CMC · California Mechanical Code

How to size openings and account for louvers, grilles, and screens (net free area)?

Q: Are insect screens allowed, and do they change the sizing?

Yes — screens are allowed but must be at least 1/4 in (6.4 mm) mesh where required; they reduce net free area, so include the screen in the NFA calculation ( § 701.10.1 , § 701.10 ) .

Q: Can motorized louvers be used for combustion air?

Yes, but they must be interlocked and proven fully open before ignition and during burner operation, with means to prevent ignition or shut down if they fail ( § 701.10.2 ) .

For homeowners: the code requires that vents and openings for combustion or ventilation be sized using the actual unobstructed airflow area after louvers, grilles, or screens are in place (net free area). If the product’s free‑area rating is unknown, assume **25%** free area for wood louvers or **75%** for metal grilles; screens must be at least **1/4‑inch** mesh and motorized louvers must be interlocked to stay fully open before and during appliance operation (CMC **§ 701.10–701.10.2**) .

Ask a question free See coverage

Last reviewed: July 6, 2026

What the code requires — 2-4 sentences

The California Mechanical Code says you must size openings for combustion, ventilation, and dilution air based on the net free area of the opening — not the gross opening area. This is the rule in § 701.10 . Where the manufacturer or louver/grille design gives a known free-area percentage, use that value; otherwise use the prescribed assumptions in § 701.10 . Screens and louvers have minimum and control requirements in § 701.10.1 and § 701.10.2 respectively .

Always size openings by the net free area (actual unobstructed airflow area). If the free-area of the grille/louver is unknown, assume 25% for wood louvers and 75% for metal louvers/grilles. Screens must be at least 1/4 in (6.4 mm) mesh. (Plain English restatement of the rule in § 701.10–701.10.2.)

Requirements in detail

Key terms (first-use bolded)

Net free area (NFA) — the actual unobstructed airflow area through an opening after accounting for louvers, grilles, or screens (see § 701.10).
Motorized louver — a louver that opens and closes by mechanical means and which must be interlocked with the appliance per § 701.10.2.

What to base your sizing on

Use the net free area of each opening when calculating required opening area for combustion, ventilation, or dilution air (§ 701.10) .
If the manufacturer provides the free-area (NFA) for the specific louver/grille/screen, use that value directly (§ 701.10) .
If the design or free-area is not known, assume default free-area percentages: wood louvers — 25%, metal louvers/grilles — 75%; and nonmotorized louvers/grilles must be fixed in the open position (§ 701.10) .

Screens and motorized louvers

Screens: Minimum mesh size is 1/4 inch (6.4 mm); do not use finer mesh (see § 701.10.1) .
Motorized louvers: Must be interlocked so they are proven fully open before main burner ignition and remain open during operation; there must be means to prevent ignition if the louver fails to open or to shut down the burner if the louver closes (§ 701.10.2) .

Decision-relevant dimensions / values

Decision item	Value / rule	Code Reference
Basis for sizing	Use Net Free Area (NFA) of each opening	§ 701.10
Default NFA — wood louvers	25% free area (if design not known)	§ 701.10
Default NFA — metal louvers/grilles	75% free area (if design not known)	§ 701.10
Nonmotorized louvers/grilles	Must be fixed in the open position	§ 701.10
Minimum screen mesh size	1/4 in (6.4 mm)	§ 701.10.1
Motorized louvers	Must be interlocked; proven fully open before ignition and during operation	§ 701.10.2

Exceptions & special cases

If the louver/grille/screen manufacturer supplies the actual free-area value for that specific product, use that measured/declared NFA rather than the default percentages in § 701.10 .
Nonmotorized louvers and grilles used for required openings must be fixed open; they are not permitted to be operable in the closed position when relied on for combustion/ventilation air (§ 701.10) .
Motorized louvers may be used only when properly interlocked to ensure they are fully open at appliance ignition and during operation; you must provide automatic prevention/shutdown means if they fail (§ 701.10.2) .
Screens finer than 1/4 in mesh are not permitted where the minimum screen size is required (§ 701.10.1) .

If a detail or exception you need is not covered in § 701.10–701.10.2, the CMC text for those precise circumstances is not available in the retrieved excerpts — consult the full code text or manufacturer data for that design-specific information.

Common mistakes

Using the gross opening area (frame-to-frame) instead of the net free area (actual unobstructed area through louvers/grilles/screens) — the code requires NFA (§ 701.10) .
Applying the 25% / 75% defaults when the louver manufacturer provides a specific free-area rating — the manufacturer value controls and must be used if known (§ 701.10) .
Forgetting that multiple layers reduce NFA (e.g., louver plus insect screen) — each obstruction reduces the net free area; always compute the combined NFA. The code requires basing size on the NFA actually provided (§ 701.10) .
Installing nonmotorized louvers as operable (not fixed open) when they are credited for permanent openings — the code requires nonmotorized louvers/grilles to be fixed in the open position if relied on for required openings (§ 701.10) .
Failing to interlock motorized louvers with combustion appliances — § 701.10.2 requires proof of full open position prior to ignition and during operation, plus shutdown/prevention if they fail .

Worked example — concrete scenario

Scenario: A water‑heater installation calls for an outdoor opening that must provide 100 in² of net free area (NFA) to meet combustion/ventilation air requirements (required NFA supplied by other CMC calculation).

Option A — unknown louver free-area; using a wood louver (assume 25% free area per § 701.10):
- Required gross opening area = required NFA ÷ free‑area fraction = 100 in² ÷ 0.25 = 400 in² (frame/gross opening). § 701.10 .
Option B — unknown louver free-area; using a metal louver/grille (assume 75% free area per § 701.10):
- Required gross opening area = 100 in² ÷ 0.75 = 133.3 in² (round up to practical opening). § 701.10 .
Option C — manufacturer data available showing the louver has 40% free area:
- Required gross opening area = 100 in² ÷ 0.40 = 250 in². Use manufacturer NFA when known (§ 701.10) .
Screens: if you also add an insect screen, verify the screen mesh and manufacturer free‑area. The screen must be at least 1/4 in (6.4 mm) mesh where the code requires it (§ 701.10.1) — and the screen will further reduce the NFA, so include that in the combined calculation (§ 701.10) .
Motorized louvers: if you use a motorized solution, it must be interlocked so that the louver is confirmed fully open before ignition and during operation; provide means to prevent ignition or shut down the burner if the louver fails to open or closes during operation (§ 701.10.2) .

Related provisions

§ 701.7.3 — minimum dimension of air openings not less than 3 inches (76 mm) (context for outdoor opening sizing) .
§ 701.8 — engineered combustion air installations (when engineering methods determine required air) .
§ 701.9 — mechanical combustion air supply rates and interlock requirements for mechanical systems .
§ 701.11 — combustion air ducts (materials, termination, and other duct requirements) — relevant when combustion air is ducted rather than through direct openings .

Code references

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

CMC § 506.4 High relevance — show source text
(3) The minimum size of outdoor opening(s) shall be the full size of outdoor opening(s) calculated in accordance with Section 506.4, multiplied by the reduction factor. The minimum dimension of air

openings shall not be less than 3 inches (76 mm).

[NFPA 54:9.3.4(3)]

506.6 Engineered Installations. Engineered combustion air installations shall provide an adequate supply of combustion, ventilation, and dilution air determined using engineering methods. [NFPA 54:9.3.5]

506.7 Mechanical Combustion Air Supply. Where all combustion air is provided by a mechanical air supply system, the combustion air shall be supplied from outdoors at the minimum rate of 0.35 cubic feet per minute per 1000 Btu/h [0.034 (m [3] /min)/kW] for all appliances located within the space. [NFPA 54:9.3.6]

506.7.1 Exhaust Fans. Where exhaust fans are installed, additional air shall be provided to replace the exhausted air. [NFPA 54:9.3.6.1]

506.7.2 Interlock. Each of the appliances served shall be interlocked to the mechanical air supply system to prevent main burner operation where the mechanical air supply system is not in operation. [NFPA 54:9.3.6.2]

506.7.3 Specified Combustion Air. Where combustion air is provided by the building’s mechanical ventilation system, the system shall provide the specified combustion air rate in addition to the required ventilation air. [NFPA 54:9.3.6.3]

506.8 Louvers, Grilles, and Screens. The required size of openings for combustion, ventilation, and dilution air shall be based on the net free area of each opening. Where the free area through a design of louver, grille, or screen is known, it shall be used in calculating the size opening required to provide the free area specified. Where the louver and grille design and free area are not known, it shall be assumed that wood louvers have 25 percent free area, and metal louvers and grilles have 75 percent free area. Nonmotorized louvers and grilles shall be fixed in the open position. [NFPA 54:9.3.7.1]

506.8.1 Minimum Screen Mesh Size. Screens shall not be smaller than [1] ⁄ 4 of an inch (6.4 mm) mesh. [NFPA 54:9.3.7.2]

506.8.2 Motorized Louvers. Motorized louvers shall be interlocked with the appliance so they are proven in the full open position prior to main burner ignition and during main burner operation. Means shall be provided to prevent the main burner from igniting should the louver fail to open during burner startup and to shut down the main burner if the louvers close during burner operation.

[NFPA 54:9.3.7.3]

506.9 Combustion Air Ducts. Combustion air ducts shall comply with the following [NFPA 54:9.3.8]:

(1) Ducts shall be constructed of galvanized steel or a material having equivalent corrosion resistance, strength, and rigidity.
CMC § 9.3.4 High relevance — show source text
[NFPA 54:9.3.4(3)] 701.8 Engineered Installations. Engineered combustion air installations shall provide an adequate supply of combustion, ventilation, and dilution air determined using engineering methods. [NFPA 54:9.3.5] 701.9 Mechanical Combustion Air Supply. Where all combustion air is provided by a mechanical air supply system, the combustion air shall be supplied from outdoors at the minimum rate of not less than 0.35 cubic feet per minute per 1000 Btu/h [0.034 (m [3] /min)/kW] for all appliances located within the space. [NFPA 54:9.3.6]

701.9.1 Exhaust Fans. Where exhaust fans are installed, additional air shall be provided to replace the exhausted air. [NFPA 54:9.3.6.1]

701.9.2 Interlock. Each of the appliances served shall be interlocked to the mechanical air supply system to prevent main burner operation where the mechanical air supply system is not in operation. [NFPA 54:9.3.6.2] 701.9.3 Specified Combustion Air. Where combustion air is provided by the building’s mechanical ventilation system, the system shall provide the specified combustion air rate in addition to the required ventilation air. [NFPA 54:9.3.6.3] 701.10 Louvers, Grilles, and Screens. The required size of openings for combustion, ventilation, and dilution air shall be based on the net free area of each opening. Where the free area through a design of louver, grille, or screen is known, it shall be used in calculating the size opening required to provide the free area specified. Where the louver and grille design and free area are not known, it shall be assumed that wood louvers have 25 percent free area and metal louvers and grilles have 75 percent free area. Nonmotorized louvers and grilles shall be fixed in the open position. [NFPA 54:9.3.7.1]

701.10.1 Minimum Screen Mesh Size. Screens shall not be smaller than [1] ⁄ 4 of an inch (6.4 mm) mesh. [NFPA 54:9.3.7.2]

701.10.2 Motorized Louvers. Motorized louvers shall be interlocked with the appliance so they are proven in the full open position prior to main burner ignition and during main burner operation. Means shall be provided to prevent the main burner from igniting should the louver fail to open during burner startup and to shut down the main burner if the louvers close during burner operation.

[NFPA 54:9.3.7.3]

2025 CALIFORNIA MECHANICAL CODE 149

), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.

COMBUSTION AIR

701.11 Combustion Air Ducts. Combustion air ducts shall comply with the following [NFPA 54:9.3.8]:

(1) Ducts shall be constructed of galvanized steel or a material having equivalent corrosion resistance, strength, and rigidity.
CMC § 9.1 High relevance — show source text
(14)ASHRAE 170. Section 9.1a -- Modify as follows: Replace reference to Table 9-1 with reference to Table 4-A.

2025 CALIFORNIA MECHANICAL CODE 69

), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.

VENTILATION AIR

«

«

402.2.1.5 Corner Openings. For zones with operable openings on two adjacent sides of a zone, the distance from the operable openings shall be not more than 5 H along a line drawn between the two openings that are farthest apart. Floor area outside that line shall comply with Section 402.2.1.1 as a zone having openings on only one side of the zone.

[ASHRAE 62.1:6.4.1.5]

402.2.1.6 Location and Size of Openings. Zones or portions of zones to be naturally ventilated shall have a permanently open airflow path to openings directly connected to the outdoors. The minimum flow rate to the zone shall be determined in accordance with

Section 403.2.1. This flow rate shall be used to deter mine the required openable area of openings, accounting only for buoyancy-driven flow. Wind-driven flow shall be used only where it can be demonstrated that the minimum flow rate is provided during all occupied hours. Openings shall be sized in accordance with Section 402.2.1.6(A). Permanently open airflow path shall include, but not be limited to, pathways that would allow airflow unimpeded by partitions, walls, and furnishings. {ASHRAE 62.1:6.4.1.6}

402.2.1.6(A) Sizing Openings. Where the zone is ventilated using a single opening or multiple single openings located at the same elevation, the openable area as a percent of the net occupiable floor area shall be greater than or equal to the value indicated in Table 402.2.1.6(A)(1). Where the zone is ventilated using two openings located at different elevations or multiple pairs of such openings, the openable area as a percent of the net occupiable floor area shall be greater than or equal to the value indicated in Table 402.2.1.6(A)(2).

Where openings are obstructed by louvers or screens, the openable area shall be based on the net free area of the opening. Where interior zones, or portions of zones, without direct openings to the outdoors are ventilated through adjoining zones, the opening between zones shall be permanently unobstructed and have a free area of not less than twice the percent of occupiable floor area used to determine the opening size of adjacent exterior zones, or 25 square feet (2.3 m [2] ), whichever is greater. Table 402.2.1.6(A)(1) and Table 402.2.1.6(A)(2) are based on buoyancy-driven flow and shall not address thermal comfort. [ASHRAE 62.1:6.4.1.6.1]

402.2.2 Engineered System Compliance Path. For an engineered natural ventilation system, the following shall be included:
CMC § 9.3.7.1 High relevance — show source text
Where the free area through a design of louver, grille, or screen is known, it shall be used in calculating the size opening required to provide the free area specified. Where the louver and grille design and free area are not known, it shall be assumed that wood louvers have 25 percent free area and metal louvers and grilles have 75 percent free area. Nonmotorized louvers and grilles shall be fixed in the open position. [NFPA 54:9.3.7.1]

701.10.1 Minimum Screen Mesh Size. Screens shall not be smaller than [1] ⁄ 4 of an inch (6.4 mm) mesh. [NFPA 54:9.3.7.2]

701.10.2 Motorized Louvers. Motorized louvers shall be interlocked with the appliance so they are proven in the full open position prior to main burner ignition and during main burner operation. Means shall be provided to prevent the main burner from igniting should the louver fail to open during burner startup and to shut down the main burner if the louvers close during burner operation.

[NFPA 54:9.3.7.3]

2025 CALIFORNIA MECHANICAL CODE 149

), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.

COMBUSTION AIR

701.11 Combustion Air Ducts. Combustion air ducts shall comply with the following [NFPA 54:9.3.8]:

(1) Ducts shall be constructed of galvanized steel or a material having equivalent corrosion resistance, strength, and rigidity.

Exception: Within dwellings units, unobstructed stud and joist spaces shall not be prohibited from conveying combustion air, provided that not more than one fireblock is removed. [NFPA 54:9.3.8.1]

(2) Ducts shall terminate in an unobstructed space, allowing free movement of combustion air to the appliances.

[NFPA 54:9.3.8.2]

(3) Ducts shall serve a single space. [NFPA 54:9.3.8.3]

(4) Ducts shall not serve both upper and lower combustion air openings where both such openings are used. The separation between ducts serving upper and lower combustion air openings shall be maintained to the source of combustion air. [NFPA 54:9.3.8.4]

(5) Ducts shall not be screened where terminating in an attic space. [NFPA 54:9.3.8.5]

(6) Horizontal upper combustion air ducts shall not slope downward toward the source of combustion air. [NFPA 54:9.3.8.6]

(7) The remaining space surrounding a chimney liner, gas vent, special gas vent, or plastic piping installed within a masonry, metal, or factory built chimney shall not be used to supply combustion air.

Exception: Direct vent appliances designed for installation in a solid fuel-burning fireplace where installed in accordance with the manufacturer’s installation instructions. [NFPA 54:9.3.8.7]

(8) Combustion air intake openings located on the exterior of the building shall have the lowest side of the combustion air intake openings located at least 12 inches (305 mm) vertically from the adjoining finished ground level.
CMC § 506.8 High relevance — show source text
506.8 Louvers, Grilles, and Screens. The required size of openings for combustion, ventilation, and dilution air shall be based on the net free area of each opening. Where the free area through a design of louver, grille, or screen is known, it shall be used in calculating the size opening required to provide the free area specified. Where the louver and grille design and free area are not known, it shall be assumed that wood louvers have 25 percent free area, and metal louvers and grilles have 75 percent free area. Nonmotorized louvers and grilles shall be fixed in the open position. [NFPA 54:9.3.7.1]

506.8.1 Minimum Screen Mesh Size. Screens shall not be smaller than [1] ⁄ 4 of an inch (6.4 mm) mesh. [NFPA 54:9.3.7.2]

506.8.2 Motorized Louvers. Motorized louvers shall be interlocked with the appliance so they are proven in the full open position prior to main burner ignition and during main burner operation. Means shall be provided to prevent the main burner from igniting should the louver fail to open during burner startup and to shut down the main burner if the louvers close during burner operation.

[NFPA 54:9.3.7.3]

506.9 Combustion Air Ducts. Combustion air ducts shall comply with the following [NFPA 54:9.3.8]:

(1) Ducts shall be constructed of galvanized steel or a material having equivalent corrosion resistance, strength, and rigidity.

Exception: Within dwellings units, unobstructed stud and joist spaces shall not be prohibited from conveying combustion air, provided that not more than one fireblock is removed. [NFPA 54:9.3.8.1]

2025 CALIFORNIA PLUMBING CODE 87

), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.

WATER HEATERS

(2) Ducts shall terminate in an unobstructed space, allowing free movement of combustion air to the appliances.

[NFPA 54:9.3.8.2]

(3) Ducts shall serve a single space. [NFPA 54:9.3.8.3]

(4) Ducts shall not serve both upper and lower combustion air openings where both such openings are used. The separation between ducts serving upper and lower combustion air openings shall be maintained to the source of combustion air. [NFPA 54:9.3.8.4]

(5) Ducts shall not be screened where terminating in an attic space. [NFPA 54:9.3.8.5]

(6) Combustion air intake openings located on the exterior of the building shall have the lowest side of the combustion air intake openings located at least 12 inches (305 mm) vertically from the adjoining finished ground level.

[NFPA 54:9.3.8.8]

(7) Horizontal upper combustion air ducts shall not slope downward toward the source of combustion air. [NFPA 54:9.3.8.6]
CMC § 402.2.1.6 High relevance — show source text
402.2.1.6(A) Sizing Openings. Where the zone is ventilated using a single opening or multiple single openings located at the same elevation, the openable area as a percent of the net occupiable floor area shall be greater than or equal to the value indicated in Table 402.2.1.6(A)(1). Where the zone is ventilated using two openings located at different elevations or multiple pairs of such openings, the openable area as a percent of the net occupiable floor area shall be greater than or equal to the value indicated in Table 402.2.1.6(A)(2).

Where openings are obstructed by louvers or screens, the openable area shall be based on the net free area of the opening. Where interior zones, or portions of zones, without direct openings to the outdoors are ventilated through adjoining zones, the opening between zones shall be permanently unobstructed and have a free area of not less than twice the percent of occupiable floor area used to determine the opening size of adjacent exterior zones, or 25 square feet (2.3 m [2] ), whichever is greater. Table 402.2.1.6(A)(1) and Table 402.2.1.6(A)(2) are based on buoyancy-driven flow and shall not address thermal comfort. [ASHRAE 62.1:6.4.1.6.1]

402.2.2 Engineered System Compliance Path. For an engineered natural ventilation system, the following shall be included:

(1) Determine hourly environmental conditions, including outdoor air dry-bulb temperature; dew-point temperature; outdoor concentration of contaminants, including PM2.5, PM10, and ozone where data are available; wind speed and direction; and internal heat gains during expected hours of natural ventilation operation.

»

»

»

»

402.2 Natural Ventilation Procedure. [Not permitted for OSHPD 1, 2, 3, 4 & 5] Natural ventilation systems shall comply with the requirements of either Section 402.2.1 through Section 402.2.1.6(A) or Section 402.2.2. Designers shall provide interior air barriers, insulation, or other means that separate naturally ventilated spaces from mechanically cooled spaces to prevent high-dew-point outdoor air from coming into contact with mechanically cooled surfaces.

[ASHRAE 62.1:6.4]

402.2.1 Prescriptive Compliance Path. Any zone designed for natural ventilation shall include a mechanical ventilation system designed in accordance with Section 403.0.

Exceptions:

(1) Zones in buildings that have all of the following:

(a) Natural ventilation openings that comply with the requirements of Section 402.2.1.

(b) Controls that prevent the natural ventilation openings from being closed during periods of expected occupancy, or natural ventilation openings that are permanently open.

(2) Zones that are not served by heating or cooling equipment. {ASHRAE 62.1:6.4.1}

402.2.1.1 Ceiling Height. For ceilings that are parallel to the floor, the ceiling height ( H ) to be used in Section 402.2.1.3 through Section 402.2.1.5 shall be the minimum ceiling height in the zone.
CMC § 701.6.1 High relevance — show source text
148 2025 CALIFORNIA MECHANICAL CODE

), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.

COMBUSTION AIR

FIGURE 701.6.1(3) ALL COMBUSTION AIR FROM OUTDOORS

THROUGH HORIZONTAL DUCTS

[NFPA 54: FIGURE A.9.3.3.1(2)]

FIGURE 701.6.2

ALL COMBUSTION AIR FROM OUTDOORS THROUGH

SINGLE COMBUSTION AIR OPENING

[NFPA 54: FIGURE A.9.3.3.2]

701.7.1 Indoor Openings. Where used, openings connecting the interior spaces shall comply with Section 701.5. [NFPA 54:9.3.4(1)]

701.7.2 Outdoor Opening(s) Location. Outdoor opening(s) shall be located in accordance with Section 701.6. [NFPA 54:9.3.4(2)]

701.7.3 Outdoor Opening(s) Size. The outdoor opening(s) size shall be calculated in accordance with the following:

(1) The ratio of the interior spaces shall be the available volume of all communicating spaces divided by the required volume.

(2) The outdoor size reduction factor shall be 1 minus the ratio of interior spaces.

(3) The minimum size of outdoor opening(s) shall be the full size of outdoor opening(s) calculated in accordance with Section 701.6, multiplied by the reduction factor. The minimum dimension of air openings shall not be less than 3 inches (76 mm).

[NFPA 54:9.3.4(3)] 701.8 Engineered Installations. Engineered combustion air installations shall provide an adequate supply of combustion, ventilation, and dilution air determined using engineering methods. [NFPA 54:9.3.5] 701.9 Mechanical Combustion Air Supply. Where all combustion air is provided by a mechanical air supply system, the combustion air shall be supplied from outdoors at the minimum rate of not less than 0.35 cubic feet per minute per 1000 Btu/h [0.034 (m [3] /min)/kW] for all appliances located within the space. [NFPA 54:9.3.6]

701.9.1 Exhaust Fans. Where exhaust fans are installed, additional air shall be provided to replace the exhausted air. [NFPA 54:9.3.6.1]

701.9.2 Interlock. Each of the appliances served shall be interlocked to the mechanical air supply system to prevent main burner operation where the mechanical air supply system is not in operation. [NFPA 54:9.3.6.2] 701.9.3 Specified Combustion Air. Where combustion air is provided by the building’s mechanical ventilation system, the system shall provide the specified combustion air rate in addition to the required ventilation air. [NFPA 54:9.3.6.3] 701.10 Louvers, Grilles, and Screens. The required size of openings for combustion, ventilation, and dilution air shall be based on the net free area of each opening.

CMC § 1803.5.1. High relevance — show source text

Exception: A drainage system for area wells is not required where the foundation is on well-drained soil or sand-gravel mixture soils in accordance with the United Soil Classification System, Group I Soils, in accordance with Section 1803.5.1.

1031.6 Bars, grilles, covers and screens. Where bars, grilles, covers, screens or similar devices are placed over emergency escape and rescue openings or area wells that serve such openings, the minimum net clear opening size shall comply with Sections 1031.3 and 1031.5. Such devices shall be releasable or removable from the inside without the use of a key, tool or force greater than that which is required for normal operation of the emergency escape and rescue opening.

2025 CALIFORNIA BUILDING CODE 10-61