CWUIC · California Wildland-Urban Interface Code

How must roof covering voids, underlayment and airspace be constructed?

If a roofing profile creates a space above a combustible deck, CWUIC requires either a 72‑lb mineral cap sheet (ASTM D3909) or a listed Class A underlayment (ASTM E108/UL 790) installed per its listing, plus bird stops at eaves and mudded hip/ridge caps. Exceptions allow 1" mineral wool board in lieu of the cap sheet or an FRT exterior‑rated sheathing to waive the Class A underlayment requirement; see **§ 504.2.1**, **§ 504.2.1.1**, and **§ 504.2.1.2**.

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Last reviewed: July 6, 2026

What the code requires — plain English

When a roof covering creates a void or airspace over a combustible deck, the roof must either have a 72 lb. (32.7 kg) mineral-surfaced cap sheet installed over the deck or a listed, Class A roof underlayment installed as part of a tested Class A roof assembly. Also, bird stops at eaves and hip/ridge caps mudded in are required to reduce ember/debris entry. These requirements are set out in § 504.2.1, § 504.2.1.1 and § 504.2.1.2 of the California Wildland‑Urban Interface Code.

If a roofing profile leaves a space between roofing and a combustible roof deck, either protect the deck with the specified mineral cap sheet (or equivalent noncombustible board) or install a listed Class A underlayment — and keep bird stops and mudded hip/ridge caps in place to block embers.

Requirements in detail

When the roof covering creates an airspace (combustible deck + profile creates void)

  • Install a 72 lb. (32.7 kg) cap sheet complying with ASTM D3909 over the roof deck. § 504.2.1.1.
  • Exception: the cap sheet is not required where at least 1 inch of mineral wool board or other noncombustible material is located between the roofing and the wood framing or deck. § 504.2.1.1 (Exception).
  • Install bird stops at the eaves where the profile fits, to prevent entry of debris at the eave. § 504.2.1.1.
  • Hip and ridge caps shall be mudded in to prevent intrusion of fire or embers. § 504.2.1.1.

When using roof underlayment to address the void

  • Install a listed and labeled roof underlayment that was tested as part of the roof assembly to ASTM E108 or UL 790 and shown to achieve a Class A fire classification. Install in accordance with the underlayment manufacturer’s installation instructions and listing. § 504.2.1.2.
  • Bird stops at the eaves and mudded hip/ridge caps are also required when the profile fits. § 504.2.1.2.
  • Exception: If the roof sheathing is fire-retardant‑treated (FRT) wood labeled for exterior use and it complies with Section 2303.2 of the California Building Code, the underlayment is not required to have a Class A classification. § 504.2.1.2 (Exception).

Quick-reference table (decision-relevant dimensions/values)

Item Value / Requirement Code Reference
Cap sheet weight 72 lb. (32.7 kg) mineral‑surfaced cap sheet; comply with ASTM D3909 § 504.2.1.1
Noncombustible board exception Not less than 1 inch of mineral wool board or equivalent noncombustible material in lieu of cap sheet § 504.2.1.1 (Exception)
Underlayment fire classification Listed/labeled underlayment tested with the roof assembly to ASTM E108 or UL 790 for Class A § 504.2.1.2
Underlayment exception (FRT) Fire-retardant-treated wood sheathing labeled for exterior use per CBC § 2303.2 — Class A underlayment not required § 504.2.1.2 (Exception)
Eave protection Bird stops at eaves where profile fits § 504.2.1.1 and § 504.2.1.2
Hip & ridge protection Hip and ridge caps shall be mudded in to resist ember intrusion § 504.2.1.1 and § 504.2.1.2

Exceptions & special cases

  • The cap sheet requirement can be avoided if you provide a minimum 1‑inch layer of mineral wool board (or other noncombustible material) between roofing and the combustible deck (§ 504.2.1.1).
  • The Class A underlayment requirement is waived when the roof sheathing is FRT wood labeled for exterior use and meeting CBC § 2303.2 — confirm labeling and compliance before relying on this exception (§ 504.2.1.2 Exception).
  • The code does not state a minimum numeric thickness for the “airspace” itself or a minimum clearance dimension created by the roofing profile; it only triggers the cap‑sheet/underlayment requirements when such a void exists (§ 504.2.1). If you need a numeric clearance for structural/ventilation reasons, consult the roof‑covering manufacturer and the Building Code provisions that govern decks and ventilation.

Common mistakes

  • Assuming any underlayment is acceptable — the underlayment must be listed and tested with the roof assembly to achieve Class A unless the FRT exception applies (§ 504.2.1.2).
  • Installing a cap sheet lighter than 72 lb. AST M D3909 or using a nonconforming material in place of the specified cap sheet (§ 504.2.1.1).
  • Forgetting bird stops at the eaves or failing to mudd hip/ridge caps — both are explicit ember‑ingress controls required by the sections. § 504.2.1.1 / § 504.2.1.2.
  • Relying on a manufacturer’s general underlayment instructions but not verifying the listing showing testing with the roof assembly to ASTM E108 or UL 790. The listing is required by § 504.2.1.2.

Worked example

Scenario: You are reroofing a home in the WUI. The roof covering is a profiled metal shingle that leaves an airspace over a combustible wood structural deck.

  1. Because the roofing profile creates a void over a combustible deck, § 504.2.1 applies and you must comply with § 504.2.1.1 or § 504.2.1.2.
  2. Option A — cap sheet route: install a 72 lb. mineral‑surfaced cap sheet (ASTM D3909) over the wood deck (meets § 504.2.1.1). Add bird stops at the eaves where the profile fits and mud hip and ridge caps.
  3. Option B — underlayment route: install a listed Class A underlayment (tested as part of the roof assembly to ASTM E108 or UL 790) in accordance with the listing and manufacturer instructions; also install bird stops and mudded hip/ridge caps. Verify the underlayment’s listing specifically shows it as part of a Class A assembly with your roof covering. § 504.2.1.2.
  4. If the roof sheathing is FRT wood, labeled for exterior use and meeting CBC § 2303.2, you may choose Option B without requiring Class A underlayment — but only after verifying the sheathing labeling and applicability of the CBC section. § 504.2.1.2 (Exception).

Related provisions

  • § 504.2.1 — Roof covering voids (general trigger for § 504.2.1.1 / 504.2.1.2).
  • § 504.2.1.1 — Airspace under roof covering (cap sheet, 1" mineral wool exception, bird stops, mudded hip/ridge).
  • § 504.2.1.2 — Roof underlayment (listed Class A underlayment requirement; FRT exception).
  • § 504.2.2 — Roof valleys (valley flashing and mineral cap sheet requirement).
  • § 504.3 — Protection of enclosed eaves (materials and performance options for underside protection).

Code references

Grounded in the retrieved California Wildland-Urban Interface Code — click a citation to read the verbatim passage:

  • CWUIC § 1.1 High relevance — show source text

    For roof slopes of 4 units vertical in 12 units horizontal (4:12)
    or greater, underlayment shall be one layer applied in the
    following manner: underlayment shall be applied shingle
    fashion, parallel to and starting from the eave and lapped 2
    inches. End laps shall be 4 inches and shall be offset by 6 feet.
    3. A single layer of self-adhering polymer modified bitumen
    underlayment complying with ASTM D1970, installed in
    accordance with the underlayment and roof covering manu-
    facturer’s installation instructions for the deck material, roof
    ventilation configuration and climate exposure of the roof
    covering.|Underlayment shall be one of the following:
    1. Two layers of mechanically fastened underlayment applied in
    the following manner: Apply a strip of underlayment felt that is
    half the width of a full sheet parallel to and starting at the
    eaves, fastened sufficiently to hold in place. Starting at the
    eave, apply full width sheets of underlayment, overlapping
    successive sheets half the width of a full sheet plus 2 inches.
    Distortionsin the underlayment shall not interferewith the abil-
    ity of the shingles to seal. End laps shall be 4 inches and shall
    be offset by 6 feet.
    2. A minimum 4-inch-wide strip of self-adhering polymer modi-
    fied bitumen underlayment complying with ASTM D1970,
    installed in accordance with the manufacturer’s installation
    instructions for the deck material, shall be applied over all
    joints in the roof decking. An approved underlayment comply-
    ing with Table R905.1.1(1) for the applicable roof covering shall
    be applied over the entire roof over the 4-inch-wide
    membrane strips.
    3. A single layer of self-adhering polymer modified bitumen
    underlayment complying with ASTM D1970, installed in accor-
    dance
    with
    the
    underlayment
    and
    roof
    covering
    manufacturer’s installation instructions for the deck material,
    roof ventilation configuration and climate exposure of the roof
    covering.| |Metal roof
    shingles|R905.4|Apply in accordance with the manufacturer’s installation
    instructions.|Underlayment shall be one of the following:
    1. Two layers of mechanically fastened underlayment applied in
    the following manner: Apply a strip of underlayment that is
    half the width of a full sheet parallel to and starting at the
    eaves, fastened sufficiently to hold in place. Starting at the
    eave, apply full width sheets of underlayment, overlapping
    successive sheets half the width of a full sheet plus 2 inches.
    End laps shall be 4 inches and shall be offset by 6 feet.
    2. A minimum 4-inch-wide strip of self-adhering polymer modified
    bitumen underlayment complying with ASTM D1970, installed
    in accordance with the manufacturer’s installation instructions
    for the deck material, shall be applied over all joints in the roof
    decking. An approved underlayment complying with Table
    R905.1.1(1) for the applicable roof covering shall be applied
    over the entire roof over the 4-inch-wide membrane strips.
    3.

  • CWUIC § 504.2.1.1 High relevance — show source text

    504.2.1.1 Airspace under roof covering. Where the roof covering is installed over a combustible deck and the roofing profile creates an airspace under the roof covering, the installation shall comply with the following: 1. A 72 lb. (32.7 kg) cap sheet complying with ASTM D3909 Standard Specification for “Asphalt Rolled Roofing (Glass Felt) Surfaced with Mineral Granules,” shall be installed over the roof deck.

    Exception: Cap sheet is not required where not less than 1 inch of mineral wool board or other noncombustible material is located between the roofing material and wood framing or deck. 2. Bird stops shall be used at the eaves where the profile fits, to prevent the entry of debris at the eave. 3. Hip and ridge caps shall be mudded in to prevent intrusion of fire or embers.

    504.2.1.2 Roof underlayment. A listed and labeled roof underlayment that has been tested as part of the roof assembly in accor- dance with ASTM E108 or UL 790 for Class A fire classification shall be installed in accordance with the underlayment manufacturer’s installation instructions and listing. Bird stops shall be used at the eaves when the profile fits, to prevent the entry of debris at the eave. Hip and ridge caps shall be mudded in to prevent intrusion of fire or embers.

    Exception: Where the roof sheathing is fire-retardant-treated wood labeled for exterior use and complies with Section 2303.2 of the California Building Code, the underlayment shall not be required to comply with a Class A fire classification.

    504.2.2 Roof valleys. Where provided, valley flashings shall be not less than 0.019 inch (0.48 mm) (No. 26 galvanized sheet gage) corrosion-resistant metal installed over a minimum 36-inch-wide (914 mm) underlayment consisting of one layer of 72-pound (32.4 kg) mineral-surfaced, nonperforated cap sheet complying with ASTM D3909 running the full length of the valley.

    504.3 Protection of enclosed eaves. The exposed underside of enclosed roof eaves and soffits shall be protected on the exposed underside by one or more of the following:

    1. Noncombustible materials.

    2. Ignition-resistant building materials. 3. Materials approved for not less than 1-hour fire-resistance-rated construction on the exterior side, as tested in accordance with ASTM E119 or UL 263.

    4. 2-inch (51 mm) nominal dimension lumber . 5. 1-inch (25 mm) nominal fire-retardant-treated lumber or [3] / 4 -inch (19.1 mm) nominal fire-retardant-treated plywood, identified for exterior use and complying with Section 2303.2 of the California Building Code . 6. Boxed-in roof eave soffit assemblies with an underside that meets the performance criteria in Section 504.7.2 when tested in accordance with the test procedures set forth in ASTM E2957. 7. Boxed-in roof eave soffit assemblies with an underside that meets the performance criteria in accordance with the test proce- dures set forth in SFM Standard 12-7A-3.

  • CWUIC § 504.1 High relevance — show source text

    � SECTION 504—IGNITION-RESISTANT CONSTRUCTION

    504.1 General. Ignition-resistant construction shall be in accordance with Sections 504.2 through 504.11.6 .

    5-4 2025 CALIFORNIA WILDLAND-URBAN INTERFACE CODE

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

    SPECIAL BUILDING CONSTRUCTION REGULATIONS

    504.2 Roof assembly. Roofs shall have a roof assembly that complies with a Class A fire classification when tested in accordance with ASTM E108 or UL 790.

    Exceptions: The following assemblies are exempt from testing and shall be considered as equivalent to the Class A fire classification:

    1. Roof assemblies with coverings of brick, masonry or an exposed concrete roof deck.
    2. Roof assemblies with ferrous or copper shingles or sheets, metal sheets and shingles, clay or concrete roof tile or slate installed on noncombustible decks or ferrous, copper or metal sheets installed without a roof deck on noncombustible framing.
    3. Roof assemblies with minimum 16 oz/sq ft (0.0416 kg/m [2] ) copper sheets installed over combustible roof decks. 4. Roof assemblies of slate roof covering installed over ASTM D226, Type II underlayment over combustible decks.

    504.2.1 Roof covering voids. Where there is a void under the roof covering it shall comply with Section 504.2.1.1 or 504.2.1.2.

    504.2.1.1 Airspace under roof covering. Where the roof covering is installed over a combustible deck and the roofing profile creates an airspace under the roof covering, the installation shall comply with the following: 1. A 72 lb. (32.7 kg) cap sheet complying with ASTM D3909 Standard Specification for “Asphalt Rolled Roofing (Glass Felt) Surfaced with Mineral Granules,” shall be installed over the roof deck.

    Exception: Cap sheet is not required where not less than 1 inch of mineral wool board or other noncombustible material is located between the roofing material and wood framing or deck. 2. Bird stops shall be used at the eaves where the profile fits, to prevent the entry of debris at the eave. 3. Hip and ridge caps shall be mudded in to prevent intrusion of fire or embers.

    504.2.1.2 Roof underlayment. A listed and labeled roof underlayment that has been tested as part of the roof assembly in accor- dance with ASTM E108 or UL 790 for Class A fire classification shall be installed in accordance with the underlayment manufacturer’s installation instructions and listing. Bird stops shall be used at the eaves when the profile fits, to prevent the entry of debris at the eave. Hip and ridge caps shall be mudded in to prevent intrusion of fire or embers.

    Exception: Where the roof sheathing is fire-retardant-treated wood labeled for exterior use and complies with Section 2303.2 of the California Building Code, the underlayment shall not be required to comply with a Class A fire classification.

  • CWUIC § 0.0134 High relevance — show source text

    Mechanically fastened underlayment shall be
    fastened using annular ring or deformed shank nails
    with 1-inch diameter metal or plastic caps. Metal caps
    shall have a thickness of not less than 32-gage (0.0134
    inch) sheet metal. Power-driven metal caps shall have a
    minimum thickness of 0.010 inch. Minimum thickness of
    the outside edge of plastic caps shall be 0.035 inch. The
    cap nail shank shall be not less than 0.083 inch. The cap
    nail shank shall have a length sufficient to penetrate
    through the roof sheathing or not less than3/4 inch into
    the roof sheathing.
    Self-adhering polymer modified bitumen underlayment
    shall be installed in accordance with the underlayment
    and roof covering manufacturers’ installation instruc-
    tions for the deck material, roof ventilation
    configuration and climate exposure of the roof
    covering.| |Clay and concrete tile|1507.3|1507.3|1507.3| |BIPV roof coverings|1507.16|1507.16|1507.16| |Metal roof panels|1507.4|Manufacturer’s installation instructions|Mechanically fastened underlayment shall be fastened
    with corrosion-resistant fasteners in a grid pattern of not
    greater than 12 inches horizontally and vertically between
    side laps with a 6-inch spacing at side and end laps.
    Mechanically fastened underlayment shall be fastened
    using annular ring or deformed shank nails with 1-inch
    diameter metal or plastic caps. Metal caps shall have a
    thickness of not less than 32-gage sheet metal. Power-
    driven metal caps shall have a minimum thickness of
    0.010 inch. Minimum thickness of the outside edge of
    plastic caps shall be 0.035 inch. The cap nail shank shall
    be not less than 0.083 inch. The cap nail shank shall have a
    length sufficient to penetrate through the roof sheathing
    or not less than3/4 inch into the roof sheathing.
    Self-adhering polymer modified bitumen underlayment
    shall be installed in accordance with the underlayment
    and roof covering manufacturers’ installation instruc-
    tions for the deck material, roof ventilation configuration
    and climate exposure of the roof covering.| |Metal roof shingles|1507.5|1507.5|1507.5| |Mineral-surfaced roll
    roofing|1507.6|1507.6|1507.6| |Slate shingles|1507.7|1507.7|1507.7| |Wood shingles|1507.8|1507.8|1507.8| |Wood shakes|1507.9|1507.9|1507.9| |For SI: 1 inch = 25.4 mm, 1 mile per hour = 0.447 m/s.|For SI: 1 inch = 25.4 mm, 1 mile per hour = 0.447 m/s.|For SI: 1 inch = 25.4 mm, 1 mile per hour = 0.447 m/s.|For SI: 1 inch = 25.4 mm, 1 mile per hour = 0.447 m/s.|

  • CWUIC § 1.1 High relevance — show source text

    Distortions
    in the underlayment shall not interfere with the ability of the
    shingles to seal. End laps shall be 4 inches and shall be offset
    by 6 feet.
    2. For roof slopes of 4 units vertical in 12 units horizontal (4:12)
    or greater, underlayment shall be one layer applied in the
    following manner: underlayment shall be applied shingle
    fashion, parallel to and starting from the eave and lapped 2
    inches. Distortions in the underlayment shall not interfere
    with the ability of the shingles to seal. End laps shall be 4
    inches and shall be offset by 6 feet.
    3. A single layer of self-adhering polymer modified bitumen
    underlayment complying with ASTM D1970, installed in
    accordance with the underlayment and roof covering manu-
    facturer’s installation instructions for the deck material, roof
    ventilation configuration and climate exposure of the roof
    covering.|Underlayment shall be one of the following:
    1. Two layers of mechanically fastened underlayment applied in
    the following manner: Apply a strip of underlayment that is
    half the width of a full sheet parallel to and starting at the
    eaves, fastened sufficiently to hold in place. Starting at the
    eave, apply full-width sheets of underlayment, overlapping
    successive sheets half the width of a full sheet plus 2 inches.
    Distortions in the underlayment shall not interfere with the
    ability of the shingles to seal. End laps shall be 4 inches and
    shall be offset by 6 feet.
    2. A minimum 4-inch-wide strip of self-adhering polymer modi-
    fied bitumen underlayment complying with ASTM D1970,
    installed in accordance with the manufacturer’s installation
    instructions for the deck material, shall be applied over all
    joints in the roof decking. An approved underlayment comply-
    ing with Table R905.1.1(1) for the applicable roof covering shall
    be applied over the entire roof over the 4-inch- wide
    membrane strips.
    3. A single layer of self-adhering polymer modified bitumen
    underlayment complying with ASTM D1970, installed in accor-
    dance
    with
    the
    underlayment
    and
    roof
    covering
    manufacturer’s installation instructions for the deck material,
    roof ventilation configuration and climate exposure of the roof
    covering.| |Clay and
    concrete tile|R905.3|Underlayment shall be one of the following:
    1. For roof slopes from 21/2 units vertical in 12 units horizontal
    (21/2:12), up to 4 units vertical in 12 units horizontal (4:12),
    underlayment shall be two layers applied in the following
    manner: apply a strip of underlayment that is half the width
    of a full sheet parallel to and starting at the eaves, fastened
    sufficiently to hold in place. Starting at the eave, apply full-
    width sheets of underlayment, overlapping successive
    sheets half the width of a full sheet plus 2 inches. End laps
    shall be 4 inches and shall be offset by 6 feet.
    2.

  • CWUIC § 1507.1.1 High relevance — show source text

    fastened
    sufficiently to hold in place. Starting at the eaves,
    apply full-width sheets of underlayment,
    overlapping successful sheets half the width of a full
    sheet plus 2 inches. Distortions in the underlayment
    shall not interfere with the ability of the shingles to
    seal. End laps shall be 4 inches and shall be offset by
    6 feet.
    2.
    A strip not less than 4 inches in width of self-
    adhering polymer modified bitumen underlayment
    complying with ASTM D1970, installed in
    accordance with the manufacturer’s installation
    instructions for the deck material, shall be applied
    over all joints in the roof decking. An approved
    underlayment complying with Table 1507.1.1(1) for
    the applicable roof covering and basic wind speed
    shall be applied over the entire roof over the 4-inch-
    wide membrane strips. Underlayment shall be
    applied in accordance with this table using the
    application requirements for where the maximum
    basic wind speed is less than 130 mph.
    3.
    A single layer of self-adhering polymer modified
    bitumen underlayment complying with ASTM
    D1970, installed in accordance with the
    underlayment and roof covering manufacurers’
    installation instructions for the deck material, roof
    ventilation configuration, and climate exposure of
    the roof covering.| |Metal roof
    shingles|1507.5|1507.5|1507.5| |Mineral-
    surfaced
    roll roofing|1507.6|1507.6|1507.6| |Slate
    shingles|1507.7|1507.7|1507.7| |Wood
    shingles|1507.8|1507.8|1507.8| |Wood
    shakes|1507.9|1507.9|1507.9| |BIPV roof
    coverings|1507.16|Underlayment shall be one of the following:
    1.
    For roof slopes from 3 units vertical in 12 units
    horizontal (3:12) to 4 units vertical in 12 units
    horizontal (4:12), underlayment shall be two layers
    applied in the following manner: Apply a strip of
    underlayment that is half the width of a full sheet
    parallel to and starting at the eaves. Starting at the
    eaves, apply full-width sheets of underlayment,
    overlapping successive sheets half the width of a full
    sheet plus 2 inches. End laps shall be 4 inches and
    shall be offset by 6 feet. Distortions in the
    underlayment shall not interfere with the ability of
    the shingles to seal.
    2.
    For roof slopes of 4 units vertical in 12 units
    horizontal (4:12) or greater, underlayment shall be
    one layer applied as follows: Underlayment shall be
    applied shingle fashion, parallel to and starting from
    the eaves and lapped 2 inches. Distortions in the
    underlayment shall not interfere with the ability of
    the shingles to seal.

  • CWUIC § 1507.5 High relevance — show source text

    5|1507.5| |Mineral-
    surfaced
    roll roofing|1507.6|1507.6|1507.6| |Slate
    shingles|1507.7|1507.7|1507.7| |Wood
    shingles|1507.8|1507.8|1507.8| |Wood
    shakes|1507.9|1507.9|1507.9| |BIPV roof
    coverings|1507.16|Underlayment shall be one of the following:
    1.
    For roof slopes from 3 units vertical in 12 units
    horizontal (3:12) to 4 units vertical in 12 units
    horizontal (4:12), underlayment shall be two layers
    applied in the following manner: Apply a strip of
    underlayment that is half the width of a full sheet
    parallel to and starting at the eaves. Starting at the
    eaves, apply full-width sheets of underlayment,
    overlapping successive sheets half the width of a full
    sheet plus 2 inches. End laps shall be 4 inches and
    shall be offset by 6 feet. Distortions in the
    underlayment shall not interfere with the ability of
    the shingles to seal.
    2.
    For roof slopes of 4 units vertical in 12 units
    horizontal (4:12) or greater, underlayment shall be
    one layer applied as follows: Underlayment shall be
    applied shingle fashion, parallel to and starting from
    the eaves and lapped 2 inches. Distortions in the
    underlayment shall not interfere with the ability of
    the shingles to seal. End laps shall be 4 inches and
    shall be offset by 6 feet.
    3.
    A single layer of self-adhering polymer modified
    bitumen underlayment complying with ASTM
    D1970, installed in accordance with the
    underlayment and roof covering manufacturers’
    installation instructions for the deck material, roof
    ventilation configuration, and climate exposure of
    the roof covering.|Underlayment shall be one of the following:
    1.
    Two layers of mechanically fastened underlayment
    applied in the following manner: Apply a strip of
    underlayment that is half the width of a full sheet
    parallel to and starting at the eaves, fastened
    sufficiently to hold in place. Starting at the eaves,
    apply full-width sheets of underlayment,
    overlapping successive sheets half the width of a full
    sheet plus 2 inches. Distortions in the underlayment
    shall not interfere with the ability of the shingles to
    seal. End laps shall be 4 inches and shall be offset by
    6 feet.
    2.
    A strip not less than 4 inches in width of self-
    adhering polymer modified bitumen underlayment
    complying with ASTM D1970, installed in
    accordance with the manufacturer’s installation
    instructions for the deck material, shall be applied
    over all joints in the roof decking. An approved
    underlayment complying with Table 1507.1.1(1) for
    the applicable roof covering and basic wind speed
    shall be applied over the entire roof over the 4-inch-
    wide membrane strips.

  • CWUIC § 0.447 High relevance — show source text

    447 m/s.|For SI: 1 mile per hour = 0.447 m/s.|For SI: 1 mile per hour = 0.447 m/s.|For SI: 1 mile per hour = 0.447 m/s.|

    2025 CALIFORNIA RESIDENTIAL CODE 9-5

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

    ROOF ASSEMBLIES

    TABLE R905.1.1(2)—UNDERLAYMENT APPLICATION Col2 Col3 Col4
    ROOF
    COVERING    		 SECTION AREAS WHERE WIND DESIGN IS NOT REQUIRED IN
    ACCORDANCE WITH FIGURE R301.2.1.1    		 AREAS WHERE WIND DESIGN IS REQUIRED IN
    ACCORDANCE WITH FIGURE R301.2.1.1
    Asphalt
    shingles    		 R905.2 Underlayment shall be one of the following:
    1. For roof slopes from 2 units vertical in 12 units horizontal
    (2:12), up to 4 units vertical in 12 units horizontal (4:12),
    underlayment shall be two layers applied in the following
    manner: apply a strip of underlayment that is half the width
    of a full sheet parallel to and starting at the eaves, fastened
    sufficiently to hold in place. Starting at the eave, apply full-
    width sheets of underlayment, overlapping successive
    sheets half the width of a full sheet plus 2 inches. Distortions
    in the underlayment shall not interfere with the ability of the
    shingles to seal. End laps shall be 4 inches and shall be offset
    by 6 feet.
    2. For roof slopes of 4 units vertical in 12 units horizontal (4:12)
    or greater, underlayment shall be one layer applied in the
    following manner: underlayment shall be applied shingle
    fashion, parallel to and starting from the eave and lapped 2
    inches. Distortions in the underlayment shall not interfere
    with the ability of the shingles to seal. End laps shall be 4
    inches and shall be offset by 6 feet.
    3. A single layer of self-adhering polymer modified bitumen
    underlayment complying with ASTM D1970, installed in
    accordance with the underlayment and roof covering manu-
    facturer’s installation instructions for the deck material, roof
    ventilation configuration and climate exposure of the roof
    covering.    		 Underlayment shall be one of the following:
    1. Two layers of mechanically fastened underlayment applied in
    the following manner: Apply a strip of underlayment that is
    half the width of a full sheet parallel to and starting at the
    eaves, fastened sufficiently to hold in place. Starting at the
    eave, apply full-width sheets of underlayment, overlapping
    successive sheets half the width of a full sheet plus 2 inches.
    Distortions in the underlayment shall not interfere with the
    ability of the shingles to seal. End laps shall be 4 inches and
    shall be offset by 6 feet.
    2.
  • CWUIC § 15-7 High relevance — show source text

    2025 CALIFORNIA BUILDING CODE 15-7

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

    ROOF ASSEMBLIES AND ROOFTOP STRUCTURES

    TABLE 1507.1.1(2)—UNDERLAYMENT APPLICATION Col2 Col3 Col4
    ROOF
    COVERING    		 SECTION MAXIMUM BASIC WIND SPEED,****V < 130 MPH IN
    HURRICANE-PRONE REGIONS ORV < 140 MPH
    OUTSIDE HURRICANE-PRONE REGIONS    		 MAXIMUM BASIC WIND SPEED,****V ≥ 130 MPH IN
    HURRICANE-PRONE REGIONS ORV ≥ 140 MPH
    OUTSIDE HURRICANE-PRONE REGIONS
    Asphalt
    shingles    		 1507.2 Underlayment shall be one of the following:
    1.
    For roof slopes from 2 units vertical in 12 units
    horizontal (2:12) to 4 units vertical in 12 units
    horizontal (4:12), underlayment shall be two layers
    applied in the following manner: Apply a strip of
    underlayment that is half the width of a full sheet
    parallel to and starting at the eaves. Starting at the
    eaves, apply full-width sheets of underlayment,
    overlapping successive sheets half the width of a full
    sheet plus 2 inches. End laps shall be 4 inches and
    shall be offset by 6 feet. Distortions in the
    underlayment shall not interfere with the ability of
    the shingles to seal.
    2.
    For roof slopes of 4 units vertical in 12 units
    horizontal (4:12) or greater, underlayment shall be
    one layer applied as follows: Underlayment shall be
    applied shingle fashion, parallel to and starting from
    the eaves and lapped 2 inches. Distortions in the
    underlayment shall not interfere with the ability of
    the shingles to seal. End laps shall be 4 inches and
    shall be offset by 6 feet.
    3.
    A single layer of self-adhering polymer modified
    bitumen underlayment complying with ASTM
    D1970, installed in accordance with the
    underlayment and roof covering manufacturers’
    installation instructions for the deck material, roof
    ventilation configuration, and climate exposure of
    the roof covering.    		 Underlayment shall be one of the following:
    1.
    Two layers of mechanically fastened underlayment
    applied in the following manner: Apply a strip of
    underlayment felt that is half the width of a full
    sheet parallel to and starting at the eaves, fastened
    sufficiently to hold in place. Starting at the eaves,
    apply full-width sheets of underlayment overlapping
    successive sheets half the width of a full sheet plus 2
    inches. Distortions in the underlayment shall not
    interfere with the ability of the shingles to seal.
  • CWUIC § 4.1 High relevance — show source text

    For roof slopes of 3 units vertical in 12 units horizontal (25-percent slope) and greater, valley flashing shall have a 36-inch-wide (914 mm) underlayment of one layer of Type I underlayment running the full length of the valley, in addition to other required underlayment. In areas where the average daily temperature in January is 25°F (-4°C) or less, metal valley flashing underlayment shall be solid-cemented to the roofing underlayment for slopes less than 7 units vertical in 12 units horizontal (58-percent slope) or be of self-adhering polymer-modified bitumen sheet.

    R905.4 Metal roof shingles. The installation of metal roof shingles shall comply with the provisions of this section.

    R905.4.1 Sheathing requirements. Metal roof shingles shall be fastened to wood structural panels, solid lumber sheathing or closely fitted lumber sheathing, except where the roof covering is specifically designed to be applied to spaced lumber sheathing.

    R905.4.2 Slope. Metal roof shingles shall not be installed on roof slopes below 3 units vertical in 12 units horizontal (25-percent slope).

    R905.4.3 Underlayment. Underlayment shall comply with Section R905.1.1.

    R905.4.3.1 Ice barrier. Where required, ice barriers shall comply with Section R905.1.2.

    R905.4.4 Material standards. Metal roof shingle roof coverings shall comply with Table R905.10.3(1). The materials used for metal roof shingle roof coverings shall be naturally corrosion resistant or be made corrosion resistant in accordance with the standards and minimum thicknesses listed in Table R905.10.3(2).

    R905.4.4.1 Wind resistance of metal roof shingles. Metal roof shingles fastened to wood structural panels, solid lumber sheathing or closely fitted lumber sheathing shall be tested in accordance with ASTM D3161, FM 4474, UL 580 or UL 1897. Metal roof shingles tested in accordance with ASTM D3161 shall meet the classification requirements of Table R905.4.4.1 for the appropriate maximum basic wind speed and the metal shingle packaging shall bear a label to indicate compliance with ASTM D3161 and the required classification in Table R905.2.4.1.

    9-10 2025 CALIFORNIA RESIDENTIAL CODE

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

    ROOF ASSEMBLIES

  • CWUIC § 1507.1.1 High relevance — show source text

    End laps shall be 4 inches and
    shall be offset by 6 feet.
    3.
    A single layer of self-adhering polymer modified
    bitumen underlayment complying with ASTM
    D1970, installed in accordance with the
    underlayment and roof covering manufacturers’
    installation instructions for the deck material, roof
    ventilation configuration, and climate exposure of
    the roof covering.|Underlayment shall be one of the following:
    1.
    Two layers of mechanically fastened underlayment
    applied in the following manner: Apply a strip of
    underlayment that is half the width of a full sheet
    parallel to and starting at the eaves, fastened
    sufficiently to hold in place. Starting at the eaves,
    apply full-width sheets of underlayment,
    overlapping successive sheets half the width of a full
    sheet plus 2 inches. Distortions in the underlayment
    shall not interfere with the ability of the shingles to
    seal. End laps shall be 4 inches and shall be offset by
    6 feet.
    2.
    A strip not less than 4 inches in width of self-
    adhering polymer modified bitumen underlayment
    complying with ASTM D1970, installed in
    accordance with the manufacturer’s installation
    instructions for the deck material, shall be applied
    over all joints in the roof decking. An approved
    underlayment complying with Table 1507.1.1(1) for
    the applicable roof covering and basic wind speed
    shall be applied over the entire roof over the 4-inch-
    wide membrane strips. Underlayment shall be
    applied in accordance with this table using the
    application requirements for where the maximum
    basic wind speed is less than 130 mph.
    3.
    A single layer of self-adhering polymer modified
    bitumen underlayment complying with ASTM
    D1970, installed in accordance with the
    underlayment and roof covering manufacturers’
    installation instructions for the deck material, roof
    ventilation configuration, and climate exposure of
    the roof covering.| |For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 mile per hour = 0.447 m/s.|For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 mile per hour = 0.447 m/s.|For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 mile per hour = 0.447 m/s.|For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 mile per hour = 0.447 m/s.|

    2025 CALIFORNIA BUILDING CODE 15-9

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

    ROOF ASSEMBLIES AND ROOFTOP STRUCTURES

  • CWUIC § 1507.1.1 High relevance — show source text

    End laps shall be 4
    inches and shall be offset by 6 feet.
    3.
    A single layer of self-adhering polymer modified
    bitumen underlayment complying with ASTM
    D1970, installed in accordance with the
    underlayment and roof covering manufacturers’
    installation instructions for the deck material, roof
    ventilation configuration, and climate exposure of
    the roof covering.|Underlayment shall be one of the following:
    1.
    Two layers of mechanically fastened underlayment
    applied in the following manner: Apply a strip of
    underlayment that is half the width of a full sheet
    parallel to and starting at the eaves, fastened
    sufficiently to hold in place. Starting at the eaves,
    apply full-width sheets of underlayment,
    overlapping successive sheets half the width of a full
    sheet plus 2 inches. Distortions in the underlayment
    shall not interfere with the ability of the shingles to
    seal. End laps shall be 4 inches and shall be offset by
    6 feet.
    2.
    A strip not less than 4 inches in width of self-
    adhering polymer modified bitumen underlayment
    complying with ASTM D1970, installed in
    accordance with the manufacturer’s installation
    instructions for the deck material, shall be applied
    over all joints in the roof decking. An approved
    underlayment complying with Table 1507.1.1(1) for
    the applicable roof covering and basic wind speed
    shall be applied over the entire roof over the 4-inch-
    wide membrane strips. Underlayment shall be
    applied in accordance with this table using the
    application requirements for where the maximum
    basic wind speed is less than 130 mph.
    3.
    A single layer of self-adhering polymer modified
    bitumen underlayment complying with ASTM
    D1970, installed in accordance with the
    underlayment and roof covering manufacurers’
    installation instructions for the deck material, roof
    ventilation configuration, and climate exposure of
    the roof covering.|

    15-8 2025 CALIFORNIA BUILDING CODE

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

    ROOF ASSEMBLIES AND ROOFTOP STRUCTURES

    TABLE 1507.1.1(2)—UNDERLAYMENT APPLICATION—continued Col2 Col3 Col4
    ROOF
    COVERING    		 SECTION MAXIMUM BASIC WIND SPEED,****V < 130 MPH IN
    HURRICANE-PRONE REGIONS ORV < 140 MPH
    OUTSIDE HURRICANE-PRONE REGIONS    		 MAXIMUM BASIC WIND SPEED,****V ≥ 130 MPH IN
    HURRICANE-PRONE REGIONS ORV ≥ 140 MPH
    OUTSIDE HURRICANE-PRONE REGIONS
    Metal roof
    panels    		 1507.4 Apply in accordance with the manufacturer’s installa-
    tion instructions    		 Underlayment shall be one of the following:
    1.

Frequently asked questions

What exactly is considered a "void" or "airspace" under § 504.2.1?

A "void" or "airspace" is created when the roof covering/profile does not sit directly on the combustible deck (i.e., a gap exists under the covering). The CWUIC triggers the protective measures when such a void exists but does not prescribe a numeric clearance dimension in the cited sections.

Can I use self‑adhering underlayment instead of the 72 lb. cap sheet?

Yes — but only if the underlayment is listed and labeled and tested as part of the roof assembly to ASTM E108 or UL 790 for Class A assembly performance, and it is installed per its listing and manufacturer instructions (see § 504.2.1.2).

When is the 1‑inch mineral wool board option acceptable?

The 1‑inch mineral wool board (or other noncombustible material) may replace the 72 lb. cap sheet where at least 1 inch of that material is located between the roofing material and wood framing or deck (see § 504.2.1.1 Exception).

Do bird stops and mudded hip/ridge caps apply even when I use a Class A underlayment?

Yes — both § 504.2.1.1 and § 504.2.1.2 require bird stops at eaves (where the profile fits) and mudding at hip/ridge caps to prevent ember/debris intrusion.

If my sheathing is fire‑retardant‑treated wood, do I still need a Class A underlayment?

If the sheathing is FRT wood labeled for exterior use and complies with CBC § 2303.2, the underlayment is not required to be Class A under § 504.2.1.2 (Exception) — but you must verify the sheathing’s labeling and compliance.

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