CBC · California Building Code

What is Type IV‑HT (heavy timber) and applicable protection rules?

Type IV‑HT is the CBC classification that lets you build with heavy timber interiors (sized per Table 2304.11) while requiring noncombustible exterior walls or specified protected CLT/FRT wood assemblies; concealed spaces and finish protections are tightly limited and must meet the options listed in § 602.4.4 and § 2304.11.

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Last reviewed: July 5, 2026

What the code requires — 2-4 sentences

Type IV‑HT is a variant of Type IV (mass timber) in which exterior walls are noncombustible but the interior building elements are heavy timber — solid sawn, glued‑laminated, or structural composite lumber — and there are no combustible concealed spaces except as allowed in § 602.4.4. See § 602.4.4 for the defining limitations and cross references to required heavy‑timber details in § 2304.11.

Requirements in detail

Basic definition and scope

Type IV‑HT is defined in § 602.4.4: exterior walls must be of noncombustible materials; interior elements must be solid wood, laminated heavy timber or SCL; and concealed spaces are either not permitted or must comply with the protections in § 602.4.4.3. § 602.4.4 also requires that minimum dimensions and other heavy‑timber details comply with § 2304.11.

Exterior wall options for Type IV‑HT

Fire‑retardant‑treated wood framing/sheathing is permitted within exterior wall assemblies that are rated 2 hours or less, per § 602.4.4.1.
Cross‑laminated timber (CLT) ≥ 4 in. thick is permitted in exterior walls with a 2‑hour rating or less; CLT and any heavy‑timber members appurtenant to that exterior wall must meet Table 2304.11 dimensions and be fire‑resistance rated as required by the exterior wall. The CLT exterior face must be protected by either fire‑retardant‑treated wood sheathing (≥ 15/32 in.), gypsum board (≥ 1/2 in.), or a noncombustible material (§ 602.4.4.2).

Concealed spaces (key protection options)

Concealed spaces in Type IV‑HT are tightly controlled. Per § 602.4.4.3, combustible concealed spaces are not allowed except where one of these is provided:

The building is fully sprinklered in accordance with § 903.3.1.1 and the concealed space itself is sprinklered;
The concealed space is completely filled with noncombustible insulation; or
The combustible surfaces in the concealed space are fully sheathed with not less than 5/8‑inch Type X gypsum board. An exception removes this requirement when the interior wall or partition is already 1‑hour fire‑resistance‑rated in conformance with § 2304.11.2.2.

Interior walls and partitions

Interior walls and partitions in Type IV‑HT must be either heavy timber constructed per § 2304.11 (two layers of 1‑inch matched boards, laminated 4‑inch construction, or otherwise conforming) or be 1‑hour fire‑resistance rated as required by § 2304.11.2. § 602.4.4 explicitly references that interior walls/partitions not less than 1‑hour FRR or heavy timber per § 2304.11.2.2 are permitted.

Minimum member dimensions (decision‑relevant sizes)

Use Table 2304.11 for minimum nominal sizes of heavy timber members (solid sawn, glued‑laminated, and SCL). Key values you will use frequently are shown below.

Element / condition	Minimum nominal dimension (solid sawn)	Typical alternate (glued‑lam, SCL)	Code Reference
Columns (floor loads or combined floor & roof)	8 in. × 8 in.	glulam 6 3/4 in. × 8 1/4 in.; SCL 7 in. × 7 1/2 in.	§ 2304.11; Table 2304.11
Wood beams & girders (floor loads)	6 in. × 10 in.	glulam 5 in. × 10 1/2 in.; SCL 5 1/4 in. × 9 1/2 in.	§ 2304.11; Table 2304.11
Roof framing (framed trusses / roof framing)	4 in. × 6 in. (typical minimum where allowed)	smaller minimums allowed where sprinklers protect roof deck per table notes	§ 2304.11; Table 2304.11
Cross‑laminated timber (CLT) — exterior walls and floors	CLT ≥ 4 in. thickness	—	§ 602.4.4.2; § 2304.11.3.1
Interior wall heavy‑timber construction	two layers of 1‑inch matched boards or laminated 4 in. thick	or 1‑hour FRR	§ 2304.11.2.2
CLT exterior surface protection options	FRT sheathing ≥ 15/32 in.; gypsum board ≥ 1/2 in.; or noncombustible material	—	§ 602.4.4.2
Concealed space protection	sprinkler the space OR fill with noncombustible insulation OR sheath with 5/8 in. Type X gypsum	—	§ 602.4.4.3

(Values in the table above come directly from § 2304.11 and § 602.4.4 and Table 2304.11; consult the full table for every element and the code notes that modify dimensions for sprinklered roofs or special spacing.)

Fire‑resistance rating and related determinations

The required fire‑resistance rating for elements in Type IV‑HT is determined according to the general mass‑timber provisions (see § 602.4 and cross reference to § 703.2 for how protection contribution is calculated). Type IV‑HT relies on heavy timber sizing (Table 2304.11) and, where required, fire‑resistance ratings specified elsewhere in the code.

Exterior exposed heavy timber

Wood columns and arches sized per § 2304.11 are permitted to be used externally where the fire separation distance (FSD) is 20 feet or more, per § 602.4.4.4.

Exceptions & special cases

Type IV‑HT is an explicit exception to the general requirement in § 602.4 that exterior walls be mass timber or noncombustible; § 602.4.4 allows certain exterior wall treatments (FRT wood or CLT with protections) for Type IV‑HT.
Where interior walls/partitions are already 1‑hour fire‑resistance rated per § 2304.11.2.2, the concealed‑space protection requirements of § 602.4.4.3 do not apply to those partitions (exception in § 602.4.4.3).
Roof and framing minimums in Table 2304.11 include notes that reduce sizes when automatic sprinklers are provided under the roof deck — always check the table notes.

If you need an exhaustive list of the table notes, sprinkler‑adjusted sizes, or the specific fire‑resistance times assigned in Table 722.7.1(1), those are specified in other tables and sections of the CBC; the excerpts available here reference those tables but do not reproduce every table entry. Consult the code directly for the full tables.

Common mistakes

Assuming CLT of any thickness is acceptable for exterior walls — the code requires CLT ≥ 4 in. when used in exterior walls for Type IV‑HT (§ 602.4.4.2).
Forgetting that CLT or heavy timber used in an exterior wall must be protected on the exterior face by one of the listed options (FRT sheathing, gypsum, or noncombustible material). § 602.4.4.2.
Overlooking concealed‑space rules: combustible concealed spaces are restricted and must meet one of the protective options in § 602.4.4.3. Designers sometimes assume standard wall cavities are allowed; they are not unless they meet the listed exceptions.
Misreading Table 2304.11: the table shows several alternative minimum sizes for solid sawn, glulam and SCL — be sure to pick the correct column and check the table notes for sprinkler reductions.

Worked example — applying the rule with numbers

Scenario: a three‑story residential apartment building (R‑2) is proposed as Type IV‑HT. The design team proposes solid sawn heavy timber interior beams and columns and CLT exterior wall panels.

Exterior wall: CLT panels 4 in. thick used in an exterior wall assembly with a required 2‑hour rating. Because CLT is ≥ 4 in., it is permitted in the exterior wall per § 602.4.4.2; the CLT face will be covered with 1/2‑inch gypsum board on the exterior face to meet the allowed protection options.
Appurtenant heavy‑timber columns supporting combined floor and roof loads: select 8 in. × 8 in. solid sawn columns (meets Table 2304.11 minimum for combined loads). § 2304.11 / Table 2304.11.
Beams/girders carrying floor loads: select 6 in. × 10 in. solid sawn beams (meets Table 2304.11). § 2304.11 / Table 2304.11.
Interior partitions: use heavy timber partitions built as laminated 4‑inch construction (complies with § 2304.11.2.2), in lieu of providing a 1‑hour rated partition assembly. § 2304.11.2.2.
Concealed spaces above the ceiling: the project elects to sprinkler the entire building per § 903.3.1.1 and to provide sprinkler heads in the concealed spaces; this satisfies one of the options in § 602.4.4.3 so combustible concealed spaces created by the heavy‑timber floor/ceiling system are permitted. § 602.4.4.3.

This configuration follows the primary dimensions and protection paths required by § 602.4.4 and § 2304.11. Final compliance also requires checking applicable fire‑resistance times in Tables 601 and 722.7 and confirming the CLT product labeling requirements (ANSI/APA PRG 320) where applicable.

Related provisions

§ 602.4 (Type IV general requirements)
§ 602.4.4 (Type IV‑HT — definition and overall requirements)
§ 602.4.4.1 (Fire‑retardant‑treated wood in exterior walls)
§ 602.4.4.2 (CLT in exterior walls and required exterior protection)
§ 602.4.4.3 (Concealed space protections for Type IV‑HT)
§ 602.4.4.4 (Exterior heavy timber members allowed at FSD ≥ 20 ft)
§ 2304.11 (Details, partitions, floors, and Table 2304.11 minimum sizes)
§ 2304.11.2 (Interior walls and partitions heavy‑timber / 1‑hour alternatives)
§ 703.2 (how fire‑resistance contributions are determined for mass timber / protected elements) — see cross references in § 602.4.

Code references

Grounded in the retrieved California Building Code — click a citation to read the verbatim passage:

CBC § 602.4.3.2 High relevance — show source text
The ASTM E1354 test shall be conducted on specimens at the thickness intended for use, in the horizontal orientation and at an incident radiant heat flux of 50 kW/m [2] .

602.4.3.2 Interior protection. Mass timber elements are permitted to be unprotected.

602.4.3.3 Floors. Floor finishes in accordance with Section 804 shall be permitted on top of the floor construction.

602.4.3.4 Roof coverings. Roof coverings in accordance with Chapter 15 shall be permitted on the outside surface of the roof assembly.

602.4.3.5 Concealed spaces. Concealed spaces shall not contain combustibles other than electrical, mechanical, fire protection, or plumbing materials and equipment permitted in plenums in accordance with the California Mechanical Code, and shall comply with all applicable provisions of Section 718. Combustible construction forming concealed spaces shall be protected with noncombustible protection with a minimum assigned time of 40 minutes, as specified in Table 722.7.1(1).

602.4.3.6 Shafts. Shafts shall be permitted in accordance with Sections 713 and 718. Shafts and elevator hoistway and interior exit stairway enclosures shall be protected with noncombustible protection with a minimum assigned time of 40 minutes, as specified in Table 722.7.1(1), on both the inside of the shaft and the outside of the shaft.

602.4.4 Type IV-HT. Type IV-HT (Heavy Timber) construction is that type of construction in which the exterior walls are of noncombustible materials and the interior building elements are of solid wood, laminated heavy timber or structural composite lumber (SCL), without concealed spaces or with concealed spaces complying with Section 602.4.4.3. The minimum dimensions for permitted materials including solid timber, glued-laminated timber, SCL and cross-laminated timber (CLT) and the details of Type IV construction shall comply with the provisions of this section and Section 2304.11. Exterior walls complying with Section 602.4.4.1 or 602.4.4.2 shall be permitted. Interior walls and partitions not less than 1-hour fire-resistance rated or heavy timber conforming with Section 2304.11.2.2 shall be permitted.

602.4.4.1 Fire-retardant-treated wood in exterior walls. Fire-retardant-treated wood framing and sheathing complying with Section 2303.2 shall be permitted within exterior wall assemblies with a 2-hour rating or less.

602.4.4.2 Cross-laminated timber in exterior walls. Cross-laminated timber (CLT) not less than 4 inches (102 mm) in thickness complying with Section 2303.1.4 shall be permitted within exterior wall assemblies with a 2-hour rating or less. Heavy timber structural members appurtenant to the CLT exterior wall shall meet the requirements of Table 2304.11 and be fire-resistance rated as required for the exterior wall. The exterior surface of the cross-laminated timber and heavy timber elements shall be protected by one of the following:
1. Fire-retardant-treated wood sheathing complying with Section 2303.2 and not less than [15] / 32 inch (12 mm) thick.
2. Gypsum board not less than [1] / 2 inch (12.7 mm) thick.
3. A noncombustible material.
CBC § 602.1.1 High relevance — show source text
3.2. The distance between solar photovoltaic array structures is a minimum of 10 feet clear. 3.3. The driveway aisle separating solar photovoltaic array structures has a minimum width of 25 feet clear. 3.4. Solar photovoltaic array structure is used only for parking purposes with no storage. 3.5. Completely open on all sides (other than necessary structural supports) with no interior partitions.

602.1.1 Minimum requirements. A building or portion thereof shall not be required to conform to the details of a type of construction higher than that type which meets the minimum requirements based on occupancy even though certain features of such a building actually conform to a higher type of construction.

602.2 Types I and II. Types I and II construction are those types of construction in which the building elements specified in Table 601 are of noncombustible materials, except as permitted in Section 603 and elsewhere in this code.

602.3 Type III. Type III construction is that type of construction in which the exterior walls are of noncombustible materials and the interior building elements are of any material permitted by this code. Fire-retardant-treated wood framing and sheathing complying with Section 2303.2 shall be permitted within exterior wall assemblies of a 2-hour rating or less.

602.4 Type IV. Type IV construction is that type of construction in which the building elements are mass timber or noncombustible materials and have fire-resistance ratings in accordance with Table 601. Mass timber elements shall meet the fire-resistance-rating requirements of this section based on either the fire-resistance rating of the noncombustible protection, the mass timber, or a combination of both and shall be determined in accordance with Section 703.2. The minimum dimensions and permitted materials for building elements shall comply with the provisions of this section and Section 2304.11. Mass timber elements of Types IV-A, IV-B and IV-C construction shall be protected with noncombustible protection applied directly to the mass timber in accordance with Sections 602.4.1 through 602.4.3. The time assigned to the noncombustible protection shall be determined in accordance with Section 703.6 and comply with Section 722.7.

Cross-laminated timber shall be labeled as conforming to ANSI/APA PRG 320 as referenced in Section 2303.1.4.

Exterior load-bearing walls and nonload-bearing walls shall be mass timber construction, or shall be of noncombustible construction.

Exception: Exterior load-bearing walls and nonload-bearing walls of Type IV-HT Construction in accordance with Section 602.4.4.

The interior building elements, including nonload-bearing walls and partitions, shall be of mass timber construction or of noncombustible construction.

Exception: Interior building elements and nonload-bearing walls and partitions of Type IV-HT construction in accordance with Section 602.4.4.

Combustible concealed spaces are not permitted except as otherwise indicated in Sections 602.4.1 through 602.4.4. Combustible stud spaces within light frame walls of Type IV-HT construction shall not be considered concealed spaces, but shall comply with Section 718.
CBC § 703.2. High relevance — show source text
Mass timber elements shall meet the fire-resistance-rating requirements of this section based on either the fire-resistance rating of the noncombustible protection, the mass timber, or a combination of both and shall be determined in accordance with Section 703.2. The minimum dimensions and permitted materials for building elements shall comply with the provisions of this section and Section 2304.11. Mass timber elements of Types IV-A, IV-B and IV-C construction shall be protected with noncombustible protection applied directly to the mass timber in accordance with Sections 602.4.1 through 602.4.3. The time assigned to the noncombustible protection shall be determined in accordance with Section 703.6 and comply with Section 722.7.

Cross-laminated timber shall be labeled as conforming to ANSI/APA PRG 320 as referenced in Section 2303.1.4.

Exterior load-bearing walls and nonload-bearing walls shall be mass timber construction, or shall be of noncombustible construction.

Exception: Exterior load-bearing walls and nonload-bearing walls of Type IV-HT Construction in accordance with Section 602.4.4.

The interior building elements, including nonload-bearing walls and partitions, shall be of mass timber construction or of noncombustible construction.

Exception: Interior building elements and nonload-bearing walls and partitions of Type IV-HT construction in accordance with Section 602.4.4.

Combustible concealed spaces are not permitted except as otherwise indicated in Sections 602.4.1 through 602.4.4. Combustible stud spaces within light frame walls of Type IV-HT construction shall not be considered concealed spaces, but shall comply with Section 718.

In buildings of Type IV-A, IV-B, and IV-C construction with an occupied floor located more than 75 feet (22 860 mm) above the lowest level of fire department vehicle access, up to and including 12 stories or 180 feet (54 864 mm) above grade plane, mass timber interior exit and elevator hoistway enclosures shall be protected in accordance with Section 602.4.1.2. In buildings greater than 12 stories or 180 feet (54 864 mm) above grade plane, interior exit and elevator hoistway enclosures shall be constructed of noncombustible materials.

602.4.1 Type IV-A. Building elements in Type IV-A construction shall be protected in accordance with Sections 602.4.1.1 through 602.4.1.6. The required fire-resistance rating of noncombustible elements and protected mass timber elements shall be determined in accordance with Section 703.2.

602.4.1.1 Exterior protection. The outside face of exterior walls of mass timber construction shall be protected with noncombustible protection with a minimum assigned time of 40 minutes, as specified in Table 722.7.1(1). Components of the exterior wall covering shall be of noncombustible material except water-resistive barriers having a peak heat release rate of less than 150kW/m [2], a total heat release of less than 20 MJ/m [2] and an effective heat of combustion of less than 18MJ/kg as determined in accordance with ASTM E1354 and having a flame spread index of 25 or less and a smoke-developed index of 450 or less as

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CBC § 2.1 High relevance — show source text
Fire-retardant-treated wood members shall be
allowed to be used for such unprotected members.
2. For Group A, E, I, L, R-1, R-2 and R-2.1 occupancies, high-rise buildings, and other applications listed in Section 1.11 regulated by the Office of the State Fire Marshal, fire protec-
tion of members other than the primary structural frame shall not be required, including protection of roof framing and decking where every part of the roof construction is 20
feet or more above any floor immediately below. Fire-retardant-treated wood members shall be allowed to be used for such unprotected members.
3. One-story portions of Group A and E assembly occupancies the roof-framing system of Type II A or Type III A construction may be of unprotected construction when such roof-
framing system is open to the assembly area and does not contain concealed spaces.
c. In all occupancies, heavy timber complying with Section 2304.11 shall be allowed for roof construction, including primary structural frame members, where a 1-hour or less
fire-resistance rating is required.
d. Not less than the fire-resistance rating required by other sections of this code.
e. Not less than the fire-resistance rating based on fire separation distance (see Table 705.5).
f. Not less than the fire-resistance rating as referenced in Section 704.9.
g. Heavy timber bearing walls supporting more than two floors or more than a floor and a roof shall have a fire-resistance rating of not less than 1 hour.|For SI: 1 foot = 304.8 mm.
a. Roof supports: Fire-resistance ratings of primary structural frame and bearing walls are permitted to be reduced by 1 hour where supporting a roof only.
b._ 1._ Except in Group_A, E,_ F-1, H,I, L,M, R-1, R-2, R-2.1 and S-1 occupancies,high-rise buildings, and other applications listed in Section 1.11 regulated by the Office of the State Fire
_Marshal, _fire protection of structural members in roof construction shall not be required, including protection of primary structural frame members, roof framing and
decking where every part of the roof construction is 20 feet or more above any floor or mezzanine immediately below. Fire-retardant-treated wood members shall be
allowed to be used for such unprotected members.
2. For Group A, E, I, L, R-1, R-2 and R-2.1 occupancies, high-rise buildings, and other applications listed in Section 1.11 regulated by the Office of the State Fire Marshal, fire protec-
tion of members other than the primary structural frame shall not be required, including protection of roof framing and decking where every part of the roof construction is 20
feet or more above any floor immediately below. Fire-retardant-treated wood members shall be allowed to be used for such unprotected members.
3. One-story portions of Group A and E assembly occupancies the roof-framing system of Type II A or Type III A construction may be of unprotected construction when such roof-
framing system is open to the assembly area and does not contain concealed spaces.
c. In all occupancies, heavy timber complying with Section 2304.11 shall be allowed for roof construction, including primary structural frame members, where a 1-hour or less
fire-resistance rating is required.
d.
CBC § 705.2.1 High relevance — show source text
705.2.1 Types I and II construction. Projections from walls of Type I or II construction shall be of noncombustible materials or combustible materials as allowed by Sections 705.2.3.1 and 705.2.4.

705.2.2 Type III, IV or V construction. Projections from walls of Type III, IV or V construction shall be of any approved material.

705.2.3 Projection protection. Projections extending to 5 feet (1524 mm) or less from the line used to determine the fire separation distance shall be one of the following:
1. Noncombustible materials.
2. Combustible materials of not less than 1-hour fire-resistance-rated construction.
3. Heavy timber construction complying with Section 2304.11.
4. Fire-retardant-treated wood.
5. As permitted by Section 705.2.3.1.
Exception: Type VB construction shall be allowed for combustible projections in Group R-3 and U occupancies with a fire separation distance greater than or equal to 5 feet (1524 mm).

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705.2.3.1 Balconies and similar projections. Balconies and similar projections of combustible construction other than fireretardant-treated wood shall be fire-resistance rated where required by Table 601 for floor construction or shall be of heavy timber construction in accordance with Section 2304.11. The aggregate length of the projections shall not exceed 50 percent of the building’s perimeter on each floor.

Exceptions:
1. On buildings of Types I and II construction, three stories or less above grade plane, fire-retardant-treated wood shall be permitted for balconies, porches, decks and exterior stairways not used as required exits.
2. Untreated wood and plastic composites that comply with ASTM D7032 and Section 2612 are permitted for pickets, rails and similar guard components that are limited to 42 inches (1067 mm) in height.
3. Balconies and similar projections on buildings of Types III, IV-HT and V construction shall be permitted to be of Type V construction and shall not be required to have a fire-resistance rating where sprinkler protection is extended to these areas.
4. Where sprinkler protection is extended to the balcony areas, the aggregate length of the balcony on each floor shall not be limited.
705.2.3.2 Vents. [SFM] Vents required by Section 2304.12.2.5 in fire-rated exterior balconies or elevated walkway surfaces shall be designed where the voids created at the intersection of the exterior curtain wall and the balcony floor are sealed with an approved material or system to retard the interior spread of flame, hot gases and products of combustion. Rated assemblies shall comply with Section 715. Ventilation openings shall comply with the fire sprinkler protection as required by Section 903.3.1.1 or 903.3.1.2 and the reference standard.

705.2.4 Bay and oriel windows. Bay and oriel windows constructed of combustible materials shall conform to the type of construction required for the building to which they are attached.

Exception: Fire-retardant-treated wood shall be permitted on buildings three stories or less above grade plane of Type I, II, III or IV construction.
CBC § 6-1 High relevance — show source text
Equation 6-1 ( U tc / U ac ) + ( U tw / U aw ) ≤ 1

where:

U tc = Total unprotected mass timber ceiling areas. U ac = Allowable unprotected mass timber ceiling area conforming to Exception 1.1 of Section 602.4.2.2.2.

U tw = Total unprotected mass timber wall areas.

U aw = Allowable unprotected mass timber wall area conforming to Exception 1.2 of Section 602.4.2.2.2.

602.4.2.2.4 Separation distance between unprotected mass timber elements. In each dwelling unit or fire area, unprotected portions of mass timber walls shall be not less than 15 feet (4572 mm) from unprotected portions of other walls measured horizontally along the floor.

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602.4.2.3 Floors. The floor assembly shall contain a noncombustible material not less than 1 inch (25 mm) in thickness above the mass timber. Floor finishes in accordance with Section 804 shall be permitted on top of the noncombustible material. Except where unprotected mass timber ceilings are permitted in Section 602.4.2.2.2, the underside of floor assemblies shall be protected in accordance with Section 602.4.1.2.

602.4.2.4 Roofs. The interior surfaces of roof assemblies shall be protected in accordance with Section 602.4.2.2 except, in nonoccupiable spaces, they shall be treated as a concealed space with no portion left unprotected. Roof coverings in accordance with Chapter 15 shall be permitted on the outside surface of the roof assembly.

602.4.2.5 Concealed spaces. Concealed spaces shall not contain combustibles other than electrical, mechanical, fire protection, or plumbing materials and equipment permitted in plenums in accordance with the California Mechanical Code, and shall comply with all applicable provisions of Section 718. Combustible construction forming concealed spaces shall be protected in accordance with Section 602.4.1.2.

602.4.2.6 Shafts. Shafts shall be permitted in accordance with Sections 713 and 718. Both the shaft side and room side of mass timber elements shall be protected in accordance with Section 602.4.1.2.

602.4.3 Type IV-C. Building elements in Type IV-C construction shall be protected in accordance with Sections 602.4.3.1 through 602.4.3.6. The required fire-resistance rating of building elements shall be determined in accordance with Section 703.2.

602.4.3.1 Exterior protection. The exterior side of walls of combustible construction shall be protected with noncombustible protection with a minimum assigned time of 40 minutes, as determined in Table 722.7.1(1).
CBC § 2304.11. High relevance — show source text
The minimum dimensions for permitted materials including solid timber, glued-laminated timber, SCL and cross-laminated timber (CLT) and the details of Type IV construction shall comply with the provisions of this section and Section 2304.11. Exterior walls complying with Section 602.4.4.1 or 602.4.4.2 shall be permitted. Interior walls and partitions not less than 1-hour fire-resistance rated or heavy timber conforming with Section 2304.11.2.2 shall be permitted.

602.4.4.1 Fire-retardant-treated wood in exterior walls. Fire-retardant-treated wood framing and sheathing complying with Section 2303.2 shall be permitted within exterior wall assemblies with a 2-hour rating or less.

602.4.4.2 Cross-laminated timber in exterior walls. Cross-laminated timber (CLT) not less than 4 inches (102 mm) in thickness complying with Section 2303.1.4 shall be permitted within exterior wall assemblies with a 2-hour rating or less. Heavy timber structural members appurtenant to the CLT exterior wall shall meet the requirements of Table 2304.11 and be fire-resistance rated as required for the exterior wall. The exterior surface of the cross-laminated timber and heavy timber elements shall be protected by one of the following:
1. Fire-retardant-treated wood sheathing complying with Section 2303.2 and not less than [15] / 32 inch (12 mm) thick.
2. Gypsum board not less than [1] / 2 inch (12.7 mm) thick.
3. A noncombustible material.
602.4.4.3 Concealed spaces. Concealed spaces shall not contain combustible materials other than building elements and electrical, mechanical, fire protection, or plumbing materials and equipment permitted in plenums in accordance with the California Mechanical Code . Concealed spaces shall comply with applicable provisions of Section 718. Concealed spaces shall be protected in accordance with one or more of the following:
1. The building shall be sprinklered throughout in accordance with Section 903.3.1.1 and automatic sprinklers shall also be provided in the concealed space.
2. The concealed space shall be completely filled with noncombustible insulation.
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1. Combustible surfaces within the concealed space shall be fully sheathed with not less than [5] / 8 -inch Type X gypsum board.
Exception: Concealed spaces within interior walls and partitions with a 1-hour or greater fire-resistance rating complying with Section 2304.11.2.2 shall not require additional protection.

602.4.4.4 Exterior structural members. Where a fire separation distance of 20 feet (6096 mm) or more is provided, wood columns and arches conforming to heavy timber sizes complying with Section 2304.11 shall be permitted to be used externally.

602.5 Type V. Type V construction is that type of construction in which the structural elements, exterior walls and interior walls are of any materials permitted by this code.

SECTION 603—COMBUSTIBLE MATERIAL IN TYPES I AND II CONSTRUCTION

**603.1 Allowable materials.
CBC § 705.5 High relevance — show source text
b|2a, b, c|1b, c|0c|1b, c|0|3a|2a|2a|HT|1b, c|0| |Bearing walls||||||||||||| |Exteriore, f|3|2|1|0|2|2|3|2|2|2|1|0| |Interior|3a|2a|1|0|1|0|3|2|2|1/HTg|1|0| |Nonbearing walls and partitions
Exterior|See Table 705.5|See Table 705.5|See Table 705.5|See Table 705.5|See Table 705.5|See Table 705.5|See Table 705.5|See Table 705.5|See Table 705.5|See Table 705.5|See Table 705.5|See Table 705.5| |Nonbearing walls and partitions
Interiord|0|0|0|0|0|0|0|0|0|See
Section
2304.11.2|0|0| |Floor construction and associated
secondary structural members (see
Section 202)|2|2|1|0|1|0|2|2|2|HT|1|0| |Roof construction and associated
secondary structural members
(see Section 202)|11/2
b|1b, c|1b, c|0c|1b, c|0|11/2|1|1|HT|1b, c|0| |For SI: 1 foot = 304.8 mm.
a. Roof supports: Fire-resistance ratings of primary structural frame and bearing walls are permitted to be reduced by 1 hour where supporting a roof only.
b._ 1._ Except in Group_A, E,_ F-1, H,I, L,M, R-1, R-2, R-2.1 and S-1 occupancies,high-rise buildings, and other applications listed in Section 1.11 regulated by the Office of the State Fire
_Marshal, _fire protection of structural members in roof construction shall not be required, including protection of primary structural frame members, roof framing and
decking where every part of the roof construction is 20 feet or more above any floor or mezzanine immediately below. Fire-retardant-treated wood members shall be
allowed to be used for such unprotected members.
2. For Group A, E, I, L, R-1, R-2 and R-2.1 occupancies, high-rise buildings, and other applications listed in Section 1.11 regulated by the Office of the State Fire Marshal, fire protec-
tion of members other than the primary structural frame shall not be required, including protection of roof framing and decking where every part of the roof construction is 20
feet or more above any floor immediately below. Fire-retardant-treated wood members shall be allowed to be used for such unprotected members.
3. One-story portions of Group A and E assembly occupancies the roof-framing system of Type II A or Type III A construction may be of unprotected construction when such roof-
framing system is open to the assembly area and does not contain concealed spaces.
c. In all occupancies, heavy timber complying with Section 2304.11 shall be allowed for roof construction, including primary structural frame members, where a 1-hour or less
fire-resistance rating is required.
d.

CBC § 2304.11 High relevance — show source text

TABLE 2304.11—MINIMUM DIMENSIONS OF HEAVY TIMBER STRUCTURAL MEMBERS	Col2	Col3	Col4	Col5	Col6	Col7	Col8
		MINIMUM NOMI- NAL SOLID SAWN SIZE	MINIMUM NOMI- NAL SOLID SAWN SIZE	MINIMUM GLUED- LAMINATED NET SIZE	MINIMUM GLUED- LAMINATED NET SIZE	MINIMUM STRUC- TURAL COMPOSITE LUMBER NET SIZE	MINIMUM STRUC- TURAL COMPOSITE LUMBER NET SIZE
SUPPORTING	HEAVY TIMBER STRUCTURAL ELEMENTS	Width, inch	Depth, inch	Width, inch	Depth, inch	Width, inch	Depth, inch
Floor loads only or combined floor and roof loads	Columns; Framed sawn or glued-lami- nated timber arches that spring from the floor line; Framed timber trusses	8	8	63/4	81/4	7	71/2
Floor loads only or combined floor and roof loads	Wood beams and girders	6	10	5	101/2	51/4	91/2
Roof loads only	Columns (roof and ceiling loads); Lower half of: wood-frame or glued-laminated arches that spring from the floor line or from grade	6	8	5	81/4	51/4	71/2
Roof loads only	Upper half of: wood-frame or glued-lami- nated arches that spring from the floor line or from grade	6	6	5	6	51/4	51/2
Roof loads only	Framed timber trusses and other roof framing;a Framed or glued-laminated arches that spring from the top of walls or wall abutments	4b	6	3b	67/8	31/2 b	51/2
For SI: 1 inch = 25.4 mm. a. Spaced members shall be permitted to be composed of two or more pieces not less than 3 inches nominal in thickness where blocked solidly throughout their intervening spaces or where spaces are tightly closed by a continuous wood cover plate of not less than 2 inches nominal in thickness secured to the underside of the members. Splice plates shall be not less than 3 inches nominal in thickness. b. Where protected by approved automatic sprinklers under the roof deck, framing members shall be not less than 3 inches nominal in width.	For SI: 1 inch = 25.4 mm. a. Spaced members shall be permitted to be composed of two or more pieces not less than 3 inches nominal in thickness where blocked solidly throughout their intervening spaces or where spaces are tightly closed by a continuous wood cover plate of not less than 2 inches nominal in thickness secured to the underside of the members.

CBC § 3102.2 Medium relevance — show source text
3102.2 Tensile membrane structures and air-supported structures. Tensile membrane structures and air-supported structures, including permanent and temporary structures, shall be designed and constructed in accordance with ASCE 55. The provisions in Sections 3102.3 through 3102.6 shall apply.

3102.3 Type of construction. Noncombustible membrane structures shall be classified as Type IIB construction. Noncombustible frame or cable-supported structures covered by an approved membrane in accordance with Section 3102.3.1 shall be classified as Type IIB construction. Heavy timber frame-supported structures covered by an approved membrane in accordance with Section 3102.3.1 shall be classified as Type IV-HT construction. Other membrane structures shall be classified as Type V construction.

Exception: Plastic less than 30 feet (9144 mm) above any floor used in greenhouses, where occupancy by the general public is not authorized, and for aquaculture pond covers is not required to meet the fire propagation performance criteria of Test Method 1 or Test Method 2, as appropriate, of NFPA 701.

3102.3.1 Membrane and interior liner material. Membranes and interior liners shall be either noncombustible as set forth in Section 703.3 shall be flame resistant in accordance with the provisions set forth in CCR, Title 19, Division 1, Chapter 8. Tops and side- walls shall be made either from fabric that has been flame resistant treated with an approved exterior chemical process by an approved application concern, or from inherently flame resistant fabric approved and listed by the State Fire Marshal (see CCR, Title 19, Division 1, Chapter 8).

Exception: Plastic less than 20 mil (0.5 mm) in thickness used in greenhouses, where occupancy by the general public is not authorized, and for aquaculture pond covers is not required to meet the fire propagation performance criteria of Test Method 1 or Test Method 2, as appropriate, of NFPA 701.

3102.4 Allowable floor areas. The area of a membrane structure shall not exceed the limitations specified in Section 506.

3102.5 Maximum height. Membrane structures shall not exceed one story nor shall such structures exceed the height limitations in feet specified in Section 504.3.

Exception: Noncombustible membrane structures serving as roofs only.

3102.6 Mixed construction. Membrane structures shall be permitted to be utilized as specified in this section as a portion of buildings of other types of construction. Height and area limits shall be as specified for the type of construction and occupancy of the building.

3102.6.1 Noncombustible membrane. A noncombustible membrane shall be permitted for use as the roof or as a skylight of any building or atrium of a building of any type of construction provided that the membrane is not less than 20 feet (6096 mm) above any floor, balcony or gallery.

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SPECIAL CONSTRUCTION

3102.6.1.1 Membrane. A membrane meeting the fire propagation performance criteria of Test Method 1 or Test Method 2, as appropriate, of NFPA 701 shall be permitted to be used as the roof or as a skylight on buildings of Type IIB, III, IV-HT and V construction, provided that the membrane is not less than 20 feet (6096 mm) above any floor, balcony or gallery.
CBC § 602.4.2.4 Medium relevance — show source text
602.4.2.4 Roofs. The interior surfaces of roof assemblies shall be protected in accordance with Section 602.4.2.2 except, in nonoccupiable spaces, they shall be treated as a concealed space with no portion left unprotected. Roof coverings in accordance with Chapter 15 shall be permitted on the outside surface of the roof assembly.

602.4.2.5 Concealed spaces. Concealed spaces shall not contain combustibles other than electrical, mechanical, fire protection, or plumbing materials and equipment permitted in plenums in accordance with the California Mechanical Code, and shall comply with all applicable provisions of Section 718. Combustible construction forming concealed spaces shall be protected in accordance with Section 602.4.1.2.

602.4.2.6 Shafts. Shafts shall be permitted in accordance with Sections 713 and 718. Both the shaft side and room side of mass timber elements shall be protected in accordance with Section 602.4.1.2.

602.4.3 Type IV-C. Building elements in Type IV-C construction shall be protected in accordance with Sections 602.4.3.1 through 602.4.3.6. The required fire-resistance rating of building elements shall be determined in accordance with Section 703.2.

602.4.3.1 Exterior protection. The exterior side of walls of combustible construction shall be protected with noncombustible protection with a minimum assigned time of 40 minutes, as determined in Table 722.7.1(1). Components of the exterior wall covering shall be of noncombustible material except water-resistive barriers having a peak heat release rate of less than 150 kW/m [2], a total heat release of less than 20 MJ/m [2] and an effective heat of combustion of less than 18 MJ/kg as determined in accordance with ASTM E1354 and having a flame spread index of 25 or less and a smoke-developed index of 450 or less as determined in accordance with ASTM E84 or UL 723. The ASTM E1354 test shall be conducted on specimens at the thickness intended for use, in the horizontal orientation and at an incident radiant heat flux of 50 kW/m [2] .

602.4.3.2 Interior protection. Mass timber elements are permitted to be unprotected.

602.4.3.3 Floors. Floor finishes in accordance with Section 804 shall be permitted on top of the floor construction.

602.4.3.4 Roof coverings. Roof coverings in accordance with Chapter 15 shall be permitted on the outside surface of the roof assembly.

602.4.3.5 Concealed spaces. Concealed spaces shall not contain combustibles other than electrical, mechanical, fire protection, or plumbing materials and equipment permitted in plenums in accordance with the California Mechanical Code, and shall comply with all applicable provisions of Section 718. Combustible construction forming concealed spaces shall be protected with noncombustible protection with a minimum assigned time of 40 minutes, as specified in Table 722.7.1(1).

602.4.3.6 Shafts. Shafts shall be permitted in accordance with Sections 713 and 718. Shafts and elevator hoistway and interior exit stairway enclosures shall be protected with noncombustible protection with a minimum assigned time of 40 minutes, as specified in Table 722.7.1(1), on both the inside of the shaft and the outside of the shaft.

**602.4.4 Type IV-HT.
CBC § 602.4.1 Medium relevance — show source text
602.4.1 Type IV-A. Building elements in Type IV-A construction shall be protected in accordance with Sections 602.4.1.1 through 602.4.1.6. The required fire-resistance rating of noncombustible elements and protected mass timber elements shall be determined in accordance with Section 703.2.

602.4.1.1 Exterior protection. The outside face of exterior walls of mass timber construction shall be protected with noncombustible protection with a minimum assigned time of 40 minutes, as specified in Table 722.7.1(1). Components of the exterior wall covering shall be of noncombustible material except water-resistive barriers having a peak heat release rate of less than 150kW/m [2], a total heat release of less than 20 MJ/m [2] and an effective heat of combustion of less than 18MJ/kg as determined in accordance with ASTM E1354 and having a flame spread index of 25 or less and a smoke-developed index of 450 or less as

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TYPES OF CONSTRUCTION

determined in accordance with ASTM E84 or UL 723. The ASTM E1354 test shall be conducted on specimens at the thickness intended for use, in the horizontal orientation and at an incident radiant heat flux of 50 kW/m [2] .

602.4.1.2 Interior protection. Interior faces of all mass timber elements, including the inside faces of exterior mass timber walls and mass timber roofs, shall be protected with materials complying with Section 703.3.

602.4.1.2.1 Protection time. Noncombustible protection shall contribute a time equal to or greater than times assigned in Table 722.7.1(1), but not less than 80 minutes. The use of materials and their respective protection contributions specified in Table 722.7.1(2) shall be permitted to be used for compliance with Section 722.7.1.

602.4.1.3 Floors. The floor assembly shall contain a noncombustible material not less than 1 inch (25 mm) in thickness above the mass timber. Floor finishes in accordance with Section 804 shall be permitted on top of the noncombustible material. The underside of floor assemblies shall be protected in accordance with Section 602.4.1.2.

602.4.1.4 Roofs. The interior surfaces of roof assemblies shall be protected in accordance with Section 602.4.1.2. Roof coverings in accordance with Chapter 15 shall be permitted on the outside surface of the roof assembly.

602.4.1.5 Concealed spaces. Concealed spaces shall not contain combustibles other than electrical, mechanical, fire protection, or plumbing materials and equipment permitted in plenums in accordance with the California Mechanical Code, and shall comply with all applicable provisions of Section 718. Combustible construction forming concealed spaces shall be protected in accordance with Section 602.4.1.2.

602.4.1.6 Shafts. Shafts shall be permitted in accordance with Sections 713 and 718. Both the shaft side and room side of mass timber elements shall be protected in accordance with Section 602.4.1.2.

Frequently asked questions

What minimum CLT thickness is required for exterior walls in Type IV‑HT?

CLT used in exterior walls for Type IV‑HT must be not less than 4 inches thick, per § 602.4.4.2.

Can I use fire‑retardant‑treated (FRT) wood in an exterior wall for Type IV‑HT?

Yes — FRT wood framing and sheathing complying with § 2303.2 is permitted within exterior wall assemblies that are rated 2 hours or less under § 602.4.4.1.

Are combustible concealed spaces allowed in Type IV‑HT?

Only where they meet one of the protections in § 602.4.4.3: the building and the concealed space are sprinklered, the space is filled with noncombustible insulation, or the space is fully sheathed with ≥ 5/8‑inch Type X gypsum.

Where do I find the minimum heavy timber sizes?

Minimum nominal sizes for heavy timber members are in Table 2304.11 and the surrounding text of § 2304.11. Use that table to select column, beam and roof member sizes.

Does Type IV‑HT remove the need to check fire‑resistance tables elsewhere in the code?

No. Type IV‑HT relies on the heavy‑timber sizing and the specific protections in § 602.4.4/§ 2304.11, but required fire‑resistance times and contributions are still determined per the code’s fire‑resistance provisions (see the cross references to § 703.2 and the fire‑resistance tables).

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Mass timber (Type IV) protection requirements and allowable unprotected areas

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Fire‑resistance ratings and Type‑specific protections

California Building Code