction Types (Types I-V) & Fire-Resistance Ratings

3.2 Construction Types (Types I-V) & Fire-Resistance Ratings

[!NOTE] The Philosophy of Structural Survivability: IBC Chapter 6 establishes five basic construction types (Types I through V) calibrated to prevent catastrophic structural collapse during a fire, limit thermal transmission between spaces, and provide sufficient structural integrity for occupant evacuation and firefighting operations. Construction types are divided into two fundamental parameters: the combustibility of materials (noncombustible vs. combustible) and the degree of passive fire protection applied to building elements (protected Subtype A vs. unprotected Subtype B).

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Combustibility Standards: ASTM E136 & Fire-Retardant-Treated Wood (FRTW)

A primary axis of building classification is whether structural elements are composed of noncombustible or combustible materials.

Noncombustible Materials and ASTM E136

Under IBC Section 703.3, a material is classified as noncombustible only if it successfully passes testing in accordance with ASTM E136 (Standard Test Method for Assessing Combustibility of Materials in a Vertical Tube Furnace at 750°C). In this test, material specimens are subjected to a furnace temperature of 1,382°F (750°C). To pass, the material must not ignite, flame, exhibit significant temperature rise, or lose excessive mass.

• Noncombustible Materials: Reinforced concrete, structural steel, concrete masonry units (CMU), brick, stone, and structural iron.
• Combustible Materials: Dimension lumber, engineered wood products (glulam, LVL, CLT), wood structural panels (plywood, OSB), architectural plastics, and paper-faced gypsum products.

Fire-Retardant-Treated Wood (FRTW)

Fire-retardant-treated wood is lumber or wood structural panels pressure-impregnated with fire-retardant chemicals that reduce surface flame spread and rate of fuel contribution. To qualify under IBC Section 2303.2, FRTW must achieve a flame spread index of 25 or less when tested in accordance with ASTM E84 or UL 723, with no evidence of significant progressive combustion when the test is continued for an additional 20-minute period.

+--------------------------------------------------------------------------------+
|                       FRTW Permitted Applications in IBC                       |
+--------------------------------------------------------------------------------+
| Type III Construction | Permitted in exterior bearing and nonbearing walls     |
| Type IV Construction  | Permitted in exterior bearing and nonbearing walls     |
| Type I & II Buildings | Permitted in nonbearing interior partitions (IBC 603)  |
| Type I & II Roofs     | Permitted in roof framing >20 ft above floor (IBC 603) |
+--------------------------------------------------------------------------------+

[!WARNING] FRTW Structural Limitations: Although FRTW is permitted in exterior walls of Type III and Type IV construction, it is never permitted to replace primary structural frame members (such as interior columns or transfer girders) in Type I-A or Type I-B construction. Treating wood with fire retardants reduces its combustibility, but it does not convert wood into a noncombustible material under ASTM E136.

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The Five Construction Types Breakdown (IBC Chapter 6)

    [ Most Fire-Resistive / Noncombustible ]
                      │
                  Type I-A  ── 3-hr Structural Frame, 2-hr Floors
                  Type I-B  ── 2-hr Structural Frame, 2-hr Floors
                  Type II-A ── 1-hr Protected Noncombustible
                  Type II-B ── 0-hr Unprotected Noncombustible (Steel)
                  Type III-A── Noncombustible Exterior / 1-hr Protected Interior
                  Type III-B── Noncombustible Exterior / 0-hr Unprotected Interior
                  Type IV-HT── Heavy Timber (8x8 cols, 6x10 beams, no hollows)
                  Type V-A  ── 1-hr Protected Wood Light-Frame
                  Type V-B  ── 0-hr Unprotected Wood Light-Frame
                      │
       [ Least Fire-Resistive / Combustible ]

1. Type I Construction (Fire-Resistive / Noncombustible Protected)

All building elements in Type I construction must be constructed of approved noncombustible materials. Type I structures represent the highest level of passive fire resistance and are mandatory for high-rise towers, large regional hospitals, and massive public assembly halls.

• Type I-A: The primary structural frame, exterior bearing walls, and interior bearing walls must have a minimum 3-hour fire-resistance rating. Floor construction and secondary members require a 2-hour rating, while roof construction requires a 1.5-hour rating. Type I-A buildings receive unlimited allowable height and unlimited floor area for most occupancy groups.
• Type I-B: The primary structural frame, exterior bearing walls, interior bearing walls, and floor construction require a 2-hour fire-resistance rating. Roof construction requires a 1-hour rating. Typical of mid-to-high-rise residential towers and major commercial offices.

2. Type II Construction (Noncombustible)

Building elements in Type II construction must be composed of noncombustible materials, but require lower passive protection ratings than Type I.

• Type II-A (Protected): Primary structural frame, bearing walls, floor assemblies, and roof assemblies require a minimum 1-hour fire-resistance rating. Steel framing is typically protected with spray-applied fire-resistive materials (SFRM), intumescent coatings, or gypsum board encasement.
• Type II-B (Unprotected): Primary structural frame, bearing walls, floors, and roofs have a 0-hour fire-resistance rating. Structural steel columns, open-web steel joists, and metal roof decks remain completely exposed. Widely utilized in single-story big-box retail stores, suburban warehouses, and light commercial facilities.

3. Type III Construction (Ordinary Construction)

Type III construction, historically referred to as "ordinary construction" or "brick-and-joist," requires noncombustible exterior walls (concrete, masonry, or permitted FRTW framing). Interior structural elements (columns, beams, floors, and roofs) may be constructed of any material permitted by the code, including dimension lumber, light-gauge steel, or heavy timber.

• Type III-A (Protected): Exterior bearing walls require a 2-hour rating. Interior structural frame, interior bearing walls, floors, and roofs require a 1-hour fire-resistance rating.
• Type III-B (Unprotected): Exterior bearing walls require a 2-hour rating. Interior structural elements, floors, and roofs have a 0-hour rating. Seen in traditional urban main street commercial buildings with masonry perimeter walls and wood floor framing.

4. Type IV Construction (Heavy Timber & Mass Timber)

Type IV construction relies on large-mass timber members whose structural survivability is derived from the charring mechanics of heavy wood. When exposed to fire, timber chars at a predictable rate (approximately 1.5 inches per hour), creating an insulating carbon char layer that protects the interior structural wood core.

Traditional Heavy Timber (Type IV-HT) Minimum Dimensions

Under IBC Section 2304.11 and Table 601, wood members in Type IV-HT must meet strict nominal dimensional minimums:

• Columns Supporting Floor Loads: Minimum 8" x 8" nominal (solid sawn or glued-laminated timber).
• Columns Supporting Roof/Ceiling Loads Only: Minimum 6" x 8" nominal.
• Beams and Girders Supporting Floors: Minimum 6" nominal width by 10" nominal depth.
• Beams and Girders Supporting Roofs Only: Minimum 4" nominal width by 6" nominal depth.
• Floor Decking: Minimum 3" nominal splined or tongue-and-groove planks, overlaid with 1" nominal finish wood flooring or 15/32" wood structural panels.
• Roof Decking: Minimum 2" nominal splined or tongue-and-groove planks, or 1-1/8" structural glued-laminated timber decking.
• Prohibition of Concealed Spaces: Traditional Type IV-HT prohibits hollow concealed spaces within walls, floors, and partitions unless fully protected by an approved automatic sprinkler system or fireblocked with noncombustible materials.

Mass Timber Subtypes in IBC 2021 (IV-A, IV-B, IV-C)

The 2021 IBC introduced three advanced mass timber construction classifications utilizing Cross-Laminated Timber (CLT) and Glulam:

• Type IV-A: Highest allowable mass timber height (up to 18 stories / 270 ft). Primary structural frame requires a 3-hour rating, and 100% of mass timber surfaces must be protected by noncombustible gypsum board encasements.
• Type IV-B: Mid-rise mass timber (up to 12 stories / 180 ft). Primary structural frame requires a 2-hour rating. Limited exposed mass timber is permitted; under IBC 602.4.2.2 the unprotected portions are measured as a percentage of the floor area of the dwelling unit or fire area — not as a percentage of the ceiling or wall itself (broadly, unprotected ceilings up to 20% of the floor area, or unprotected walls up to 40% of the floor area, with a lower combined allowance when both are exposed).
• Type IV-C: Low-rise mass timber (up to 9 stories / 85 ft). Primary structural frame requires a 2-hour rating, and interior mass timber may be fully exposed (0% noncombustible protection required), achieving fire resistance through sacrificial char depth.

5. Type V Construction (Wood Light-Frame / Any Permitted Materials)

Type V structures permit structural elements, exterior walls, and interior walls to be built of any material permitted by code, universally implemented as wood light-frame stud walls, wood I-joists, and light wood roof trusses.

• Type V-A (Protected): Primary structural frame, exterior bearing walls, interior bearing walls, floors, and roofs require a minimum 1-hour fire-resistance rating, typically achieved with 5/8" Type X gypsum board assemblies.
• Type V-B (Unprotected): All building elements have a 0-hour fire-resistance rating (except exterior walls governed by Fire Separation Distance). Common in single-family residential homes and small detached commercial buildings.

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IBC Table 601: Fire-Resistance Rating Requirements for Building Elements

IBC Table 601 establishes the baseline fire-resistance ratings (in hours) for every primary building element across all construction types:

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IBC Table 705.5: Exterior Wall Fire-Resistance Ratings & Fire Separation Distance

While Table 601 establishes ratings based on internal structural requirements, IBC Table 705.5 establishes fire-resistance ratings for exterior walls based on their exposure to external risks, measured by Fire Separation Distance (FSD).

[!IMPORTANT] The Renumbering Trap: In the 2018 IBC and earlier editions this table was numbered Table 602 and lived in Chapter 6. In the 2021 IBC there is no Table 602 — Section 602.1 simply states that exterior walls "shall have a fire-resistance rating not less than that specified in Table 705.5," and the table itself sits in Chapter 7. Prep material written against older editions still says "Table 602." Both refer to the same requirement; cite Table 705.5 when working from the 2021 IBC.

                 [ Centerline of Street / Alley ]
                                │
                                │  <── Fire Separation Distance (FSD) ──>
                                │
                         [ Property Line ]
                                │
                                │  <── Fire Separation Distance (FSD) ──>
                                │
                       [ Exterior Building Face ]

Fire Separation Distance is the distance measured at right angles from the building face to the closest interior lot line, to the centerline of an adjacent street, alley, or public way, or to an imaginary line between two buildings on the same parcel.

Note the four distance bands — there is no 15-foot breakpoint. Within the 5–10 ft and 10–30 ft bands the required rating also depends on the construction type, and walls with an FSD of 10 feet or less must be rated for exposure to fire from both sides, while walls farther than 10 feet are rated for exposure from the inside only (IBC 705.5).

[!TIP] Exterior Wall Openings (IBC Table 705.8): The percentage of allowable unprotected and protected exterior wall openings (windows and doors) is directly proportional to Fire Separation Distance. At an FSD of less than 3 feet, no openings are permitted in exterior walls. Between 3 feet and 5 feet, only protected openings (fire-rated glass, fire dampers) are permitted, capped at small surface area percentages.