93-Tall-Mass-Timber-Buildings 93-Tall-Mass-Timber-Buildings Questions and Answers

The fire resistance of a protected mass timber element is determined by adding the inherent fire resistance of the mass timber to the protection time contributed by the approved noncombustible protection. Therefore, option D is correct.

The mass timber component provides structural endurance through its large cross section and predictable charring behavior. The noncombustible protection delays heating, ignition, and charring of the protected timber surfaces. Its contribution may be established by testing or by assigned values permitted under IBC Section 722.7. The combined value must equal or exceed the rating required for the building element.

Option A ignores the additional protection contribution. Option B ignores the structural endurance provided by the timber after the protective membrane has performed its assigned function. Option C incorrectly substitutes “finish rating time.” A finish rating measures temperature rise behind a protective membrane and is not automatically the same as the added fire-resistance contribution established for protected mass timber.

Inspection must confirm that the protection system matches the approved calculation or tested configuration, including material type, thickness, layer count, fastening, edge treatment, joints, penetrations, and continuity.

References/Topics: 2021 IBC Sections 703.6, 703.8 and 722.7; protected mass timber performance method and noncombustible protection contribution.

The intended code exception is shafts and vertical exit enclosures , making option B correct. The governing construction-phase provision is 2021 IFC Section 3303.5, which applies to buildings of Types IV-A, IV-B, and IV-C designed to exceed six stories above grade plane.

Where Section 602.4 of the IBC requires noncombustible protection, at least one layer must be installed on building elements located on floor levels—including mezzanines—more than four levels below active mass timber construction before additional floor levels can be erected. The provision is intended to prevent a tall mass timber project from accumulating an excessive number of substantially unprotected combustible stories during erection.

Section 3303.5 expressly states that shafts and vertical exit enclosures are not considered part of the active mass timber construction for application of this construction-progress requirement. Therefore, B is the examination answer.

Mezzanines are expressly included in the protected-level requirement. Required protection for floors, walls, and ceilings under IBC Section 602.4 progresses with construction subject to the IFC provision. The same section also requires standpipes, an approved water supply for fire department operations, and timely installation of required exterior wall coverings.

References/Topics: 2021 IFC Section 3303.5; 2021 IBC Section 602.4; fire safety during construction of tall mass timber buildings.

IBC Table 601, Footnote “a,” permits a one-hour reduction in the required fire-resistance rating of the primary structural frame and bearing walls where those elements support a roof only. Option C is therefore correct.

For Type IV-C construction, the normal Table 601 requirement for bearing walls is two hours. Where a bearing wall qualifies as supporting only roof construction, application of the footnote permits that rating to be reduced from two hours to one hour.

The limitation “supporting a roof only” is critical. The reduction cannot be applied simply because a wall is located at the uppermost occupied level. If the wall carries a floor, mezzanine, transfer load, occupied rooftop structure, or another structural element beyond qualifying roof construction, the condition for the reduction is not satisfied.

Options A and B are not reductions prescribed by Table 601. Option D would effectively eliminate the entire normal two-hour Type IV-C bearing-wall rating, which the footnote does not permit.

For inspection purposes, the approved construction documents and structural load path must substantiate use of the reduction. The fire-protection installation should then be checked against the actual reduced rating specifically authorized by the design.

References/Topics: 2021 IBC Table 601, Footnote “a”; Type IV-C bearing walls supporting roof construction only.

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ICC-ES ESR-1539 is the applicable evaluation report, making option C correct. ESR-1539 addresses nails and staples manufactured by participating companies and evaluates their use in engineered and nonengineered structural connections.

An ICC-ES Evaluation Report provides evidence that a proprietary product or system complies with specified building-code requirements where the product does not rely solely on the generic prescriptive provisions of the IBC. For proprietary nails, the report provides the technical basis needed to establish characteristics such as dimensions, materials, recognized structural applications, allowable design values, installation limitations, and conditions of use.

The current ICC-ES document is published as ESR-1539P, reflecting subsequent administrative updates to the report, but the identifying evaluation-report number remains 1539 , matching option C and the examination question.

For special inspection, merely recognizing the ESR number is not enough. The inspector should verify that the installed proprietary fastener is actually included in the report, that its diameter and length correspond to the approved construction documents, and that installation spacing, penetration, orientation, substrate, and other report conditions are satisfied.

References/Topics: ICC-ES ESR-1539, structural nails and staples; proprietary fastener evaluation and installation requirements. ICC-ES states that the nails and staples covered by ESR-1539 are used in engineered and prescriptive structural connections.

Option A reproduces one of the two permitted exterior gypsum-board fastening methods in 2021 IBC Section 722.7.2.2. Type X gypsum board used as noncombustible protection on the exterior side of mass timber walls may be attached using galvanized nails of minimum 12 gage with a 7/16-inch head and sufficient length to penetrate the mass timber at least 1 inch .

The code also permits ASTM C1002 Type S, W, or G screws, but those screws must likewise penetrate the mass timber a minimum of 1 inch . Options C and D are therefore incorrect because they specify only 3/4-inch and 7/8-inch penetration, respectively. Option B is incorrect because it substitutes 16-gage nails for the minimum 12-gage requirement.

Section 722.7.2.2 additionally requires exterior Type X gypsum-board layers to be fastened 12 inches on center each way and 6 inches on center at joints or ends. Panel-edge fasteners must be located at least 1 inch and not more than 2 inches from the edge.

These fastening details are part of the conditions necessary to claim the protection contribution in Table 722.7.1(2).

References/Topics: 2021 IBC Sections 722.7.1 and 722.7.2.2; exterior Type X gypsum-board protection.

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A separate inspection by the building official is required where the fire resistance of a structural connection in Type IV-A, IV-B, or IV-C construction is provided by wood cover calculated in accordance with Section 2304.10.1. Therefore, option C is correct.

The required inspection occurs after the calculated wood cover has been installed but before additional coverings or finishes conceal the connection. This sequencing allows the building official to verify the actual cover thickness, geometry, continuity, fit, fastening, and correspondence with the approved fire-resistance design.

The requirement is distinct from routine Chapter 17 special inspection. Although a qualified special inspector may observe connection installation and mass timber erection, Section 110.3.5 expressly identifies a required building-department inspection for this calculated wood-cover condition. The inspection cannot be bypassed merely because the connection was observed during structural installation.

A rated joint system under Section 715 is governed by its tested or listed assembly requirements but is not the circumstance identified by this specific inspection provision. The other options do not state the code trigger. Inspectors should therefore coordinate notification before the connection is enclosed because subsequent destructive exposure may otherwise be necessary.

References/Topics: 2021 IBC Sections 110.3.5 and 2304.10.1; inspection of calculated wood cover protecting mass timber connections.

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The highlighted exposed surfaces in this figure are mass timber wall surfaces . Under 2021 IBC Section 602.4.2.2.2, Exception 1.2, unprotected portions of Type IV-B mass timber walls, including attached columns, are permitted up to an area equal to 40 percent of the floor area in the applicable dwelling unit or fire area. Option D is therefore correct.

This should be distinguished from the corresponding ceiling limitation. Unprotected mass timber ceilings, including attached beams, are limited to 20 percent of floor area. The larger 40-percent allowance applies specifically to wall exposure.

Where only wall surfaces are exposed, the allowable wall area is calculated directly from the base floor area. For example, a 1,000-square-foot fire area would permit up to 400 square feet of exposed wall surface under this exception. However, Section 602.4.2.2.4 additionally requires unprotected wall portions to maintain the prescribed 15-foot separation from other applicable exposed wall or ceiling portions.

Where exposed walls and exposed ceilings occur together, the individual 40- and 20-percent limits cannot simply be maximized simultaneously; the mixed-exposure equation in Section 602.4.2.2.3 governs.

References/Topics: 2021 IBC Sections 602.4.2.2.2 through 602.4.2.2.4; Type IV-B wall exposure.

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Option C is not an IFC requirement. The 2021 IFC requires inspection records to be maintained on-site, but it does not require those records to be secured in a fire- and water-resistant safe.

Section 3303.2 requires the owner to designate a site safety director. Section 3303.3 then makes that director responsible for completing a daily fire safety inspection of the project site. Every day, building and outdoor construction areas must be evaluated against the prescribed fire-safety inspection items.

The results of each inspection must be documented and retained on-site until a certificate of occupancy has been issued . The records must also be immediately available at the site for presentation to the fire code official upon request. Options A, B, and D therefore accurately reflect the code requirements.

The daily inspection program covers practical hazards such as hot work, temporary heating, combustible debris, temporary electrical wiring, hazardous-material storage, fire apparatus access, hydrants, fire department connections, standpipe availability, and portable extinguishers.

For tall mass timber projects, this documentation is particularly important because substantial combustible construction may be present before permanent fire protection systems are complete.

References/Topics: 2021 IFC Sections 3303.2 and 3303.3; site safety director; daily fire-safety inspection and documentation.

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IBC Table 601 requires floor construction and associated secondary structural members in Types IV-A, IV-B, and IV-C construction to have a two-hour fire-resistance rating. Therefore, option B is correct.

This requirement applies to the complete floor assembly, not only the timber panel. The required two-hour performance may be established through the calculated fire resistance of the mass timber, the contribution of approved noncombustible protection, or a permitted combination of both. Any topping, protective membrane, connection protection, and edge condition that forms part of the approved rated design must be installed accordingly.

The ratings for other building elements should not be transferred to the floor. A three-hour rating applies to the primary structural frame and bearing walls of Type IV-A, but its floor construction remains two hours. Type IV-B and Type IV-C floors also require two hours, even though those construction types permit different degrees of exposed mass timber.

The special inspector must verify panel dimensions, approved layup, protection layers, joints, penetrations, connections, topping thickness, and other details that influence the floor’s rated performance.

References/Topics: 2021 IBC Sections 601 and 602.4; Table 601, floor construction and associated secondary structural members.

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The IBC does not require structural composite lumber to retain the same nominal dimensions as the solid-sawn member it replaces. Instead, the minimum finished dimensions are determined from the heavy-timber dimensional equivalency provisions in Section 2304.11.

For a solid-sawn member having nominal dimensions of 8 inches by 8 inches, the corresponding minimum structural composite lumber net dimensions are 7 inches in width by 7-1/2 inches in depth. Therefore, option B is correct. The dimensional conversion recognizes that nominal solid-sawn dimensions differ from actual manufactured dimensions and that SCL products are produced to controlled net dimensions.

The 6-3/4-inch by 8-1/4-inch dimensions in option C correspond to the minimum glued-laminated timber equivalency for an 8-inch-by-8-inch nominal solid-sawn member, not the SCL equivalency. Option D incorrectly assumes that the SCL member must retain the original nominal dimensions, while option A does not match the required equivalency for either SCL or glued-laminated timber.

The substitution must also be approved by the registered design professional because dimensional compliance alone does not establish equivalent structural capacity, connection behavior, fire resistance, or load-path performance.

References/Topics: 2021 IBC Section 2304.11; heavy-timber minimum dimensions; minimum structural composite lumber net-size table.

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The applicable requirement is found in 2021 IBC Section 504.3 and Table 504.3. The table regulates building height in feet above grade plane by occupancy, construction type, and sprinkler condition. For a Group I-4 occupancy equipped throughout with a Section 903.3.1.1 sprinkler system, the applicable row is the sprinklered “S” row.

Reading across that row to Type IV-B construction gives a maximum building height of 120 feet. Therefore, option B is correct. The 180-foot value applies to Type IV-A construction for the same occupancy and sprinkler condition. The 85-foot value, which is not offered, applies to Type IV-C and Type IV-HT. The 270-foot value generally appears for certain lower-hazard occupancies—such as Groups A, B, E, F, M, S and U—when constructed as sprinklered Type IV-A, but it is not permitted for Group I-4.

This distinction reflects the additional life-safety considerations associated with custodial-care occupancies. A special inspector should verify that the approved construction documents identify both the correct occupancy classification and construction type because either classification directly changes the permitted height.

References/Topics: 2021 IBC Section 504.3; Table 504.3, Group I-4, sprinklered Type IV-B construction; Section 903.3.1.1.

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IBC Table 601 requires both exterior and interior bearing walls in Type IV-C construction to have a minimum two-hour fire-resistance rating. Option B is correct.

For exterior bearing walls, the Table 601 rating is not the only applicable criterion. Section 602.1 also requires exterior walls to satisfy Table 705.5 based on occupancy and fire separation distance. Where Table 705.5 or another provision imposes a more restrictive rating, the higher requirement governs.

The two-hour rating may be achieved through the calculated structural fire resistance of the mass timber, approved noncombustible protection, or an approved combination of the two. Connections, intersections, penetrations, and abutting edges must not create a premature failure path or compromise the rated assembly.

A one-hour reduction can be permitted for bearing walls supporting a roof only under Table 601, Footnote “a,” but that exception does not apply to a wall carrying floors or other qualifying structural loads. The inspector must confirm the actual load path before accepting any reduced protection at the uppermost level.

References/Topics: 2021 IBC Sections 601, 602.1 and 602.4; Tables 601 and 705.5; Type IV-C bearing walls.

The required minimum fire flow is 500 gallons per minute , making option C correct.

2021 IFC Section 3313.1 requires an approved temporary or permanent water supply for fire protection to become available at specified stages of construction, including when combustible building materials arrive on the site. Section 3313.2 then establishes the specific requirement: when combustible building materials for the building under construction are delivered, a minimum fire flow of 500 gpm must be provided.

The hydrant supplying this flow must be located within 500 feet of the combustible building materials as measured along an approved fire apparatus access lane. If a single hydrant cannot provide the required coverage because of site configuration, additional hydrants are required.

The 500-gpm value in this question is the construction-stage minimum associated with combustible-material delivery. It should not be mistaken for the final required fire flow of the completed building. Depending on construction type, fire separation distance, building area, and subsequent phases of vertical construction, the required available water supply can be greater.

For mass timber projects, verifying this water supply before large timber packages arrive is a critical construction-phase fire-safety control.

References/Topics: 2021 IFC Sections 3313.1 and 3313.2; combustible building materials; minimum construction fire flow.

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Type IV-C generally permits interior mass timber surfaces to remain exposed, but shaft and elevator hoistway enclosures are a specific exception to that broad exposure allowance. Section 602.4.3.6 requires shafts, elevator hoistways, and interior exit stairway enclosures constructed with mass timber to receive noncombustible protection having a minimum assigned protection time of 40 minutes . Option A is therefore correct.

Critically, protection is required on both the inside and outside of the shaft enclosure . The 40-minute value is an assigned contribution from the noncombustible protection; the complete enclosure must still satisfy whatever overall fire-resistance rating is required by Sections 713, 713.4, Table 601, or other applicable provisions.

Option B incorrectly applies the general Type IV-C exposed-interior allowance to a specifically regulated shaft condition. Option C is incomplete because 1/2-inch Type X gypsum board provides only 25 minutes under Table 722.7.1(2), not the required 40 minutes. A 5/8-inch Type X board provides a recognized 40-minute contribution when installed in accordance with Section 722.7.2. Option D does not state the prescribed Type IV-C shaft requirement.

References/Topics: 2021 IBC Sections 602.4.3.2 and 602.4.3.6; Tables 722.7.1(1) and 722.7.1(2).

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Type IV-HT construction permits concealed spaces where those spaces comply with the specific protection alternatives prescribed for traditional heavy-timber construction. One permitted method is to completely fill the concealed space with noncombustible insulation. Therefore, option D is correct.

This provision should not be confused with the concealed-space requirements applicable to the tall mass timber classifications—Types IV-A, IV-B, and IV-C. Those classifications employ dedicated noncombustible protection and assigned-time requirements intended to control the contribution of concealed combustible construction during a fire. Type IV-HT follows a different concealed-space framework and allows protection through approved alternatives, including sprinkler protection, gypsum-board enclosure, or complete filling with noncombustible insulation, as applicable.

The insulation must completely occupy the concealed volume. Merely installing partial batt insulation, leaving unprotected voids, or relying on nominally fire-resistant thermal insulation does not satisfy the provision. The inspector should verify the material’s noncombustibility, continuity, placement around obstructions, and conformance with the approved construction documents before the space becomes inaccessible.

References/Topics: 2021 IBC Section 602.4.4; concealed spaces in Type IV-HT construction; noncombustible insulation protection method.

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Minimum prescriptive dimensions are not themselves a component added into a calculated fire-resistance time. Therefore, option A is the exception.

IBC Section 602.4 states that a mass timber element’s required fire resistance can be based on the fire-resistance contribution of the mass timber, the noncombustible protection, or a combination of those two components. Options B, C, and D reflect those permitted approaches.

Minimum dimensions under Section 2304.11 serve a different function. They establish the minimum cross-sectional size and construction characteristics required for qualifying heavy timber or mass timber elements. A member can satisfy the minimum dimensional requirement without automatically achieving every required one-, two-, or three-hour fire-resistance rating. The structural fire calculation must evaluate the remaining effective section after charring, elevated-temperature effects, applied loading, and any credited protective materials.

The inspector must therefore verify both requirements independently: the installed member must meet the minimum code dimensions and must match the approved fire-resistance design. Increasing a member to the minimum prescriptive size does not by itself replace the required calculation, tested assembly, or approved protection system.

References/Topics: 2021 IBC Sections 602.4, 703.2, 722.7 and 2304.11; calculated mass timber fire resistance.

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A conventional 2 × 6 solid-sawn stud is a light-frame member and does not satisfy the minimum cross-sectional dimensions associated with mass timber or heavy-timber construction. Therefore, option A is the exception.

The IBC defines mass timber as Type IV structural elements primarily composed of solid, built-up, panelized or engineered wood products that meet the minimum cross-sectional dimensions required for Type IV construction. A nominal 2 × 6 stud is substantially smaller than those dimensions and is ordinarily classified as conventional light-frame construction. Its use might be permitted in a particular nonstructural partition or another specifically authorized application, but it is not itself a qualifying mass timber structural element.

CLT is expressly recognized as a mass timber panel product. Parallel-strand lumber is a form of structural composite lumber and can qualify where it satisfies the applicable minimum dimensions, product-standard requirements and approved structural design. Dowel-laminated timber is a built-up panelized timber product and may likewise serve as a mass timber element when its dimensions, engineering and approval satisfy the construction documents and code.

The classification depends on the completed element’s dimensions and approved structural function—not merely on the fact that it is manufactured from wood.

References/Topics: 2021 IBC Chapter 2 definitions of “Mass Timber” and “Conventional Light-Frame Construction”; Sections 602.4 and 2304.11.

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Yes. The allowance applies to both Type IV-B and Type IV-C construction , making option D correct.

IBC Section 508.4.4.1 regulates the construction of required separations between adjacent occupancies. Where the required separation is formed by a fire barrier or horizontal assembly, mass timber elements are specifically permitted to serve that function in Type IV-B or Type IV-C construction provided the mass timber is separated from the building interior by an approved thermal barrier.

The prescribed thermal barrier is gypsum board not less than 1/2 inch thick , or another material that has been tested and meets both the Temperature Transmission Fire Test and Integrity Fire Test acceptance criteria of NFPA 275.

The requirement is supplemental to the actual fire-resistance rating required for the occupancy separation. In other words, simply installing 1/2-inch gypsum board does not independently create the required fire barrier; the underlying mass timber assembly must also provide the applicable rated performance.

Type IV-HT is not included in this specific Section 508.4.4.1 mass timber allowance.

References/Topics: 2021 IBC Section 508.4.4.1; Type IV-B and IV-C occupancy separations; NFPA 275 thermal barriers.

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The calculation begins with 2021 IBC Table 506.2. Because the building is a Group B occupancy, has more than one story, and is protected throughout by an automatic sprinkler system complying with Section 903.3.1.1, the applicable tabular classification is “SM”—a multistory, fully sprinklered building.

For Group B and Type IV-A construction, Table 506.2 assigns an SM allowable area factor of 324,000 square feet. However, this is not the final total allowable building area. Section 506.2.1 requires a single-occupancy building with more than three stories above grade plane to use Equation 5-2. Where no frontage increase is stated, the calculation is:

324,000 square feet × 3 = 972,000 square feet.

The story-area multiplier is limited to three even though the building contains 15 stories. The code does not permit multiplying 324,000 square feet by all 15 stories. Additionally, the actual area of an individual story cannot exceed its calculated allowable area. Option A represents a different construction-type area factor, while option B does not result from the required IBC multistory calculation. The building is not automatically unlimited merely because it is Type IV-A and sprinklered.

References/Topics: 2021 IBC Sections 506.2 and 506.2.1; Equation 5-2; Table 506.2, Group B, Type IV-A, SM area factor.

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A 7/16-inch wood structural panel is not one of the prescriptively acceptable fireblocking materials listed by IBC Section 718.2.1. Therefore, option C is correct.

Section 718.2.1 permits fireblocking to consist of several specified materials. These include one-half-inch gypsum board; properly secured batts or blankets of mineral wool, mineral fiber, or other approved materials; and mass timber complying with Section 2304.11. Options A, B, and D are consequently valid.

For wood structural panels, however, the IBC requires a substantially greater thickness: a single thickness of approximately 0.719 inch , conventionally represented as 23/32 inch, with joints backed by material of the same required thickness. A 7/16-inch panel is only approximately 0.438 inch thick and therefore does not satisfy that prescriptive fireblocking provision.

Fireblocking is intended to interrupt concealed draft openings and restrict the movement of flame and hot gases through concealed combustible spaces. It should not be confused with a tested firestop system used at penetrations through fire-resistance-rated assemblies.

Special inspection should confirm that the installed material, thickness, joint backing, retention, and location conform to the approved construction documents and the applicable code provision.

References/Topics: 2021 IBC Sections 718.2 and 718.2.1; Section 2304.11; permitted fireblocking materials.

In Type IV-C construction, combustible construction forming concealed spaces must receive noncombustible protection providing an assigned time of at least 40 minutes. Consequently, option D is correct.

Type IV-C permits substantial portions of mass timber to remain exposed within occupied interior spaces, subject to the applicable construction and fire-resistance requirements. That permission does not extend unchanged into concealed spaces. Concealed combustible surfaces create a different hazard because fire can spread above ceilings, inside shafts, or within service cavities without immediate detection. Section 602.4.3.5 therefore imposes a specific 40-minute noncombustible protection requirement.

An NFPA 13 sprinkler system is generally required for tall mass timber buildings, but sprinkler protection alone is not the prescribed substitute for this concealed-space protection. Fire-retardant-treated wood is still combustible and does not constitute the required noncombustible protective membrane. Option C is also incorrect because the concealed-space rule is more specific than the provisions governing exposed surfaces in occupied spaces.

Inspection should occur before closure so that membrane continuity, board orientation, fastener spacing, joint treatment, penetrations, and protection around connection hardware can be fully observed.

References/Topics: 2021 IBC Sections 602.4.3.2 and 602.4.3.5; Table 722.7.1(1); concealed combustible construction in Type IV-C buildings.

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