IBC Mass Timber: Type IV-A, IV-B, IV-C and IV-HT Requirements

TL;DR — Key Takeaways

• IBC 2024 Chapter 6 recognizes four Type IV subdivisions for mass timber: IV-A, IV-B, IV-C, and IV-HT (legacy heavy timber).

• Type IV-A: Full encapsulation of all exposed mass timber required; maximum 18 stories above grade.

• Type IV-B: Partial encapsulation; limited exposed mass timber permitted; maximum 12 stories above grade.

• Type IV-C: No encapsulation required; exposed mass timber throughout permitted; maximum 9 stories above grade.

• Type IV-HT: Legacy heavy timber from pre-2021 IBC — minimum 8-inch timber dimensions, no fire resistance rating required for structural members, generally 6 stories maximum under most occupancy/construction type combinations.

• All Type IV-A, B, and C buildings must be provided with NFPA 13 automatic sprinklers throughout (§602.4.1 conditions).

• "Encapsulation" means covering mass timber with Type X gypsum board — the number of layers and minimum thickness differs by subdivision.

• IBC 2024 refined the connection-protection requirements from the 2021 edition, clarifying how metallic connections at mass timber elements must be protected.

• Mass timber products eligible for Type IV include: cross-laminated timber (CLT), glued-laminated timber (glulam), laminated veneer lumber (LVL), structural composite lumber (SCL), and similar engineered mass timber products.

Background: Why the Code Changed for Mass Timber

Before IBC 2021, the only Type IV option was "Heavy Timber" — a construction type with prescriptive minimums for solid timber member dimensions (8" × 8" columns, 6" × 10" beams) that relied on char formation rather than fire resistance ratings for protection. Heavy timber was limited to about 65 feet by the height and area table.

The 2021 and 2024 editions created three new subdivisions (IV-A, IV-B, IV-C) to address modern engineered mass timber products — particularly cross-laminated timber — and to allow taller mass timber buildings with fire-resistance-rated protection systems rather than solely relying on char. The mass timber provisions represent the most significant structural change to IBC in decades, enabling mass timber high-rises up to 18 stories.

IBC construction types explained

IBC §602.4 — The Four Type IV Subdivisions

Type IV-A: Maximum Protection, Maximum Height

Story limit: 18 stories above grade (§504.3, Table 504.3)

Encapsulation: Full — all mass timber elements must be fully encapsulated with noncombustible materials (typically Type X gypsum board) to achieve the required fire resistance rating.

Fire resistance ratings (Table 601):

• Structural frame (columns, beams): 3 hours

• Floor construction: 2 hours

• Roof construction: 1.5 hours

What "full encapsulation" means in practice:

Every mass timber surface — columns, beams, floors, walls — must be covered with gypsum board or other noncombustible materials at all times during and after construction. Mass timber is not visible in a fully constructed IV-A building. The encapsulation must achieve the required fire resistance rating, which typically requires two layers of 5/8" Type X gypsum for the 2-hour floor rating and three layers for the 3-hour structural frame rating.

Sprinkler requirement: NFPA 13 sprinklers required throughout. Sprinklers alone do not satisfy the encapsulation requirement — both are required.

Typical application: Tall mass timber office and residential buildings (10–18 stories) where the mass timber is being used for structural efficiency and sustainability credentials but where the exposed aesthetic is not required.

Type IV-B: Partial Exposure, Moderate Height

Story limit: 12 stories above grade (Table 504.3)

Encapsulation: Partial — the exterior of mass timber elements may remain exposed. The floor and roof assemblies and certain wall elements must be encapsulated to achieve fire resistance ratings; interior structural members in some locations may remain exposed.

Fire resistance ratings (Table 601):

• Structural frame: 2 hours

• Floor construction: 2 hours

• Roof construction: 1.5 hours

Where exposure is permitted:

IBC §602.4.2 (Type IV-B) allows mass timber surfaces to be exposed under specific conditions — primarily at the underside of floor decks in occupied spaces, where the deck itself can be exposed to a maximum area per floor. The exterior of columns and beams in occupied spaces may be partially exposed.

Key distinction from IV-A: In IV-B, some mass timber is intentionally left exposed as an architectural feature. The exposed timber must still contribute to the fire resistance rating through its char potential — the code accounts for the charring rate of mass timber products when calculating fire resistance.

Typical application: 8–12 story mixed-use or office buildings where the mass timber aesthetic (exposed CLT ceiling, visible timber columns) is a design goal.

Type IV-C: Full Exposure, Lower Height

Story limit: 9 stories above grade (Table 504.3)

Encapsulation: None required by the construction type itself — exposed mass timber throughout is permitted.

Fire resistance ratings (Table 601):

• Structural frame: 0 hours (no rating required — relies on char performance)

• Floor construction: 0 hours

• Roof construction: 0 hours

How IV-C achieves fire protection without ratings:

Type IV-C relies on the inherent char resistance of mass timber — thick timber sections (minimum dimensions established in §602.4.3) form a protective char layer at approximately 1.5 inches per hour of fire exposure. The structural member beneath the char layer retains strength. This is the same principle as the legacy IV-HT heavy timber type, but applied to engineered mass timber products.

Minimum dimensions for Type IV-C:

Mass timber elements must meet minimum size requirements that ensure adequate char depth before structural failure:

• Columns: 8 inches × 8 inches minimum (same as IV-HT for solid members; CLT columns have equivalent performance criteria)

• Beams: 6 inches × 10 inches minimum

• Floor decks: 4 inches thick minimum (CLT or mass timber panels)

Sprinkler requirement: NFPA 13 sprinklers throughout still required for IV-C.

Typical application: 5–9 story buildings where full visual expression of the mass timber structure is the architectural intent — where exposed CLT ceilings, exposed glulam beams, and visible structural elements are desired throughout.

Type IV-HT: Legacy Heavy Timber

Story limit: Typically 65 feet (varies by occupancy and area — governed by Table 504.3, generally not more than ~6 stories for most occupancy combinations)

Encapsulation: None — heavy timber relies on minimum timber dimensions and char performance, same as IV-C

Fire resistance ratings (Table 601): None required for structural members — this is the defining characteristic that distinguishes IV-HT from all other Type IV subdivisions.

Minimum dimensions for IV-HT (§602.4.4):

• Columns: 8 inches × 8 inches minimum (or 8 inches × 6 inches if supporting floors)

• Beams: 6 inches × 10 inches minimum

• Arches: 8 inches × 8 inches

• Roof decking: 3 inches nominal (tongue and groove)

• Floor decking: 4 inches (where used as a diaphragm)

What sets IV-HT apart from IV-A/B/C:

IV-HT has no fire resistance rating requirement for structural members. IV-A, IV-B, and IV-C all have rating requirements (3-hr, 2-hr, 0-hr respectively for the frame). IV-HT also predates the 2021 IBC mass timber initiative — it existed for decades as the standard "heavy timber" construction type for mills, warehouses, and religious buildings.

Sprinkler requirement: Unlike IV-A/B/C, Type IV-HT does not universally require NFPA 13 sprinklers — sprinkler requirements follow the standard §903 occupancy triggers, not an automatic Type IV-HT mandate.

Connection Protection Requirements (IBC 2024 Update)

One of the significant IBC 2024 refinements to mass timber is clarified connection protection requirements. Metallic connectors (steel brackets, screws, bolts) conduct heat rapidly and can fail earlier than the charring mass timber around them. IBC 2024 §602.4 addresses this by requiring that metal connections in fire-resistance-rated mass timber assemblies be:

• Protected with the same noncombustible cover (gypsum board) as the surrounding mass timber member, OR

• Designed and tested as part of the fire-resistance-rated assembly (test data demonstrating the connection maintains load capacity through the required fire duration), OR

• Demonstrated through calculation per ASTM E119 or equivalent

This requirement particularly affects IV-A buildings where 3-hour ratings are required — protecting every screwed and bolted connection with three layers of gypsum board is a significant design challenge that must be coordinated between architect and structural engineer early in design.

Height and Area Comparison Table

Type	Max Stories	Frame Rating	Floor Rating	Sprinklers Mandatory	Exposed Mass Timber
IV-A	18	3 hr	2 hr	Yes (NFPA 13)	No — fully encapsulated
IV-B	12	2 hr	2 hr	Yes (NFPA 13)	Partial — limited exposure
IV-C	9	0 hr	0 hr	Yes (NFPA 13)	Yes — fully exposed
IV-HT	~6	None	None	By occupancy only	Yes

IBC building height and area limits

Eligible Mass Timber Products

IBC §602.4 and the associated definitions recognize the following products as eligible for mass timber construction:

• Cross-Laminated Timber (CLT): Layers of dimension lumber cross-oriented and glued. Used for floors, walls, and roofs. APA PRG 320 is the governing standard.

• Glued-Laminated Timber (Glulam): Horizontal layers of lumber glued with the grain parallel. Used for beams, columns, and arches. ANSI/APA EWS T300 governs.

• Laminated Veneer Lumber (LVL): Thin wood veneers glued with grain parallel. Used for beams and headers.

• Structural Composite Lumber (SCL) / Laminated Strand Lumber (LSL): Reconstituted wood strands or veneers bonded under heat and pressure. Used for beams, headers, and columns.

• Nail-laminated timber (NLT) and dowel-laminated timber (DLT): Less common; eligible where meeting dimensional and performance requirements.

The product must be listed, labeled, and tested in accordance with the applicable ANSI/APA or ASTM standard to qualify for use in Type IV-A, B, C, or HT construction.

Research Mass Timber Code Requirements for Your Project

Mass timber code compliance requires coordinating IBC Chapter 6 construction type requirements with Chapter 5 height/area limits, Chapter 9 sprinkler requirements, and local jurisdiction amendments — some states have not yet adopted IBC 2021 or 2024 and remain on earlier editions where the IV-A/B/C framework does not exist. Melt Code lets you research mass timber requirements against your adopted edition and jurisdiction in one search.

Look up mass timber construction type requirements on Melt Code Try Melt Code →

Frequently Asked Questions

Q: Does IBC 2021 have the same IV-A/B/C framework as IBC 2024?

Yes — the IV-A/B/C/HT framework was introduced in IBC 2021. IBC 2024 refined the connection protection requirements and made some adjustments to encapsulation language, but the four-subdivision structure, story limits, and fundamental approach are the same in both editions.

Q: Can a Type IV-C building expose mass timber in all spaces, including corridors and stairwells?

IBC §602.4.3 allows exposed mass timber in occupied spaces. However, exit enclosures (stairwells per §1023.5) must be constructed of noncombustible materials in most cases — they are typically built with concrete or masonry even in an otherwise exposed mass timber building. The mass timber is expressed in occupied floors, not in the fire-rated exit stairwells.

Q: Is a glulam beam exposed in a ceiling a Type IV building?

Not automatically. A building using glulam beams exposed in the ceiling must still meet the overall construction type classification. If the structure is otherwise Type IIB or VA, with glulam beams added for aesthetics, the building remains the lighter construction type — the glulam beams must meet the fire protection requirements of that type. A building only qualifies as Type IV-C when the entire structural system consists of mass timber elements meeting the dimensional minimums.

Q: What certification does mass timber need?

The product must be manufactured to the applicable ANSI/APA standard (PRG 320 for CLT, EWS T300 for glulam) and must be listed by an approved third-party inspection agency. The structural design must use published design values for the specific product. The engineer of record is responsible for specifying the required grade and certification in the construction documents.

Q: Are Type IV buildings limited to residential and office uses?

No — the IBC does not limit Type IV mass timber construction to specific occupancy groups. Height and area limits in Table 504.3/506.2 apply by occupancy group and construction type combination, just as with all other construction types. High-hazard (Group H) occupancies have stricter limits, but office, residential, educational, and assembly uses are all viable in mass timber buildings within the story limits.