CRC · California Residential Code

How is effective flue area determined and what are spacing requirements between adjacent flues?

For homeowners: the chimney flue must be large enough — usually a fraction of the fireplace opening (e.g., round = 1/12) or chosen from the code chart — and if two flues share a chimney they normally must be separated by a 4‑inch masonry wall unless the chimney vents only one appliance (then liner joints must be staggered 4 inches).

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Last reviewed: July 6, 2026

What the code requires — 2-4 sentences

  • The California Residential Code requires that the net cross‑sectional flue area for a masonry fireplace be sized either by fixed area ratios (Option 1) or by using the sizing chart (Option 2) in § R1003.15. § R1003.13 requires masonry wythes at least 4 inches thick between adjacent flue linings in a multi‑flue chimney, with a limited exception when only one appliance is vented. § R1003.14 and the tabulated flue areas provide the net areas used to select liner sizes.

The single most important rule: size the flue to the required net cross‑sectional area (by ratio or Figure R1003.15.2) and keep at least a 4‑inch masonry wythe separating adjacent flues unless the single‑appliance exception applies.

Requirements in detail

Key defined terms (first‑use)

  • Effective flue area — the net cross‑sectional area of the chimney flue used for sizing, determined from the code ratios or Figure R1003.15.2 and from the clay/liner tables.
  • Masonry wythe — the solid masonry web or partition built between flue linings inside a chimney.

How to determine minimum effective flue area (§ R1003.15)

  • Option 1 (ratio method):
    • Round flues — minimum net area = 1/12 of the fireplace opening.
    • Square flues — minimum net area = 1/10 of the fireplace opening.
    • Rectangular flues:
      • Aspect ratio < 2:1 → minimum net area = 1/10 of the opening.
      • Aspect ratio ≥ 2:1 → minimum net area = 1/8 of the opening.
  • Option 2 (chart method): use Figure R1003.15.2 to select a flue area based on fireplace opening area and chimney height (height measured from the firebox floor to the top of the chimney flue).
  • Use the net cross‑sectional areas listed in Tables R1003.14(1) and R1003.14(2) (clay liner sizes) or manufacturer‑provided net areas or measured field areas.

Spacing / separation between adjacent flues (§ R1003.13)

  • Where two or more flues are located in the same chimney, build masonry wythes not less than 4 inches (102 mm) thick and bond them into the chimney walls.
  • Exception: when the chimney is venting only one appliance, two flues may adjoin with only the flue‑lining separation between them; the joints of adjacent flue linings must be staggered not less than 4 inches (102 mm).

Tabulated liner areas (decision inputs)

  • Use the clay flue lining area tables to match a required net area to a nominal liner size (Tables R1003.14(1) & R1003.14(2)). Typical entries include round 8" → 50 in², 10" → 78 in², 12" → 113 in²; rectangular 12×12 → 102 in², 12×16 → 131 in².

Decision table (quick reference)

Decision / dimension Required value Code Reference
Minimum masonry wythe between flues 4 in (102 mm) § R1003.13
Stagger between adjacent liner joints (single‑appliance exception) 4 in (102 mm) § R1003.13
Round flue min area (Option 1) 1/12 of fireplace opening § R1003.15.1
Square flue min area (Option 1) 1/10 of fireplace opening § R1003.15.1
Rectangular flue min area (aspect <2:1) 1/10 of fireplace opening § R1003.15.1
Rectangular flue min area (aspect ≥2:1) 1/8 of fireplace opening § R1003.15.1
Example round liner areas (nominal diameter → net area) 8" → 50 in²; 10" → 78 in²; 12" → 113 in² Tables R1003.14(1) & (2)
Example rectangular liner areas 12×12 → 102 in²; 12×16 → 131 in² Tables R1003.14(2)

Exceptions & special cases

  • The § R1003.13 exception allows two flues to adjoin without a masonry wythe only when the chimney serves one appliance; in that case liner joints must be staggered at least 4 in.
  • Flues serving appliances (not fireplaces) are governed by § R1003.14 for single‑appliance connectors and by the California Mechanical Code where multiple appliance venting or mechanical draft sizing is required. If multiple appliances connect to one flue, sizing may need to follow the Mechanical Code (or CMC/NFPA rules) rather than the fireplace rules.
  • Clay liner net areas must be taken from the code tables or from manufacturer data or field measurement; do not base sizing on gross brick/masonry cavity dimensions alone.

Common mistakes

  • Treating gross internal chimney dimensions as the net flue area (ignore mortar joints, obstructions, shape losses). Always use the net area from Table R1003.14 or measured manufacturer data.
  • Forgetting to account for aspect ratio: a tall narrow rectangular flue (≥2:1) requires a larger fraction (1/8) of opening area.
  • Failing to measure the chimney height from firebox floor to top when using Figure R1003.15.2 (Option 2).
  • Omitting the 4‑inch masonry wythe between adjacent flues in a multi‑flue chimney, or not staggering liner joints when relying on the single‑appliance exception.
  • Connecting multiple appliance connectors into a single flue without using the California Mechanical Code sizing rules (or NFPA methods where required).

Worked example — concrete numbers

Scenario: masonry fireplace with opening = 6 ft² (6 × 144 = 864 in²). You need to size the flue.

  1. Option 1 (ratio method) — pick round flue: required net area = 1/12 × 864 = 72 in². From Table R1003.14(1), a 10‑inch round liner gives 78 in², which meets the requirement; an 8‑inch (50 in²) would be too small.

  2. Option 1 (rectangular with aspect ≥2:1): required net area = 1/8 × 864 = 108 in². Table R1003.14(2) shows 12×12 = 102 in² (insufficient) and 12×16 = 131 in² (sufficient). So choose 12×16 or larger.

  3. If the chimney height or other conditions suggest using Option 2, consult Figure R1003.15.2 and measure height from the firebox floor to the top of flue; select a flue with net area equal to or exceeding the figure's required area.

  4. If two flues are placed side‑by‑side in the same chimney and both will be used for separate appliances, provide a masonry wythe ≥ 4 in between the liners; if the chimney serves only one appliance you may allow them to adjoin but must stagger liner joints ≥ 4 in.

Related provisions

  • § R1003.14 — Flue area for appliances and tables of liner areas.
  • § R1003.12 — Clay flue lining installation and space around lining.
  • § R1003.11.5 / R1003.11.6 — Liner material listings and required labeling when relined for specific fuel types.
  • California Mechanical Code — sizing rules for chimneys serving appliances and for multiple appliance venting (see CMC sizing / NFPA guidance referenced from CRC).

Code references

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

  • CRC § 10-8 High relevance — show source text

    10-8 2025 CALIFORNIA RESIDENTIAL CODE

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

    CHIMNEYS AND FIREPLACES

    R1003.12.1 Listed materials. Listed materials used as flue linings shall be installed in accordance with the terms of their listings and manufacturer’s instructions.

    R1003.12.2 Space around lining. The space surrounding a chimney lining system or vent installed within a masonry chimney shall not be used to vent any other appliance.

    Exception: This shall not prevent the installation of a separate flue lining in accordance with the manufacturer’s instructions.

    R1003.13 Multiple flues. Where two or more flues are located in the same chimney, masonry wythes shall be built between adjacent flue linings. The masonry wythes shall be not less than 4 inches (102 mm) thick and bonded into the walls of the chimney.

    Exception: Where venting only one appliance, two flues shall be permitted to adjoin each other in the same chimney with only the flue lining separation between them. The joints of the adjacent flue linings shall be staggered not less than 4 inches (102 mm).

    R1003.14 Flue area (appliance). Chimney flues shall not be smaller in area than that of the area of the connector from the appliance

    [see Tables R1003.14(1) and R1003.14(2)]. The sizing of a chimney flue to which multiple appliance venting systems are connected shall be in accordance with the California Mechanical Code.

    TABLE R1003.14(1)—NET CROSS-SECTIONAL AREA OF ROUND FLUE SIZESa Col2
    ** FLUE SIZE, INSIDE DIAMETER (inches)** ** CROSS-SECTIONAL AREA (square inches)**
    6 28
    7 38
    8 50
    10 78
    103/4 90
    12 113
    15 176
    18 254
    For SI: 1 inch = 25.4 mm, 1 square inch = 645.16 mm2.
    a. Flue sizes are based on ASTM C315.    		 For SI: 1 inch = 25.4 mm, 1 square inch = 645.16 mm2.
    a. Flue sizes are based on ASTM C315.
  • CRC § 25.4 High relevance — show source text

    | |Fireplace lintel|L|Noncombustible material.| |Chimney walls with flue lining|M|Solid masonry units or hollow masonry units grouted solid with not
    less than 4-inch nominal thickness.| |Distances between adjacent flues|—|See Section R1003.13.| |Effective flue area (based on area of fireplace opening)|P|See Section R1003.15.| |Clearances
    Combustible material
    Mantel and trim
    Above roof|R|See Sections R1001.11 and R1003.18.
    See Section R1001.11, Exception 4.
    3 feet at roofline and 2 feet at 10 feet.| |Anchorageb
    Strap Number
    Embedment into chimney
    Fasten to
    Bolts|S|3/16-inch × 1-inch
    Two
    12 inches hooked around outer bar with 6-inch extension.
    4 joists
    Two1/2-inch diameter.| |Footing
    Thickness
    Width|T|12 inches min.
    6 inches each side of fireplace wall.| |For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 square foot = 0.0929 m2.
    Note: This table provides a summary of major requirements for the construction of masonry chimneys and fireplaces. Letter references are to Figure R1001.1, which shows
    examples of typical construction. This table does not cover all requirements, nor does it cover all aspects of the indicated requirements. For the actual mandatory require-
    ments of the code, see the indicated section of text.
    a. The letters refer to Figure R1001.1.
    b. Not required in Seismic Design Category A or B.|For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 square foot = 0.0929 m2.
    Note: This table provides a summary of major requirements for the construction of masonry chimneys and fireplaces. Letter references are to Figure R1001.1, which shows
    examples of typical construction. This table does not cover all requirements, nor does it cover all aspects of the indicated requirements. For the actual mandatory require-
    ments of the code, see the indicated section of text.
    a. The letters refer to Figure R1001.1.
    b. Not required in Seismic Design Category A or B.|For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 square foot = 0.0929 m2.
    Note: This table provides a summary of major requirements for the construction of masonry chimneys and fireplaces. Letter references are to Figure R1001.1, which shows
    examples of typical construction. This table does not cover all requirements, nor does it cover all aspects of the indicated requirements. For the actual mandatory require-
    ments of the code, see the indicated section of text.
    a. The letters refer to Figure R1001.1.
    b. Not required in Seismic Design Category A or B.|

    2025 CALIFORNIA RESIDENTIAL CODE 10-3

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

    CHIMNEYS AND FIREPLACES

    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm.

  • CRC § 25.4 High relevance — show source text

    SECTION R1001—MASONRY FIREPLACES

    R1001.1 General. Masonry fireplaces shall be constructed in accordance with this section and the applicable provisions of Chapters 3 and 4.

    TABLE R1001.1—SUMMARY OF REQUIREMENTS FOR MASONRY FIREPLACES AND CHIMNEYS Col2 Col3
    ** ITEM** ** LETTERa** ** REQUIREMENTS**
    Hearth slab thickness A 4 inches
    Hearth extension (each side of opening) B 8-inch fireplace opening < 6 square feet.
    12-inch fireplace opening ≥ 6 square feet.
    Hearth extension (front of opening) C 16-inch fireplace opening < 6 square feet.
    20-inch fireplace opening ≥ 6 square feet.
    Hearth slab reinforcing D Reinforced to carry its own weight and all imposed loads.
    Thickness of wall of firebox E 10-inch solid brick or 8 inches where a firebrick lining is used.
    Joints in firebrick1/4-inch maximum.
    Distance from top of opening to throat F 8 inches
    Smoke chamber wall thickness
    Unlined walls    		 G 6 inches
    8 inches
    Chimney
    Vertical reinforcingb    		 H Four No. 4 full-length bars for chimney up to 40 inches wide.
    Add two No. 4 bars for each additional 40 inches or fraction of
    width or each additional flue.
    Horizontal reinforcing J 1/4-inch ties at 18 inches and two ties at each bend in vertical steel.
    Bond beams K No specified requirements.
    Fireplace lintel L Noncombustible material.
    Chimney walls with flue lining M Solid masonry units or hollow masonry units grouted solid with not
    less than 4-inch nominal thickness.
    Distances between adjacent flues — See Section R1003.13.
    Effective flue area (based on area of fireplace opening) P See Section R1003.15.
    Clearances
    Combustible material
    Mantel and trim
    Above roof    		 R See Sections R1001.11 and R1003.18.
    See Section R1001.11, Exception 4.
    3 feet at roofline and 2 feet at 10 feet.
    Anchorageb
    Strap Number
    Embedment into chimney
    Fasten to
    Bolts    		 S 3/16-inch × 1-inch
    Two
    12 inches hooked around outer bar with 6-inch extension.
    4 joists
    Two1/2-inch diameter.
    Footing
    Thickness
    Width    		 T 12 inches min.
    6 inches each side of fireplace wall.
    For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 square foot = 0.0929 m2.
    Note: This table provides a summary of major requirements for the construction of masonry chimneys and fireplaces. Letter references are to Figure R1001.1, which shows
    examples of typical construction. This table does not cover all requirements, nor does it cover all aspects of the indicated requirements. For the actual mandatory require-
    ments of the code, see the indicated section of text.
    a. The letters refer to Figure R1001.1.
    b.
  • CRC § 25.4 High relevance — show source text

    R1003.13 Multiple flues. Where two or more flues are located in the same chimney, masonry wythes shall be built between adjacent flue linings. The masonry wythes shall be not less than 4 inches (102 mm) thick and bonded into the walls of the chimney.

    Exception: Where venting only one appliance, two flues shall be permitted to adjoin each other in the same chimney with only the flue lining separation between them. The joints of the adjacent flue linings shall be staggered not less than 4 inches (102 mm).

    R1003.14 Flue area (appliance). Chimney flues shall not be smaller in area than that of the area of the connector from the appliance

    [see Tables R1003.14(1) and R1003.14(2)]. The sizing of a chimney flue to which multiple appliance venting systems are connected shall be in accordance with the California Mechanical Code.

    TABLE R1003.14(1)—NET CROSS-SECTIONAL AREA OF ROUND FLUE SIZESa Col2
    ** FLUE SIZE, INSIDE DIAMETER (inches)** ** CROSS-SECTIONAL AREA (square inches)**
    6 28
    7 38
    8 50
    10 78
    103/4 90
    12 113
    15 176
    18 254
    For SI: 1 inch = 25.4 mm, 1 square inch = 645.16 mm2.
    a. Flue sizes are based on ASTM C315.    		 For SI: 1 inch = 25.4 mm, 1 square inch = 645.16 mm2.
    a. Flue sizes are based on ASTM C315.
    TABLE R1003.14(2)—NET CROSS-SECTIONAL AREA OF SQUARE AND RECTANGULAR FLUE SIZES Col2
    ** FLUE SIZE, OUTSIDE NOMINAL DIMENSIONS (inches)** ** CROSS-SECTIONAL AREA (square inches)**
    4.5 × 8.5 23
    4.5 × 13 34
    8 × 8 42
    8.5 × 8.5 49
    8 × 12 67
    8.5 × 13 76
    12 × 12 102
    8.5 × 18 101
    13 × 13 127
    12 × 16 131
    13 × 18 173
    16 × 16 181
    16 × 20 222
    18 × 18 233
    20 × 20 298
    20 × 24 335
    24 × 24 431
    For SI: 1 inch = 25.4 mm, 1 square inch = 645.16 mm2. For SI: 1 inch = 25.4 mm, 1 square inch = 645.16 mm2.

    R1003.15 Flue area (masonry fireplace). Flue sizing for chimneys serving fireplaces shall be in accordance with Section R1003.15.1 or R1003.15.2.

    2025 CALIFORNIA RESIDENTIAL CODE 10-9

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

    CHIMNEYS AND FIREPLACES

  • CRC § 4.5 High relevance — show source text
    TABLE R1003.14(2)—NET CROSS-SECTIONAL AREA OF SQUARE AND RECTANGULAR FLUE SIZES Col2
    ** FLUE SIZE, OUTSIDE NOMINAL DIMENSIONS (inches)** ** CROSS-SECTIONAL AREA (square inches)**
    4.5 × 8.5 23
    4.5 × 13 34
    8 × 8 42
    8.5 × 8.5 49
    8 × 12 67
    8.5 × 13 76
    12 × 12 102
    8.5 × 18 101
    13 × 13 127
    12 × 16 131
    13 × 18 173
    16 × 16 181
    16 × 20 222
    18 × 18 233
    20 × 20 298
    20 × 24 335
    24 × 24 431
    For SI: 1 inch = 25.4 mm, 1 square inch = 645.16 mm2. For SI: 1 inch = 25.4 mm, 1 square inch = 645.16 mm2.

    R1003.15 Flue area (masonry fireplace). Flue sizing for chimneys serving fireplaces shall be in accordance with Section R1003.15.1 or R1003.15.2.

    2025 CALIFORNIA RESIDENTIAL CODE 10-9

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

    CHIMNEYS AND FIREPLACES

    R1003.15.1 Option 1. Round chimney flues shall have a minimum net cross-sectional area of not less than one-twelfth of the fireplace opening. Square chimney flues shall have a minimum net cross-sectional area of one-tenth of the fireplace opening. Rectangular chimney flues with an aspect ratio less than 2 to 1 shall have a minimum net cross-sectional area of one-tenth of the fireplace opening. Rectangular chimney flues with an aspect ratio of 2 to 1 or more shall have a minimum net cross-sectional area of one-eighth of the fireplace opening. Cross-sectional areas of clay flue linings are shown in Tables R1003.14(1) and R1003.14(2) or as provided by the manufacturer or as measured in the field.

    R1003.15.2 Option 2. The minimum net cross-sectional area of the chimney flue shall be determined in accordance with Figure R1003.15.2. A flue size providing not less than the equivalent net cross-sectional area shall be used. Cross-sectional areas of clay flue linings are shown in Tables R1003.14(1) and R1003.14(2) or as provided by the manufacturer or as measured in the field. The height of the chimney shall be measured from the firebox floor to the top of the chimney flue.

    FIGURE R1003.15.2—FLUE SIZES FOR MASONRY CHIMNEYS

    Col1 Col2 Col3 Col4 Col5 Col6 Col7 Col8 Col9 Col10 Col11 Col12 Col13 Col14 3000
    224
    187
    MIN. CROSS-SECTIONAL AREA (SQ. IN.
  • CRC § 11.5 High relevance — show source text

    R1003.11.5 Oil-fired appliances approved for use with Type L vent. Flue lining and vent systems for use in masonry chimneys with oil-fired appliances approved for use with Type L vent shall be limited to the following:

    1. Flue lining systems complying with Section R1003.11.1.
    2. Listed chimney liners complying with UL 641 (see Section R1003.11.6 for marking).

    R1003.11.6 Notice of usage. Where a flue is relined with a material not complying with Section R1003.11.1, the chimney shall be plainly and permanently identified by a label attached to a wall, ceiling or other conspicuous location adjacent to where the connector enters the chimney. The label shall include the following message or equivalent language:

    THIS CHIMNEY FLUE IS FOR USE ONLY WITH [TYPE OR CATEGORY OF APPLIANCE] APPLIANCES THAT BURN [TYPE OF FUEL]. DO NOT CONNECT OTHER TYPES OF APPLIANCES.

    R1003.12 Clay flue lining (installation). Clay flue liners shall be installed in accordance with ASTM C1283 and extend from a point not less than 8 inches (203 mm) below the lowest inlet or, in the case of fireplaces, from the top of the smoke chamber to a point above the enclosing walls. The lining shall be carried up vertically, with a slope not greater than 30 degrees (0.52 rad) from the vertical.

    Clay flue liners shall be laid in medium-duty water insoluble refractory mortar conforming to ASTM C199 with tight mortar joints left smooth on the inside and installed to maintain an airspace or insulation not to exceed the thickness of the flue liner separating the flue liners from the interior face of the chimney masonry walls. Flue liners shall be supported on all sides. Only enough mortar shall be placed to make the joint and hold the liners in position.

    10-8 2025 CALIFORNIA RESIDENTIAL CODE

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

    CHIMNEYS AND FIREPLACES

    R1003.12.1 Listed materials. Listed materials used as flue linings shall be installed in accordance with the terms of their listings and manufacturer’s instructions.

    R1003.12.2 Space around lining. The space surrounding a chimney lining system or vent installed within a masonry chimney shall not be used to vent any other appliance.

    Exception: This shall not prevent the installation of a separate flue lining in accordance with the manufacturer’s instructions.

    R1003.13 Multiple flues. Where two or more flues are located in the same chimney, masonry wythes shall be built between adjacent flue linings. The masonry wythes shall be not less than 4 inches (102 mm) thick and bonded into the walls of the chimney.

    Exception: Where venting only one appliance, two flues shall be permitted to adjoin each other in the same chimney with only the flue lining separation between them. The joints of the adjacent flue linings shall be staggered not less than 4 inches (102 mm).

    R1003.14 Flue area (appliance). Chimney flues shall not be smaller in area than that of the area of the connector from the appliance

    [see Tables R1003.14(1) and R1003.14(2)]. The sizing of a chimney flue to which multiple appliance venting systems are connected shall be in accordance with the California Mechanical Code.

  • CRC § 21-17 High relevance — show source text

    2025 CALIFORNIA BUILDING CODE 21-17

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

    MASONRY

    TABLE 2113.16(2)—NET CROSS-SECTIONAL AREA OF SQUARE AND RECTANGULAR FLUE SIZES—continued Col2
    FLUE SIZE, OUTSIDE NOMINAL DIMENSIONS (inches) CROSS-SECTIONAL AREA (square inches)
    13 × 18 173
    16 × 16 181
    16 × 20 222
    18 × 18 233
    20 × 20 298
    20 × 24 335
    24 × 24 431
    For SI: 1 inch = 25.4 mm, 1 square inch = 645.16 mm2. For SI: 1 inch = 25.4 mm, 1 square inch = 645.16 mm2.

    2113.16.1 Minimum area. Round chimney flues shall have a minimum net cross-sectional area of not less than [1] / 12 of the fireplace opening. Square chimney flues shall have a minimum net cross-sectional area of not less than [1] / 10 of the fireplace opening. Rectangular chimney flues with an aspect ratio less than 2 to 1 shall have a minimum net cross-sectional area of not less than [1] / 10 of the fireplace opening. Rectangular chimney flues with an aspect ratio of 2 to 1 or more shall have a minimum net cross-sectional area of not less than [1] / 8 of the fireplace opening.

    2113.16.2 Determination of minimum area. The minimum net cross-sectional area of the flue shall be determined in accordance with Figure 2113.16. A flue size providing not less than the equivalent net cross-sectional area shall be used. Cross-sectional areas of clay flue linings are as provided in Tables 2113.16(1) and 2113.16(2) or as provided by the manufacturer or as measured in the field. The height of the chimney shall be measured from the firebox floor to the top of the chimney flue.

    2113.17 Inlet. Inlets to masonry chimneys shall enter from the side. Inlets shall have a thimble of fireclay, rigid refractory material or metal that will prevent the connector from pulling out of the inlet or from extending beyond the wall of the liner.

    2113.18 Masonry chimney cleanout openings. Cleanout openings shall be provided within 6 inches (152 mm) of the base of each flue within every masonry chimney. The upper edge of the cleanout shall be located not less than 6 inches (152 mm) below the lowest chimney inlet opening. The height of the opening shall be not less than 6 inches (152 mm). The cleanout shall be provided with a noncombustible cover.

    Exception: Chimney flues serving masonry fireplaces, where cleaning is possible through the fireplace opening.

    2113.19 Chimney clearances. Any portion of a masonry chimney located in the interior of the building or within the exterior wall of the building shall have a minimum airspace clearance to combustibles of 2 inches (51 mm). Chimneys located entirely outside the exterior walls of the building, including chimneys that pass through the soffit or cornice, shall have a minimum airspace clearance of 1 inch (25 mm). The airspace shall not be filled, except to provide fireblocking in accordance with Section 2113.20.

  • CRC § 803.0. High relevance — show source text

    (1) Those listed in Section 803.0.

    (2) The effective areas of the vent connector and chimney flue of a venting system serving a single appliance with a draft hood shall be not less than the area

    of the appliance flue collar or draft hood outlet or greater than seven times the draft hood outlet area.

    (3) The effective area of the chimney flue of a venting system serving two appliances with draft hoods shall be not less than the area of the larger draft hood outlet plus 50 percent of the area of the smaller draft hood outlet or greater than seven times the smaller draft hood outlet area.

    (4) Chimney venting systems using mechanical draft shall be sized in accordance with engineering methods.

    (5) Other engineering methods. [NFPA 54:12.6.3.1]

    2025 CALIFORNIA MECHANICAL CODE 155

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

    CHIMNEYS AND VENTS

    FIGURE 802.5.4

    TYPICAL TERMINATION LOCATIONS FOR

    CHIMNEYS AND SINGLE-WALL METAL PIPES SERVING

    RESIDENTIAL-TYPE AND LOW-HEAT APPLIANCE

    [NFPA 54: FIGURE A.12.6.2.1]

    802.5.6 Inspection of Chimneys or Vents. This inspection shall be made after chimneys, vents, or parts thereof, authorized by the permit, have been installed and before such vent or part thereof has been covered or concealed.

    802.5.7 Inspection of Chimneys. Before replacing an existing appliance or connecting a vent connector to a chimney, the chimney passageway shall be examined to ascertain that it is clear and free of obstructions and shall be cleaned if previously used for venting solid- or liquid-fuel-burning appliances or fireplaces. [NFPA 54:12.6.4.1] 802.5.7.1 Standard. Chimneys shall be lined in accordance with NFPA 211. [NFPA 54:12.6.4.2]

    802.5.7.2 Cleanouts. Cleanouts shall be examined and where they do not remain tightly closed when not in use, they shall be repaired or replaced. [NFPA 54:12.6.4.3] 802.5.7.3 Existing Chimney. When inspection reveals that an existing chimney is not safe for the intended application, it shall be repaired, rebuilt, lined, relined, or replaced with a vent or chimney to conform to NFPA 211 and shall be suitable for the appliances to be attached. [NFPA 54:12.6.4.4] 802.5.8 Chimney Serving Appliances Burning Other Fuels. An appliance shall not be connected to a chimney flue serving a separate appliance designed to burn solid fuel. [NFPA 54:12.6.5.1] 802.5.8.1 Gas and Liquid Fuel-Burning Appliances. Where one chimney serves gas appliances and liquid fuel-burning appliances, the appliances shall be connected through separate openings or connected through a single opening where joined by a suitable fitting located as close as practical to the chimney. Where two or more openings are provided into one chimney flue, they shall be at different levels.

  • CRC § 2113.14 Medium relevance — show source text

    Exception: This shall not prevent the installation of a separate flue lining in accordance with the manufacturer’s instructions.

    2113.14 Multiple flues. Where two or more flues are located in the same chimney, masonry wythes shall be built between adjacent flue linings. The masonry wythes shall be not less than 4 inches (102 mm) thick and bonded into the walls of the chimney.

    Exception: Where venting only one appliance, two flues are permitted to adjoin each other in the same chimney with only the flue lining separation between them. The joints of the adjacent flue linings shall be staggered not less than 4 inches (102 mm).

    2113.15 Flue area (appliance). Chimney flues shall not be smaller in area than the area of the connector from the appliance. Chimney flues connected to more than one appliance shall be not less than the area of the largest connector plus 50 percent of the areas of additional chimney connectors.

    Exceptions:

    1. Chimney flues serving oil-fired appliances sized in accordance with NFPA 31.
    2. Chimney flues serving gas-fired appliances sized in accordance with the California Mechanical Code .

    2113.16 Flue area (masonry fireplace). Flue sizing for chimneys serving fireplaces shall be in accordance with Section 2113.16.1 or 2113.16.2.

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    FIGURE 2113.16—FLUE SIZES FOR MASONRY CHIMNEYS

    a Note: 1 inch = 25.4 mm, 1 square inch = 645 mm [2] .

    TABLE 2113.16(1)—NET CROSS-SECTIONAL AREA OF ROUND FLUE SIZESa Col2
    FLUE SIZE, INSIDE DIAMETER (inches) CROSS-SECTIONAL AREA (square inches)
    6 28
    7 38
    8 50
    10 78
    103/4 90
    12 113
    15 176
    18 254
    For SI: 1 inch = 25.4 mm, 1 square inch = 645.16 mm2.
    a. Flue sizes are based on ASTM C315.    		 For SI: 1 inch = 25.4 mm, 1 square inch = 645.16 mm2.
    a. Flue sizes are based on ASTM C315.
    TABLE 2113.16(2)—NET CROSS-SECTIONAL AREA OF SQUARE AND RECTANGULAR FLUE SIZES Col2
    FLUE SIZE, OUTSIDE NOMINAL DIMENSIONS (inches) CROSS-SECTIONAL AREA (square inches)
    4.5 × 8.5 23
    4.5 × 13 34
    8 × 8 42
    8.5 × 8.5 49
    8 × 12 67
    8.5 × 13 76
    12 × 12 102
    8.5 × 18 101
    13 ×13 127
    12 × 16 131

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  • CRC § 2113.11.3.3 Medium relevance — show source text

    2113.11.3.3 Termination height. Concrete and masonry chimneys for high-heat appliances shall extend not less than 20 feet (6096 mm) higher than any portion of any building within 50 feet (15 240 mm).

    2113.11.3.4 Clearance. Concrete and masonry chimneys for high-heat appliances shall have approved clearance from buildings and structures to prevent overheating combustible materials, permit inspection and maintenance operations on the chimney and prevent danger of burns to persons.

    2113.12 Clay flue lining (installation). Clay flue liners shall be installed in accordance with ASTM C1283 and extend from a point not less than 8 inches (203 mm) below the lowest inlet or, in the case of fireplaces, from the top of the smoke chamber to a point above the enclosing walls. The lining shall be carried up vertically, with a maximum slope not greater than 30 degrees (0.52 rad) from the vertical.

    Clay flue liners shall be laid in medium-duty nonwater-soluble refractory mortar conforming to ASTM C199 with tight mortar joints left smooth on the inside and installed to maintain an airspace or insulation not to exceed the thickness of the flue liner separating the flue liners from the interior face of the chimney masonry walls. Flue lining shall be supported on all sides. Only enough mortar shall be placed to make the joint and hold the liners in position.

    2113.13 Additional requirements.

    2113.13.1 Listed materials. Listed materials used as flue linings shall be installed in accordance with the terms of their listings and the manufacturer’s instructions.

    2113.13.2 Space around lining. The space surrounding a chimney lining system or vent installed within a masonry chimney shall not be used to vent any other appliance.

    Exception: This shall not prevent the installation of a separate flue lining in accordance with the manufacturer’s instructions.

    2113.14 Multiple flues. Where two or more flues are located in the same chimney, masonry wythes shall be built between adjacent flue linings. The masonry wythes shall be not less than 4 inches (102 mm) thick and bonded into the walls of the chimney.

    Exception: Where venting only one appliance, two flues are permitted to adjoin each other in the same chimney with only the flue lining separation between them. The joints of the adjacent flue linings shall be staggered not less than 4 inches (102 mm).

    2113.15 Flue area (appliance). Chimney flues shall not be smaller in area than the area of the connector from the appliance. Chimney flues connected to more than one appliance shall be not less than the area of the largest connector plus 50 percent of the areas of additional chimney connectors.

    Exceptions:

    1. Chimney flues serving oil-fired appliances sized in accordance with NFPA 31.
    2. Chimney flues serving gas-fired appliances sized in accordance with the California Mechanical Code .

    2113.16 Flue area (masonry fireplace). Flue sizing for chimneys serving fireplaces shall be in accordance with Section 2113.16.1 or 2113.16.2.

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    FIGURE 2113.16—FLUE SIZES FOR MASONRY CHIMNEYS

  • CRC § 12.6.2.1 Medium relevance — show source text

    ** A chimney for residential-type or low-heat appliances shall extend at least 3 feet (914 mm) above the highest point where it passes through a roof of a building and at least 2 feet (610 mm) higher than any portion of a building within a horizontal distance of 10 feet (3048 mm). [NFPA 54:12.6.2.1] (See Figure 802.5.4) 802.5.4.1 Medium-Heat Gas Appliances. A chimney for medium-heat appliances shall extend at least 10 feet (3048 mm) higher than any portion of any building within 25 feet (7620 mm). [NFPA 54:12.6.2.2]

    802.5.4.2 Chimney Height. A chimney shall extend at least 5 feet (1524 mm) above the highest connected appliance draft hood outlet or flue collar.

    [NFPA 54:12.6.2.3]

    802.5.4.3 Decorative Shrouds. Decorative

    shrouds shall not be installed at the termination of factory-built chimneys except where such shrouds are

    listed and labeled for use with the specific factorybuilt chimney system and are installed in accordance with the manufacturer’s installation instructions.

    [NFPA 54:12.6.2.4]

    802.5.5 Size of Chimneys. The effective area of a chimney venting system serving listed appliances with draft hoods, Category I appliances, and other appliances listed for use with Type B vents shall be in accordance with one of the following methods:

    (1) Those listed in Section 803.0.

    (2) The effective areas of the vent connector and chimney flue of a venting system serving a single appliance with a draft hood shall be not less than the area

    of the appliance flue collar or draft hood outlet or greater than seven times the draft hood outlet area.

    (3) The effective area of the chimney flue of a venting system serving two appliances with draft hoods shall be not less than the area of the larger draft hood outlet plus 50 percent of the area of the smaller draft hood outlet or greater than seven times the smaller draft hood outlet area.

    (4) Chimney venting systems using mechanical draft shall be sized in accordance with engineering methods.

    (5) Other engineering methods. [NFPA 54:12.6.3.1]

    2025 CALIFORNIA MECHANICAL CODE 155

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

    CHIMNEYS AND VENTS

    FIGURE 802.5.4

    TYPICAL TERMINATION LOCATIONS FOR

    CHIMNEYS AND SINGLE-WALL METAL PIPES SERVING

    RESIDENTIAL-TYPE AND LOW-HEAT APPLIANCE

    [NFPA 54: FIGURE A.12.6.2.1]

    802.5.6 Inspection of Chimneys or Vents. This inspection shall be made after chimneys, vents, or parts thereof, authorized by the permit, have been installed and before such vent or part thereof has been covered or concealed.

  • CRC § 13.2 Medium relevance — show source text

    2113 A .13.2 Space around lining. The space surrounding a chimney lining system or vent installed within a masonry chimney shall not be used to vent any other appliance.

    Exception: This shall not prevent the installation of a separate flue lining in accordance with the manufacturer’s instructions.

    2113 A .14 Multiple flues. Where two or more flues are located in the same chimney, masonry wythes shall be built between adjacent flue linings. The masonry wythes shall be not less than 4 inches (102 mm) thick and bonded into the walls of the chimney.

    Exception: Where venting only one appliance, two flues are permitted to adjoin each other in the same chimney with only the flue lining separation between them. The joints of the adjacent flue linings shall be staggered not less than 4 inches (102 mm).

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    2113 A .15 Flue area (appliance). Chimney flues shall not be smaller in area than the area of the connector from the appliance. Chimney flues connected to more than one appliance shall be not less than the area of the largest connector plus 50 percent of the areas of additional chimney connectors.

    Exceptions:

    1. Chimney flues serving oil-fired appliances sized in accordance with NFPA 31.
    2. Chimney flues serving gas-fired appliances sized in accordance with the California Mechanical Code .

    2113 A .16 Flue area (masonry fireplace). Flue sizing for chimneys serving fireplaces shall be in accordance with Section 2113 A .16.1 or 2113 A .16.2.

    FIGURE 2113 A .16—FLUE SIZES FOR MASONRY CHIMNEYS

    a Note: 1 inch = 25.4 mm, 1 square inch = 645 mm [2] .

    TABLE 2113A.16(1)—NET CROSS-SECTIONAL AREA OF ROUND FLUE SIZESa Col2
    FLUE SIZE, INSIDE DIAMETER (inches) CROSS-SECTIONAL AREA (square inches)
    6 28
    7 38
    8 50
    10 78
    103/4 90
    12 113
    15 176
    18 254
    For SI: 1 inch = 25.4 mm, 1 square inch = 645.16 mm2.
    a. Flue sizes are based on ASTM C315.    		 For SI: 1 inch = 25.4 mm, 1 square inch = 645.16 mm2.
    a. Flue sizes are based on ASTM C315.

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Frequently asked questions

What exactly is the "fireplace opening" area you use for the 1/12 or 1/10 calculation?

Use the plan‑view net area of the fireplace opening (the clear opening through which smoke must pass). The code ratios in § R1003.15.1 apply to that opening area.

Can I use the masonry cavity size (inside brick) instead of table values for clay liners?

No — use the net cross‑sectional area from Tables R1003.14(1)/(2), manufacturer data, or field measurements of the actual liner opening. Do not assume gross cavity equals net flue area.

If two flues are next to each other, is a 2‑inch partition OK?

No — where two or more flues are in the same chimney a masonry wythe not less than 4 inches thick is required between flue linings under § R1003.13, unless the single‑appliance exception applies.

Which method should I pick — Option 1 ratios or Option 2 figure?

Either method is acceptable per § R1003.15; Option 1 is quick (shape‑based ratios), Option 2 uses the chart and accounts for chimney height. Use the method that best matches your field situation and verify with Table R1003.14 liner areas.

Does the CRC cover multi‑appliance chimney sizing?

The CRC points to the California Mechanical Code and NFPA methods for chimneys serving multiple appliances or mechanical draft systems — follow those referenced rules for multi‑appliance sizing.

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Masonry Chimneys

California Residential Code