CBC · California Building Code

What are the weather protection and water-resistive barrier requirements for exterior walls?

Your exterior walls must have a continuous water‑resistive barrier behind the cladding, integrated with flashing and a path to drain any water out of the assembly; the CBC specifies acceptable WRB materials, lap requirements (2" over lower layer; 6" at joints for certain materials), and allows an ASTM E331 test alternative in specific situations — see **§ 1402.2** and **§ 1403.2** for the exact rules and test conditions.

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Last reviewed: July 5, 2026

What the code requires — 2-4 sentences

The California Building Code requires buildings to have a weather‑resistant exterior wall assembly that prevents accumulation of water inside the wall by providing a continuous water‑resistive barrier behind the exterior veneer, flashing, and a means for draining water that enters the assembly — § 1402.2 . The CBC also prescribes the minimum materials and installation criteria for the water‑resistive barrier (WRB), including accepted materials and lap/installation conventions — § 1403.2 .

Requirements in detail

Core performance rule (what the wall must do)

  • Provide a weather‑resistant exterior wall assembly with flashing and a design that prevents water accumulation inside the assembly by:
    • Including a continuous WRB behind the veneer, and
    • Providing a means to drain any water that enters the assembly to the exterior, and
    • Controlling condensation per the code’s condensation provisions — § 1402.2 .

What the WRB must be and where it goes

  • At least one layer of WRB attached to studs or sheathing and installed with flashing (as required by the flashing section) so it is continuous behind the exterior veneer. The WRB must be continuous to the top of walls and terminated at penetrations and appendages so the assembly meets the performance required in § 1402.2 — § 1403.2 .
  • The intersection of the WRB with fenestration openings must be flashed and assembled in accordance with the fenestration manufacturer’s instructions (or other approved methods) — § 1403.2 .

Acceptable WRB materials (per CBC)

The WRB must comply with one of these (text paraphrased from the CBC):

  • No. 15 asphalt felt complying with ASTM D226, Type 1.
  • WRB complying with ASTM E2556, Type I or II.
  • Foam‑plastic insulating sheathing WRB systems installed per manufacturer’s instructions (where allowed).
  • Tested assemblies in accordance with ASTM E331 (see § 1402.2 testing exception conditions).
  • Other approved materials installed per manufacturer instructions — § 1403.2 .

Lap/installation details called out in the CBC

  • No. 15 felt and WRBs complying with ASTM E2556 shall be applied horizontally with the upper layer lapped over the lower not less than 2 inches (51 mm). Where joints occur, upper and lower layers shall be lapped not less than 6 inches (152 mm) — § 1403.2 .
  • Flashing of the exterior wall assembly is required (see flashing rules in § 1404.4) and WRB installation must be coordinated with that flashing — § 1402.2 .

Decision‑relevant dimensions, tolerances and test conditions

Item Value / requirement Code Reference
Minimum WRB layers Not fewer than one layer applied over studs or sheathing § 1403.2
Horizontal lap (upper over lower) for No.15 felt / ASTM E2556 WRB ≥ 2 in (51 mm) § 1403.2
Lap at joints (upper & lower) ≥ 6 in (152 mm) § 1403.2
ASTM E331 wind‑driven rain test — min assembly size 4 ft × 8 ft (1219 × 2438 mm) § 1402.2
ASTM E331 test — minimum differential pressure 6.24 psf (0.297 kN/m²) § 1402.2
ASTM E331 test — minimum duration 2 hours § 1402.2
ASTM E331 test — required elements in test assembly ≥ 1 opening, 1 control joint, 1 wall/eave interface, 1 wall sill § 1402.2

Drainage & flashing

  • The exterior wall assembly must provide a means for draining water that enters the assembly to the exterior; flashing is required as described in the flashing section of the code (referenced as § 1404.4) — § 1402.2 .
  • Fenestration and WRB intersections must be flashed per the fenestration manufacturer’s instructions or approved alternative methods — § 1403.2 .

Exceptions & special cases

  • No weather‑resistant exterior wall assembly requirement when the exterior is monolithic concrete or masonry walls designed under Chapters 19 and 21 — § 1402.2, Exception 1 .
  • The code permits omission of the WRB/drainage/flashing requirements if the wall assembly has been demonstrated by ASTM E331 testing (wind‑driven rain) to resist water penetration; the code sets the required test conditions and acceptance criteria (assembly size, pressure, duration, inclusion of openings and joints) — § 1402.2, Exception 2 .
  • EIFS and EIFS with drainage: EIFS is treated specially; certain EIFS systems that meet the EIFS performance/ drainage requirements are addressed in Chapter 1407 and may have particular WRB specifications (see § 1407.4.1 and § 1407.4.1.1) — § 1407.4.1 and § 1407.4.1.1 .

If a proposed WRB or wall system relies on manufacturer instructions or testing in lieu of the prescriptive options listed in § 1403.2, the installation must follow the manufacturer’s approved instructions and the wall must meet the performance requirements in § 1402.2 — § 1403.2 .

Common mistakes

  • Treating the WRB as optional on framed walls (WRB is required: at least one layer) — § 1403.2 .
  • Failing to make the WRB continuous to the top of wall or failing to properly terminate at penetrations and appendages — continuity and termination are expressly required — § 1403.2 .
  • Installing WRB laps smaller than the prescriptive laps (upper over lower < 2 in; joint laps < 6 in) for No. 15 felt or ASTM E2556 materials — check the lap dimensions in § 1403.2 .
  • Not coordinating WRB with flashing and fenestration flashing (WRB must be integrated with flashing per § 1404.4 and fenestration instructions) — § 1402.2 and § 1403.2 .
  • Assuming the ASTM E331 test exemption applies without using the prescribed test assembly and parameters (assembly size, test pressure, duration, test elements) — the code sets explicit test conditions — § 1402.2 .
  • Using foam‑plastic insulating sheathing WRB systems without following manufacturer instructions and any additional CBC limits — foam WRB systems are allowed only per the code and manufacturer instructions — § 1403.2 .

Worked example — concrete scenario with numbers

Scenario: A 2‑story wood‑framed office building with a traditional stucco veneer over sheathing.

What the CBC requires (apply § references):

  1. Install at least one continuous WRB over the sheathing (minimum: one layer) and make it continuous to the top of the wall and properly terminate at all penetrations — § 1403.2 .
  2. If using No. 15 asphalt felt as the WRB, apply it horizontally so the upper layer laps over the lower layer at least 2 in (51 mm). Where seams end, lap upper and lower layers at least 6 in (152 mm) — § 1403.2 .
  3. Install flashing where required (window sills, head flashings, base weeps, through‑wall flashings) consistent with the flashing rules referenced by the performance section — flashing per § 1404.4 must be used so water is directed to the exterior — § 1402.2 .
  4. Provide a means of drainage (weeps/weep screed or other approved drainage path) so any water that penetrates the stucco drains to the exterior — the assembled wall must not allow accumulation of water inside the wall — § 1402.2 .
  5. If the owner proposes not to use a WRB/ drainage details, they may instead show ASTM E331 test evidence that the entire assembly (including a representative opening, control joint, wall/eave interface and sill) passed the test at 6.24 psf for 2 hours with no penetration — only then may the prescriptive WRB/drainage requirements be omitted — § 1402.2 .

Related provisions (CBC sections you will commonly need)

  • § 1404.4 — Flashing requirements for exterior walls (referenced by § 1402.2) .
  • § 1404.3 — Condensation protection for exterior wall assemblies (referenced by § 1402.2) .
  • § 1402.6 — Requirements/restrictions when the WRB is combustible (vertical and lateral flame propagation limits) .
  • § 1407.4.1 — EIFS with drainage: performance and WRB specifics (EIFS are given special WRB/drainage treatment) .
  • § 1403.1 — Materials general rules for exterior wall materials (context for WRB choices) .

Code references

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

  • CBC § 1401.1 High relevance — show source text

    ICC code development note: Code change proposals to sections preceded by the designation [BS] will be considered by the IBC—Structural Code Development

    Committee during the 2025 (Group B) Code Development Cycle.

    SECTION 1401—GENERAL

    1401.1 Scope. The provisions of this chapter shall establish the minimum requirements for exterior walls, exterior wall assemblies, exterior wall coverings, exterior wall openings, exterior windows and doors, exterior soffits and fascias, and architectural trim.

    SECTION 1402—PERFORMANCE REQUIREMENTS

    1402.1 General. The provisions of this section shall apply to exterior walls, exterior wall coverings and components thereof.

    1402.2 Weather protection. Buildings shall be provided with a weather-resistant exterior wall assembly. The exterior wall assembly shall include flashing, as described in Section 1404.4. The exterior wall assembly shall be designed and constructed in such a manner as to prevent the accumulation of water within the exterior wall assembly by providing a water-resistive barrier behind the exterior veneer, as described in Section 1403.2, and a means for draining water that enters the assembly to the exterior. Protection against condensation in the exterior wall assembly shall be provided in accordance with Section 1404.3.

    Exceptions:

    1. A weather-resistant exterior wall assembly shall not be required over concrete or masonry walls designed in accordance with Chapters 19 and 21, respectively.
    2. Compliance with the requirements for a means of drainage, and the requirements of Sections 1403.2 and 1404.4, shall not be required for an exterior wall assembly that has been demonstrated through testing to resist wind-driven rain, including joints, penetrations and intersections with dissimilar materials, in accordance with ASTM E331 under the following conditions:

    The exterior wall design shall be considered to resist wind-driven rain where the results of testing, in accordance with ASTM E331, indicate that water did not penetrate control joints in the exterior wall, joints at the perimeter of openings or intersections of terminations with dissimilar materials.

    2.1. Exterior wall test assemblies shall include not fewer than one opening, one control joint, one wall/eave interface and one wall sill. Tested openings and penetrations shall be representative of the intended end-use configuration. 2.2. Exterior wall test assemblies shall be not less than 4 feet by 8 feet (1219 mm by 2438 mm) in size. 2.3. Exterior wall test assemblies shall be tested at a minimum differential pressure of 6.24 pounds per square foot (0.297 kN/m [2] ). 2.4. Exterior wall test assemblies shall be subjected to a minimum test exposure duration of 2 hours. 3. Exterior insulation and finish systems (EIFS) complying with Section 1407.4.1.

    1402.2.1 [BSC-CG] See California Green Building Standards Code, Chapter 5, Division 5.4 for additional weather protection requirements.

    [BS] 1402.3 Wind resistance. Exterior walls, exterior wall coverings, exterior soffits and fascias, and the associated openings, shall be designed and constructed to resist safely the superimposed loads required by Chapter 16.

  • CBC § 1.2 High relevance — show source text

    1_||X||||||||||||||||||||||| |1403|||X|||||||||||||||||||||| |1404|||X|||||||||||||||||||||| |1404.1.2||||||||X|X|X|X|X||X|X|||||||||| |1404.3.3|||||||||||||||||||||X|||| |1405|||X|||||||||||||||||||||| |1406|||X|||||||||||||||||||||| |1407|||X|||||||||||||||||||||| |1408|||X|||||||||||||||||||||| |1411|||X|||||||||||||||||||||| |1412|||X|||||||||||||||||||||| |1413||||||||X|X|X|X|X||X|X||||||||||

    The state agency does not adopt sections identified with the following symbol: The Office of the State Fire Marshal’s adoption of this chapter or individual sections is applicable to structures regulated by other state agencies pursuant to Section 1.11.

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    14-2 2025 CALIFORNIA BUILDING CODE

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    14 EXTERIOR WALLS

    User notes:

    About this chapter: Chapter 14 addresses requirements for exterior walls of buildings. Minimum standards for wall covering materials, such as material performance and fire resistance, installation of wall coverings and the ability of the wall to provide weather protection are provided. This chapter also contains limitations on the areas and heights of combustible wall coverings based on fire separation distances, radiant heat exposure and surface burning characteristics.

    ICC code development note: Code change proposals to sections preceded by the designation [BS] will be considered by the IBC—Structural Code Development

    Committee during the 2025 (Group B) Code Development Cycle.

    SECTION 1401—GENERAL

    1401.1 Scope. The provisions of this chapter shall establish the minimum requirements for exterior walls, exterior wall assemblies, exterior wall coverings, exterior wall openings, exterior windows and doors, exterior soffits and fascias, and architectural trim.

    SECTION 1402—PERFORMANCE REQUIREMENTS

    1402.1 General. The provisions of this section shall apply to exterior walls, exterior wall coverings and components thereof.

    1402.2 Weather protection. Buildings shall be provided with a weather-resistant exterior wall assembly. The exterior wall assembly shall include flashing, as described in Section 1404.4. The exterior wall assembly shall be designed and constructed in such a manner as to prevent the accumulation of water within the exterior wall assembly by providing a water-resistive barrier behind the exterior veneer, as described in Section 1403.2, and a means for draining water that enters the assembly to the exterior. Protection against condensation in the exterior wall assembly shall be provided in accordance with Section 1404.3.

    Exceptions:

    1. A weather-resistant exterior wall assembly shall not be required over concrete or masonry walls designed in accordance with Chapters 19 and 21, respectively.
    2. Compliance with the requirements for a means of drainage, and the requirements of Sections 1403.2 and 1404.4, shall not be required for an exterior wall assembly that has been demonstrated through testing to resist wind-driven rain, including joints, penetrations and intersections with dissimilar materials, in accordance with ASTM E331 under the following conditions:
  • CBC § 1.1 High relevance — show source text

    R703.1 General. Exterior walls shall provide the building with a weather-resistant exterior wall envelope. The exterior wall envelope shall include flashing as described in Section R703.4. Exception: Log walls designed and constructed in accordance with the provisions of ICC 400. R703.1.1 Water resistance. The exterior wall envelope shall be designed and constructed in a manner that prevents the accumulation of water within the wall assembly by providing a water-resistive barrier behind the exterior cladding as required by Section R703.2 and a means of draining to the exterior water that penetrates the exterior cladding. Exceptions:

    1. A weather-resistant exterior wall envelope shall not be required over concrete or masonry walls designed in accordance with Chapter 6 and flashed in accordance with Section R703.4 or R703.8.
    2. Compliance with the requirements for a means of drainage, and the requirements of Sections R703.2 and R703.4, shall not be required for an exterior wall envelope that has been demonstrated to resist wind-driven rain through testing of the exterior wall envelope, including joints, penetrations and intersections with dissimilar materials, in accordance with ASTM E331 under the following conditions: 2.1. Exterior wall envelope test assemblies shall include at least one opening, one control joint, one wall/eave interface and one wall sill. All tested openings and penetrations shall be representative of the intended enduse configuration. 2.2. Exterior wall envelope test assemblies shall be at least 4 feet by 8 feet (1219 mm by 2438 mm) in size. 2.3. Exterior wall assemblies shall be tested at a minimum differential pressure of 6.24 pounds per square foot (299 Pa). 2.4. Exterior wall envelope assemblies shall be subjected to the minimum test exposure for a minimum of 2 hours. The exterior wall envelope design shall be considered to resist wind-driven rain where the results of testing indicate that water did not penetrate control joints in the exterior wall envelope, joints at the perimeter of openings penetration or intersections of terminations with dissimilar materials.

    R703.1.2 Wind resistance. Wall coverings, backing materials and their attachments shall be capable of resisting wind loads in accordance with Tables R301.2.1(1) and R301.2.1(2). Wind-pressure resistance of the siding, exterior soffit and backing materials shall be determined by ASTM E330 or other applicable standard test methods. Where wind-pressure resistance is determined by design analysis, data from approved design standards and analysis conforming to generally accepted engineering practice shall be used to evaluate the siding, exterior soffit and backing material and its fastening. All applicable failure modes including bending rupture of siding, fastener withdrawal and fastener head pull-through shall be considered in the testing or design analysis. Where the wall covering, exterior soffit and backing material resist wind load as an assembly, use of the design capacity of the assembly shall be permitted. R703.2 Water-resistive barrier. Not fewer than one layer of water-resistive barrier shall be applied over studs or sheathing of all exterior walls with flashing as indicated in Section R703.4, in such a manner as to provide a continuous water-resistive barrier behind the exterior wall veneer and behind deck ledgers. The water-resistive barrier material shall be continuous to the top of walls and terminated at penetrations and building appendages in a manner to meet the requirements of the exterior wall envelope as described in Section R703.1.

  • CBC § 1403.1 High relevance — show source text

    SECTION 1403—MATERIALS

    1403.1 General. Materials used for the construction of exterior walls shall comply with the provisions of this section. Materials not prescribed herein shall be permitted, provided that any such alternative has been approved.

    1403.2 Water-resistive barrier. Not fewer than one layer of water-resistive barrier material shall be attached to the studs or sheathing, with flashing as described in Section 1404.4, in such a manner as to provide a continuous water-resistive barrier behind the exterior wall veneer. The intersection between the water-resistive barrier material and fenestration openings shall be flashed and assembled in accordance with the fenestration manufacturer’s installation instructions, or other approved methods for applications not addressed by the fenestration manufacturer’s instructions. The water-resistive barrier material shall be continuous to the top of walls and terminated at penetrations and building appendages in a manner to meet the requirements of the exterior wall envelope as described in Section 1402.2.

    Water-resistive barriers shall comply with one of the following:

    1. No. 15 felt complying with ASTM D226, Type 1.

    2. ASTM E2556, Type I or II.

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    1. Foam plastic insulating sheathing water-resistive barrier systems complying with Section 1402.2 and installed in accordance with manufacturer’s installation instructions.

    2. ASTM E331 in accordance with Section 1402.2.

    3. Other approved materials installed in accordance with the manufacturer’s installation instructions.

    No. 15 asphalt felt and water-resistive barriers complying with ASTM E2556 shall be applied horizontally with the upper layer lapped over the lower layer not less than 2 inches (51 mm). Where joints occur, the upper and lower layer shall be lapped not less than 6 inches (152 mm).

    [BS] 1403.3 Wood. Exterior walls of wood construction shall be designed and constructed in accordance with Chapter 23.

    [BS] 1403.3.1 Basic hardboard. Basic hardboard shall conform to the requirements of ANSI A135.4.

    [BS] 1403.3.2 Hardboard siding. Hardboard siding shall conform to the requirements of ANSI A135.6 and, where used structurally, shall be so identified by the label of an approved agency.

    [BS] 1403.4 Masonry. Exterior walls of masonry construction shall be designed and constructed in accordance with this section and Chapter 21. Masonry units, mortar and metal accessories used in anchored and adhered veneer shall meet the physical requirements of Chapter 21. The backing of anchored and adhered veneer shall be of concrete, masonry, steel framing or wood framing. Continuous insulation meeting the applicable requirements of this code shall be permitted between the backing and the masonry veneer.

    [BS] 1403.5 Metal. Exterior walls constructed of cold-formed or structural steel shall be designed in accordance with Chapter 22. Exterior walls constructed of aluminum shall be designed in accordance with Chapter 20.

    [BS] 1403.5.1 Aluminum siding. Aluminum siding shall conform to the requirements of AAMA 1402.

    [BS] 1403.5.2 Cold-rolled copper. Copper shall conform to the requirements of ASTM B370.

  • CBC § 7.2.1 High relevance — show source text

    On wood-frame construction with an on-grade floor slab system, exterior plaster shall be applied to cover, but not extend below, lath, paper and screed.

    The proportion of aggregate to cementitious materials shall be as set forth in Table R702.1(3).

    R703.7.2.1 Weep screeds. A minimum 0.019-inch (0.5 mm) (No. 26 galvanized sheet gage), corrosion-resistant weep screed or plastic weep screed, with a minimum vertical attachment flange of 3 [1] / 2 inches (89 mm), shall be provided at or below the foundation plate line on exterior stud walls in accordance with ASTM C926. The weep screed shall be placed not less than 4 inches (102 mm) above the earth or 2 inches (51 mm) above paved areas and shall be of a type that will allow trapped water to drain to the exterior of the building. The weather-resistant barrier shall lap the attachment flange. The exterior lath shall cover and terminate on the attachment flange of the weep screed.

    R703.7.3 Water-resistive barriers. Water-resistive barriers shall be installed as required in Section R703.2 and shall comply with Section R703.7.3.1 or R703.7.3.2.

    Exception: Sections R703.7.3.1 and R703.7.3.2 shall not apply to construction where accumulation, condensation or freezing of moisture will not damage the materials.

    R703.7.3.1 Dry climates. In Dry (B) climate zones indicated in Section R702.7.3 and Table R702.7.3, water-resistive barriers shall comply with one of the following:

    1. The water-resistive barrier shall be two layers of 10-minute Grade D paper or have a water resistance equal to or greater than two layers of a water-resistive barrier complying with ASTM E2556, Type I. The individual layers shall be installed independently such that each layer provides a separate continuous plane. Flashing installed in accordance with Section R703.4 and intended to drain to the water-resistive barrier shall be directed between the layers.
    2. The water-resistive barrier shall be 60-minute Grade D paper or have a water resistance equal to or greater than one layer of a water-resistive barrier complying with ASTM E2556, Type II. The water-resistive barrier shall be separated from the stucco by a layer of foam plastic insulating sheathing, other non-water-absorbing layer, a drainage space or means of drainage complying with Section R703.7.3.2. Flashing installed in accordance with Section 703.4 and intended to drain to the water-resistive barrier shall be directed to the exterior side of the water-resistive barrier.

    R703.7.3.2 Moist or marine climates. In the Moist (A) or Marine (C) climate zones indicated in Section R702.7.3 and Table R702.7.3, water-resistive barriers shall comply with one of the following:

    1. In addition to complying with Section R703.7.3.1, a space or drainage material not less than [3] / 16 inch (5 mm) in depth shall be added to the exterior side of the water-resistive barrier.
  • CBC § 2.1. High relevance — show source text

    The exterior wall design shall be considered to resist wind-driven rain where the results of testing, in accordance with ASTM E331, indicate that water did not penetrate control joints in the exterior wall, joints at the perimeter of openings or intersections of terminations with dissimilar materials.

    2.1. Exterior wall test assemblies shall include not fewer than one opening, one control joint, one wall/eave interface and one wall sill. Tested openings and penetrations shall be representative of the intended end-use configuration. 2.2. Exterior wall test assemblies shall be not less than 4 feet by 8 feet (1219 mm by 2438 mm) in size. 2.3. Exterior wall test assemblies shall be tested at a minimum differential pressure of 6.24 pounds per square foot (0.297 kN/m [2] ). 2.4. Exterior wall test assemblies shall be subjected to a minimum test exposure duration of 2 hours. 3. Exterior insulation and finish systems (EIFS) complying with Section 1407.4.1.

    1402.2.1 [BSC-CG] See California Green Building Standards Code, Chapter 5, Division 5.4 for additional weather protection requirements.

    [BS] 1402.3 Wind resistance. Exterior walls, exterior wall coverings, exterior soffits and fascias, and the associated openings, shall be designed and constructed to resist safely the superimposed loads required by Chapter 16.

    [BS] 1402.3.1 Attachments through exterior insulation. Where exterior wall coverings are attached to the building structure through exterior continuous insulation, furring and attachments through the exterior insulation shall be designed to resist design loads determined in accordance with Chapter 16, including support of cladding weight as applicable. Exterior wall coverings attached to the building structure through foam plastic insulating sheathing shall comply with the attachment requirements of Section 1404.5.1, 1404.5.2 or 1404.5.3.

    1402.4 Fire resistance. Exterior walls shall be fire-resistance rated as required by other sections of this code with opening protection as required by Chapter 7.

    1402.5 Vertical and lateral flame propagation. Exterior walls on buildings of Type I, II, III and IV construction that contain a combustible exterior wall covering, combustible insulation or a combustible water-resistive barrier shall comply with Sections 1402.5.1 through 1402.5.5, as applicable. Where compliance with NFPA 285 and associated acceptance criteria is required in Sections 1402.5.1 through 1402.5.5, the exterior wall assembly shall be tested in accordance with and comply with the acceptance criteria of NFPA 285.

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    1402.5.1 Combustible water-resistive barrier. Exterior walls containing a combustible water-resistive barrier shall comply with Section 1402.6.

    1402.5.2 Metal composite material (MCM). Exterior walls containing metal composite material (MCM) systems shall comply with Section 1406.

    1402.5.3 Exterior insulation and finish system (EIFS). Exterior walls containing an exterior insulation and finish system (EIFS) shall comply with Section 1407.

  • CBC § 1402.7 High relevance — show source text

    1402.7 Exterior wall veneers manufactured using combustible adhesives. Exterior wall assemblies on buildings of Type I, II, III or IV construction that are greater than 40 feet (12 192 mm) in height above grade plane and contain an exterior wall veneer manufactured using a combustible adhesive to laminate a metal core with noncombustible facing materials shall be tested in accordance with, and comply with, the acceptance criteria of NFPA 285, with the adhesive level at the maximum application rate intended for use. Combustibility shall be determined in accordance with Section 703.3.

    1402.8 Vertical and lateral flame propagation compliance methods. When exterior wall assemblies are required in this chapter to be tested for vertical and lateral flame propagation in accordance with and comply with the acceptance criteria of NFPA 285, compliance with the requirements shall be established by any of the following:

    1. An exterior wall assembly tested in accordance with and meeting the acceptance criteria of NFPA 285.
    2. An exterior wall assembly design listed by an approved agency for compliance with NFPA 285.
    3. An approved analysis based on an assembly or condition tested in accordance with and meeting the acceptance criteria of NFPA 285.

    [BS] 1402.9 Flood resistance. For buildings in flood hazard areas as established in Section 1612.3, exterior walls extending below the elevation required by Section 1612 shall be constructed with flood-damage-resistant materials.

    [BS] 1402.10 Flood resistance for coastal high-hazard areas and coastal A zones. For buildings in coastal high-hazard areas and coastal A zones as established in Section 1612.3, electrical, mechanical and plumbing system components shall not be mounted on or penetrate through exterior walls that are designed to break away under flood loads.

    SECTION 1403—MATERIALS

    1403.1 General. Materials used for the construction of exterior walls shall comply with the provisions of this section. Materials not prescribed herein shall be permitted, provided that any such alternative has been approved.

    1403.2 Water-resistive barrier. Not fewer than one layer of water-resistive barrier material shall be attached to the studs or sheathing, with flashing as described in Section 1404.4, in such a manner as to provide a continuous water-resistive barrier behind the exterior wall veneer. The intersection between the water-resistive barrier material and fenestration openings shall be flashed and assembled in accordance with the fenestration manufacturer’s installation instructions, or other approved methods for applications not addressed by the fenestration manufacturer’s instructions. The water-resistive barrier material shall be continuous to the top of walls and terminated at penetrations and building appendages in a manner to meet the requirements of the exterior wall envelope as described in Section 1402.2.

    Water-resistive barriers shall comply with one of the following:

    1. No. 15 felt complying with ASTM D226, Type 1.

    2. ASTM E2556, Type I or II.

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    1. Foam plastic insulating sheathing water-resistive barrier systems complying with Section 1402.2 and installed in accordance with manufacturer’s installation instructions.

    2. ASTM E331 in accordance with Section 1402.2.

    3. Other approved materials installed in accordance with the manufacturer’s installation instructions.

  • CBC § 6.7.5 High relevance — show source text

    FIGURE BL103.6.7.5—TYPICAL HEMP-LIME THICK LINING

    BONDING COAT, WHERE

    BL103.6.7.6 Minimum thickness at anchorage rails. The minimum thickness of hemp-lime between the exterior face of vertical anchorage rails and the finished face of hemp-lime shall be 3 inches (76 mm) or in accordance with the binder manufacturer’s specifications.

    BL103.7 Moisture control. Hemp-lime assemblies shall be protected from water intrusion and damage in accordance with Sections BL103.7.1 through BL103.7.9.

    BL103.7.1 Water-resistive barriers. Water-resistive barriers are prohibited on hemp-lime walls, except as permitted or required elsewhere in this appendix.

    BL103.7.2 Vapor retarders. Vapor retarders are prohibited on hemp-lime walls, except as permitted or required elsewhere in this appendix.

    BL103.7.3 Penetrations in hemp-lime walls. Penetrations in exterior hemp-lime walls shall be sealed with an approved sealant or gasket on the exterior side of the wall in all climate zones and on the interior side of the wall in Climate Zones 5, 6, 7, 8 and Marine 4, as defined in Chapter 11.

    BL103.7.4 Horizontal surfaces. Hemp-lime walls and other hemp-lime assemblies shall be provided with a water-resistive barrier at weather-exposed horizontal surfaces. The water-resistive barrier shall be of a material and installation that will prevent water from entering the wall system. Horizontal surfaces shall include exterior window sills and sills at exterior niches. Horizontal surfaces shall be sloped not less than 1 unit vertical in 12 units horizontal (8-percent slope) and shall drain away from hemp-lime

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    APPENDIX BL — HEMP-LIME (HEMPCRETE) CONSTRUCTION

    walls and other assemblies. Where the water-resistive barrier is below the finish material, it shall be sloped not less than 1 unit vertical in 12 units horizontal (8-percent slope) and shall drain to the exterior surface of the hemp-lime wall’s vertical finish.

    BL103.7.5 Air barrier. Exterior hemp-lime walls shall have a vapor-permeable air barrier on all exterior and interior surfaces, except as permitted or required elsewhere in this appendix. Plaster in accordance with Section BL104.3 shall be acceptable as an air barrier.

    BL103.7.6 Separation of hemp-lime and earth or paved areas. Hemp-lime shall be not less than 8 inches (203 mm) above exposed earth or paved areas.

    BL103.7.7 Separation of exterior plaster and earth or paved areas. Exterior plaster applied to hemp-lime shall be not less than 8 inches (203 mm) above exposed earth or paved areas.

    BL103.7.8 Separation of hemp-lime and exterior plaster from foundation. Hemp-lime and exterior plaster shall be separated from the foundation with an approved moisture barrier.

    BL103.7.9 Base of wall flashing. The outer face of exterior walls shall be flashed to prevent water intrusion at the base of the wall.

    SECTION BL104—FINISHES

  • CBC § 2.4. High relevance — show source text

    2.4. Exterior wall envelope assemblies shall be subjected to the minimum test exposure for a minimum of 2 hours. The exterior wall envelope design shall be considered to resist wind-driven rain where the results of testing indicate that water did not penetrate control joints in the exterior wall envelope, joints at the perimeter of openings penetration or intersections of terminations with dissimilar materials.

    R703.1.2 Wind resistance. Wall coverings, backing materials and their attachments shall be capable of resisting wind loads in accordance with Tables R301.2.1(1) and R301.2.1(2). Wind-pressure resistance of the siding, exterior soffit and backing materials shall be determined by ASTM E330 or other applicable standard test methods. Where wind-pressure resistance is determined by design analysis, data from approved design standards and analysis conforming to generally accepted engineering practice shall be used to evaluate the siding, exterior soffit and backing material and its fastening. All applicable failure modes including bending rupture of siding, fastener withdrawal and fastener head pull-through shall be considered in the testing or design analysis. Where the wall covering, exterior soffit and backing material resist wind load as an assembly, use of the design capacity of the assembly shall be permitted. R703.2 Water-resistive barrier. Not fewer than one layer of water-resistive barrier shall be applied over studs or sheathing of all exterior walls with flashing as indicated in Section R703.4, in such a manner as to provide a continuous water-resistive barrier behind the exterior wall veneer and behind deck ledgers. The water-resistive barrier material shall be continuous to the top of walls and terminated at penetrations and building appendages in a manner to meet the requirements of the exterior wall envelope as described in Section R703.1. Where the water-resistive barrier also functions as a component of a continuous air barrier, the waterresistive barrier shall be installed as an air barrier in accordance with Section N1102.5.1.1. Water-resistive barrier materials shall comply with one of the following:

    1. No. 15 felt complying with ASTM D226, Type 1.

    2. ASTM E2556, Type 1 or 2.

    3. Foam plastic insulating sheathing water-resistive barrier systems complying with Section R703.1.1 and installed in accordance with the manufacturer’s installation instructions.

    4. ASTM E331 in accordance with Section R703.1.1.

    5. Other approved materials in accordance with the manufacturer’s installation instructions. No.15 asphalt felt and water-resistive barriers complying with ASTM E2556 shall be applied horizontally, with the upper layer lapped over the lower layer not less than 2 inches (51 mm), and where joints occur, shall be lapped not less than 6 inches (152 mm). Exception: A water-resistive barrier shall not be required in unconditioned detached tool sheds, storage sheds, playhouses, and other similar accessory structures provided all of the following requirements are met:

    6. Exterior wall covering is limited to siding that is attached direct to studs.

    7. Exterior walls are uninsulated.

    8. Interior side of exterior walls has no wall covering or wall finishes. R703.3 Wall covering nominal thickness and attachments. The nominal thickness and attachment of exterior wall coverings shall be in accordance with Table R703.3(1), the wall covering material requirements of this section, and the wall covering manufacturer’s installation instructions. Cladding attachment over foam sheathing shall comply with the additional requirements and limitations of Sections R703.15 through R703.17.

  • CBC § 1402.6 High relevance — show source text

    1402.6 Water-resistive barriers. Exterior walls on buildings of Type I, II, III or IV construction that are greater than 40 feet (12 192 mm) in height above grade plane and contain a combustible water-resistive barrier shall be tested in accordance with and comply with the acceptance criteria of NFPA 285.Combustibility shall be determined in accordance with Section 703.3. For the purposes of this section, fenestration products, flashing of fenestration products and water-resistive-barrier flashing and accessories at other locations, including through wall flashings, shall not be considered part of the water-resistive barrier.

    Exceptions:

    1. Exterior walls in which the water-resistive barrier is the only combustible component and the exterior wall has an exterior wall covering of brick, concrete, stone, terra cotta, stucco or steel with minimum thicknesses in accordance with Table 1404.2.

    2. Exterior walls in which the water-resistive barrier is the only combustible component and the water-resistive barrier complies with the following: 2.1 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 when tested on specimens at the thickness intended for use, in accordance with ASTM E1354, in the horizontal orientation and at an incident radiant heat flux of 50 kW/m [2] .

    2.2 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, with test specimen preparation and mounting in accordance with ASTM E2404.

    1402.7 Exterior wall veneers manufactured using combustible adhesives. Exterior wall assemblies on buildings of Type I, II, III or IV construction that are greater than 40 feet (12 192 mm) in height above grade plane and contain an exterior wall veneer manufactured using a combustible adhesive to laminate a metal core with noncombustible facing materials shall be tested in accordance with, and comply with, the acceptance criteria of NFPA 285, with the adhesive level at the maximum application rate intended for use. Combustibility shall be determined in accordance with Section 703.3.

    1402.8 Vertical and lateral flame propagation compliance methods. When exterior wall assemblies are required in this chapter to be tested for vertical and lateral flame propagation in accordance with and comply with the acceptance criteria of NFPA 285, compliance with the requirements shall be established by any of the following:

    1. An exterior wall assembly tested in accordance with and meeting the acceptance criteria of NFPA 285.
    2. An exterior wall assembly design listed by an approved agency for compliance with NFPA 285.
    3. An approved analysis based on an assembly or condition tested in accordance with and meeting the acceptance criteria of NFPA 285.

    [BS] 1402.9 Flood resistance. For buildings in flood hazard areas as established in Section 1612.3, exterior walls extending below the elevation required by Section 1612 shall be constructed with flood-damage-resistant materials.

    [BS] 1402.10 Flood resistance for coastal high-hazard areas and coastal A zones. For buildings in coastal high-hazard areas and coastal A zones as established in Section 1612.3, electrical, mechanical and plumbing system components shall not be mounted on or penetrate through exterior walls that are designed to break away under flood loads.

    SECTION 1403—MATERIALS

  • CBC § 1406.10.3 High relevance — show source text
    1. The MCM is used as elements of balconies and similar projections, architectural trim or embellishments.

    1406.10.3 Full-scale tests. The MCM system shall be tested in accordance with, and comply with, the acceptance criteria of NFPA 285. Such testing shall be performed on the MCM system with the MCM in the maximum thickness intended for use.

    1406.11 Type V construction. MCM shall be permitted to be installed on buildings of Type V construction.

    1406.12 Foam plastic insulation. Where MCM systems are included in an exterior wall assembly containing foam plastic insulation, the exterior wall assembly shall also comply with the requirements of Section 2603.

    1406.13 Labeling. MCM shall be labeled in accordance with Section 1703.5.

    SECTION 1407—EXTERIOR INSULATION AND FINISH SYSTEMS (EIFS)

    1407.1 General. The provisions of this section shall govern the materials, construction and quality of exterior insulation and finish systems (EIFS) for use as exterior wall coverings in addition to other applicable requirements of Chapters 7, 14, 16, 17 and 26.

    1407.2 Performance characteristics. EIFS shall be constructed such that it meets the performance characteristics required in ASTM E2568.

    [BS] 1407.3 Structural design. The underlying structural framing and substrate shall be designed and constructed to resist loads as required by Chapter 16.

    1407.4 Weather resistance. EIFS shall comply with Section 1402 and shall be designed and constructed to resist wind and rain in accordance with this section and the manufacturer’s application instructions.

    1407.4.1 EIFS with drainage. EIFS with drainage shall have an average minimum drainage efficiency of 90 percent when tested in accordance the requirements of ASTM E2273 and is required on framed walls of Type V construction, Group R1, R2, R3 and R4 occupancies.

    1407.4.1.1 Water-resistive barrier. For EIFS with drainage, the water-resistive barrier shall comply with Section 1403.2 or ASTM E2570.

    1407.5 Exterior walls of buildings of any height. Exterior wall assemblies containing an EIFS exterior wall covering shall comply with Section 2603.5.

    1407.6 Installation. Installation of the EIFS and EIFS with drainage shall be in accordance with the EIFS manufacturer’s instructions.

    1407.7 Special inspections. EIFS installations shall comply with the provisions of Sections 1704.2 and 1705.17.

    SECTION 1408—HIGH-PRESSURE DECORATIVE EXTERIOR-GRADE COMPACT LAMINATES (HPL)

    1408.1 General. The provisions of this section shall govern the materials, construction and quality of High-Pressure Decorative ExteriorGrade Compact Laminates (HPL) for use as exterior wall coverings in addition to other applicable requirements of Chapters 14 and 16.

    1408.2 Exterior wall covering. HPL used as exterior wall covering or as elements of balconies and similar projections and bay and oriel windows to provide cladding or weather resistance shall comply with Sections 1408.4 through 1408.14.

    1408.3 Architectural trim and embellishments. HPL used as architectural trim or embellishments shall comply with Sections 1408.7 through 1408.14.

  • CBC § 1407.4.1 High relevance — show source text

    1407.4.1 EIFS with drainage. EIFS with drainage shall have an average minimum drainage efficiency of 90 percent when tested in accordance the requirements of ASTM E2273 and is required on framed walls of Type V construction, Group R1, R2, R3 and R4 occupancies.

    1407.4.1.1 Water-resistive barrier. For EIFS with drainage, the water-resistive barrier shall comply with Section 1403.2 or ASTM E2570.

    1407.5 Exterior walls of buildings of any height. Exterior wall assemblies containing an EIFS exterior wall covering shall comply with Section 2603.5.

    1407.6 Installation. Installation of the EIFS and EIFS with drainage shall be in accordance with the EIFS manufacturer’s instructions.

    1407.7 Special inspections. EIFS installations shall comply with the provisions of Sections 1704.2 and 1705.17.

    SECTION 1408—HIGH-PRESSURE DECORATIVE EXTERIOR-GRADE COMPACT LAMINATES (HPL)

    1408.1 General. The provisions of this section shall govern the materials, construction and quality of High-Pressure Decorative ExteriorGrade Compact Laminates (HPL) for use as exterior wall coverings in addition to other applicable requirements of Chapters 14 and 16.

    1408.2 Exterior wall covering. HPL used as exterior wall covering or as elements of balconies and similar projections and bay and oriel windows to provide cladding or weather resistance shall comply with Sections 1408.4 through 1408.14.

    1408.3 Architectural trim and embellishments. HPL used as architectural trim or embellishments shall comply with Sections 1408.7 through 1408.14.

    [BS] 1408.4 Structural design. HPL systems shall be designed and constructed to resist wind loads as required by Chapter 16 for components and cladding.

    1408.5 Approval. Results of approved tests or an engineering analysis shall be submitted to the building official to verify compliance with the requirements of Chapter 16 for wind loads.

    1408.6 Weather resistance. HPL systems shall comply with Section 1402 and shall be designed and constructed to resist wind and rain in accordance with this section and the manufacturer’s instructions.

    14-18 2025 CALIFORNIA BUILDING CODE

    on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.

    EXTERIOR WALLS

    1408.7 Durability. HPL systems shall be constructed of approved materials that maintain the performance characteristics required in Section 1408 for the duration of use.

    1408.8 Fire-resistance rating. Where HPL systems are used on exterior walls required to have a fire-resistance rating in accordance with Section 705, evidence shall be submitted to the building official that the required fire-resistance rating is maintained.

    Exception: HPL systems not containing foam plastic insulation, which are installed on the outer surface of a fire-resistance-rated exterior wall in a manner such that the attachments do not penetrate through the entire exterior wall assembly, shall not be required to comply with this section.

    1408.9 Surface-burning characteristics. Unless otherwise specified, HPL shall have a flame spread index of 75 or less and a smokedeveloped index of 450 or less when tested in the minimum and maximum thicknesses intended for use in accordance with ASTM E84 or UL 723.

Frequently asked questions

Do I always need a WRB behind every exterior cladding?

Yes — for framed walls and typical exterior veneers the CBC requires not fewer than one layer of WRB behind the veneer. The code does allow exceptions (monolithic concrete/masonry or a successful ASTM E331 wind‑driven rain test for the assembly) — § 1403.2 and § 1402.2 .

What lap sizes do I need when using No. 15 felt or ASTM E2556 WRB?

Apply horizontally with the upper layer lapped over the lower not less than 2 in (51 mm); where joints occur lap upper and lower layers not less than 6 in (152 mm) — § 1403.2 .

Can I omit flashing if I use a WRB?

No — flashing is required and the WRB must be integrated with flashing. § 1402.2 explicitly requires the exterior wall assembly to include flashing as described in § 1404.4 and to provide a drainage means — § 1402.2 .

Are foam‑plastic sheathing WRB systems allowed?

Yes, but only where the system complies with the code’s conditions (listed in § 1403.2) and is installed per the manufacturer’s instructions — § 1403.2 .

What if my wall assembly is tested per ASTM E331?

If the tested assembly (including representative openings, joints and interfaces) shows no penetration under the required test conditions (min size 4 ft × 8 ft, 6.24 psf, 2 hours), compliance with the prescriptive drainage and WRB/flashing requirements may not be required — § 1402.2 .

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