Building size and construction type

Once the occupancy is known, the next question is how big and how tall the building may be, and what it must be built of. Codes link three things: the occupancy, the size of the building (its height in stories and meters or feet, and its area) and the construction type, which describes how combustible the structure is and how long its main elements must resist fire. Sprinklers are the fourth factor, because a sprinklered building is allowed to be larger or less fire-resistant than an unsprinklered one. Getting this combination right is one of the most consequential decisions in a building's design, because it fixes the structural system, the fire-resistance ratings of floors, roofs and walls, and much of the cost.

Construction managers meet these rules in preconstruction cost studies, value engineering and product substitutions. A proposal to change from steel to wood framing, to drop the sprinkler system or to add a story all depend on them. This lesson explains how the code sets size limits, defines construction types and treats mass timber, and how firewalls divide a large building into smaller ones for code purposes.

Measuring building size

Codes measure size with specific definitions, and the results can differ from the area on a real estate plan.

• Building area, in the IBC, is the area included within surrounding exterior walls, or exterior walls and fire walls, exclusive of vent shafts and courts. Allowable area limits in IBC Chapter 5 apply to each story, and total building area is limited to a multiple of the allowable area per story.
• Building height, in the IBC, is measured in feet from grade plane to the average height of the highest roof surface, and the number of stories above grade plane is limited separately. A basement is not a story above grade plane if its floor above is less than 6 ft above grade plane, or not more than 12 ft above finished ground at any point.
• Grade plane is a reference plane representing the average of finished ground level adjoining the building at exterior walls, measured at the lowest points within 6 ft of the building or the property line if closer.

These definitions matter on sloping sites, where a basement may be partly above ground on one side, and on buildings with mezzanines, which may or may not count as stories depending on their size and openness. A construction manager who adds a mezzanine for storage during construction may unknowingly add a story for code purposes.

Types of construction in the IBC

IBC Chapter 6 defines five types of construction, numbered I to V, several divided into A and B. Type A is the protected version, with higher fire-resistance ratings, and Type B is unprotected or less protected. Table 601 sets the required fire-resistance ratings in hours for the structural frame, bearing walls, floors and roofs for each type.

• Type I (A and B): noncombustible materials with the highest ratings, typically 3 hours for the frame in Type IA and 2 hours in IB. Used for high-rises, hospitals and large buildings.
• Type II (A and B): noncombustible materials with lower ratings; Type IIB has no required rating for most elements. Common for steel-framed stores, warehouses and offices.
• Type III (A and B): noncombustible exterior walls and any permitted material, usually wood, inside. Common for low-rise apartments and mixed-use buildings.

Two further types complete the system.

Type IV covers heavy timber (Type IV-HT) and, since the 2021 IBC, three mass timber types: IV-A, IV-B and IV-C, which allow tall wood buildings of cross-laminated timber and glued-laminated members with different amounts of noncombustible protection. Type V (A and B) allows any material permitted by the code, typically light wood framing; Type VA has 1-hour ratings and Type VB none. Type VB is the most common construction for small commercial buildings and houses.

Allowable height and area

The IBC sets allowable height and area in Chapter 5 using three tables. Table 504.3 gives the maximum height in feet above grade plane, Table 504.4 the maximum number of stories and Table 506.2 the allowable area per story, each for every occupancy and construction type, with separate values for nonsprinklered (NS) buildings and sprinklered buildings: S1 for one-story and S13R for some residential buildings, SM for multistory. Sprinklered buildings may generally be 20 ft taller and one story more than nonsprinklered ones, and their allowable area per story is typically three to four times larger.

Frontage also increases the allowable area. Where a building has more than 25 percent of its perimeter on a public way or open space at least 20 ft wide, Section 506.3 allows an increase factor If = [F/P − 0.25] × W/30, where F is the building perimeter fronting the open space, P is the total perimeter and W is the open space width (capped at 30 ft for the calculation in most cases). The allowable area per story is then the tabular area At plus At × If. For multistory buildings, the total allowable building area is the per-story allowable area multiplied by the number of stories above grade plane, up to a maximum of three for sprinklered buildings or two for nonsprinklered.

Mass timber and tall wood buildings

Mass timber, including cross-laminated timber (CLT), glued-laminated timber (glulam), nail-laminated timber and laminated veneer lumber, has changed what codes allow. Large solid timber members char at a predictable rate in a fire, and the char layer insulates the wood behind it, so mass timber can achieve fire-resistance ratings that light wood framing cannot. Encapsulating the timber with gypsum board delays the onset of charring and reduces the fuel the structure contributes to a fire.

The 2021 IBC introduced three mass timber types. Type IV-A allows up to 18 stories for Group B and R-2 (270 ft maximum height), with full noncombustible protection of the timber. Type IV-B allows up to 12 stories (180 ft) with limited exposed timber, and Type IV-C allows up to 9 stories for Group B and 8 for R-2 (85 ft) with all timber permitted to be exposed. The 2024 IBC increased the amount of exposed timber permitted in Type IV-B. Mass timber types require sprinklers, noncombustible protection installed progressively during construction under IBC Chapter 33 and special inspections of connections and protection.

Fire walls

A firewall divides a building into two or more parts that are treated as separate buildings for the purposes of building area and construction requirements. A firewall must have a high fire-resistance rating and remain standing even if the structure on one side collapses, so it is usually concrete or masonry, extends through the roof or to a parapet, and is designed to resist the lateral forces a collapsing structure would place on it. Firewalls are a common way to build large warehouses, shopping centers and long apartment blocks in cheaper construction types.

In the IBC, Section 706 requires fire walls with ratings from Table 706.4, typically 3 hours for Groups A, B, E, H-4, I, R-1, R-2 and U (2 hours permitted in Types II and V construction), 3 hours for Groups F-1, H-3, H-5, M and S-1, 4 hours for H-1 and H-2, and 2 hours for F-2, S-2, R-3 and R-4. Fire walls must have structural stability so that collapse on either side does not cause the wall to collapse, must extend from the foundation to the roof and usually to a parapet at least 30 in above the roof, and have limited, protected openings.

Construction type in practice

Construction managers deal with construction type every day, often without naming it. A steel deck roof on unprotected steel joists in a Type IIB or noncombustible unrated building is cheap, but adding a story may require protecting the steel with fireproofing or gypsum board. Wood blocking, furring and roofing in a noncombustible building are permitted only within the code's limits. Combustible insulation, plastic piping and cladding containing plastics need specific approval in noncombustible buildings. Substituting a product on site, such as a foam-plastic insulated panel for a mineral-wool panel, can change compliance with the construction type and with exterior wall fire spread rules.

The construction type also drives the fire-resistance ratings that must be maintained throughout construction: rated floor assemblies need listed designs, rated columns need protection to their full height, and penetrations need firestopping. Lesson 6 covers these ratings in detail.