Section 2.2: Construction Types and Height/Area Limits

Building Construction Types and Height/Area Limitations

An essential duty of the fire plans examiner is verifying that the proposed construction type complies with the fire-resistance requirements and the height and area limits established by the building code. Chapter 6 of the 2024 International Building Code (IBC) classifies buildings into five distinct construction types (Types I through V), which are further divided into subcategories "A" and "B." This classification is based on the combustibility of the materials used and the fire-resistance rating of the primary structural elements.

Defining the Five Construction Types (IBC Section 602)

• Type I (Noncombustible, Protected): All structural elements—including the structural frame, exterior bearing walls, interior bearing walls, and floor/roof construction—must be constructed of noncombustible materials (steel, concrete, masonry). Type I-A requires a 3-hour fire-resistance rating for the structural frame and bearing walls, representing the most robust construction type. Type I-B requires a 2-hour rating.
• Type II (Noncombustible, Unprotected/Protected): Similar to Type I, all structural elements must be constructed of noncombustible materials. However, the fire-resistance ratings are lower. Type II-A requires a 1-hour rating for structural elements, while Type II-B is unprotected (0-hour rating), meaning bare structural steel may be used.
• Type III (Combustible/Noncombustible): Often called "ordinary construction," Type III buildings feature exterior walls constructed of noncombustible materials (masonry, concrete) to prevent exterior fire spread, while the interior structural elements are permitted to be of combustible materials (wood framing). Type III-A requires a 1-hour rating for structural elements, while Type III-B is unprotected.
• Type IV (Heavy Timber - HT): Type IV construction requires exterior walls to be constructed of noncombustible materials, while the interior structural elements are made of solid or laminated wood without concealed spaces. The large mass of heavy timber elements provides inherent fire resistance, as the outer charred layer insulates the inner wood.
• Type V (Combustible): Type V construction allows all structural elements to be constructed of combustible wood framing (standard stick-frame construction). Type V-A requires a 1-hour rating for all elements, while Type V-B is unprotected.

Fire-Resistance Ratings of Building Elements (IBC Table 601)

IBC Table 601 is the master matrix used by plans examiners to verify the hourly fire-resistance ratings of structural frames, bearing walls, and floor/roof construction. For instance, in a Type III-A building, the floor construction must have a 1-hour fire-resistance rating, whereas in a Type III-B building, it requires a 0-hour rating. Additionally, the plans examiner must cross-reference IBC Table 602 to determine the required fire-resistance rating of exterior walls based on the building's Fire Separation Distance (FSD). FSD is the distance measured from the building face to the closest interior property line, to the centerline of a street or public way, or to an imaginary line between two buildings on the same property. As the FSD decreases, the risk of fire spreading to adjacent structures increases. Therefore, Table 602 requires higher fire ratings for exterior walls as the FSD decreases. For example, a Group M building of Type II-B construction with an FSD of less than 5 feet requires a 2-hour rated exterior wall, whereas a building with an FSD of 30 feet or more requires a 0-hour rating.

Allowable Building Heights and Areas (IBC Chapter 5)

Once the occupancy group and construction type are verified, the plans examiner uses the tables in Chapter 5 to determine if the building's size is within code limits.

• Table 504.3: Dictates the maximum allowable height in feet.
• Table 504.4: Dictates the maximum allowable stories above grade plane.
• Table 506.2: Dictates the baseline allowable area per story (referred to as $A_t$).

These tables contain three separate rows for each occupancy group:

1. NS (Non-Sprinklered): Baseline allowances.
2. S (Sprinklered): Allowances when the building is protected by an NFPA 13 automatic sprinkler system.
3. SM (Sprinklered Multi-story): Allowances when the building is protected by an NFPA 13R sprinkler system (limited to Group R occupancies up to 4 stories).

Equipping a building with an automatic sprinkler system yields substantial increases. For example, a Group B office building of Type III-B construction has a baseline story limit of 3 stories and a height limit of 55 feet under the NS row. If it is fully sprinklered under NFPA 13 (S row), the limits increase to 4 stories and 75 feet, and the allowable area per story increases from 19,000 square feet to 57,000 square feet (a 200% increase).

Frontage Area Increases (IBC Section 506.3)

If a building is surrounded by open space, yards, or public streets, it receives an area increase because the fire department has better access to perform firefighting and rescue operations, and the risk of exposure fire spread is reduced. The frontage increase ($I_f$) is calculated using the following formula:

$I_f = (F/P - 0.25) \times W / 30$

Where:

• $I_f$ = Area increase factor (expressed as a decimal).
• $F$ = The portion of the building perimeter that faces a public way or open space with a minimum width of 20 feet.
• $P$ = The total perimeter of the building.
• $W$ = The width of the public way or open space. The value of $W$ must be at least 20 feet and is capped at 30 feet for the calculation.

Worked Example: Frontage Increase Calculation

A client submits plans for a 2-story Group B building of Type V-A construction. The total perimeter ($P$) of the building is 600 feet. The building is positioned such that:

• The front side (150 feet) faces a public street that is 40 feet wide.
• The right side (150 feet) faces an open parking lot that is 30 feet wide.
• The rear and left sides (300 feet total) are located 5 feet from the property line.

Step 1: Determine the qualifying frontage ($F$) Only open space with a width of 20 feet or more qualifies. The front street (40 feet wide) and the right parking lot (30 feet wide) qualify. The rear and left sides do not qualify because their separation distance is less than 20 feet. $F = 150 + 150 = 300\text{ feet}$

Step 2: Calculate the frontage ratio $\text{Ratio} = F/P = 300 / 600 = 0.50$ Since this ratio is greater than 0.25, the building is eligible for a frontage increase.

Step 3: Determine the average width of open space ($W$) The street width is 40 feet, which is capped at 30 feet for the formula. The parking lot width is 30 feet. $W = (30\text{ (street)} + 30\text{ (parking lot)}) / 2 = 30\text{ feet}$

Step 4: Calculate the frontage increase factor ($I_f$) $I_f = (0.50 - 0.25) \times 30 / 30 = 0.25 \times 1.0 = 0.25$

If the baseline allowable area ($A_t$) from Table 506.2 is 10,000 square feet, the frontage increase allows an additional 2,500 square feet ($0.25 \times 10,000$) per story, bringing the total allowable area per story to 12,500 square feet. If the building is sprinklered, this increase is applied to the already-increased sprinklered area.

Master Construction Types and Element Fire-Resistance Summary (IBC Table 601)