Chimney Construction, Height & Termination

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

• CMC § 12.6.2.1 High 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.

• CMC § 11.2.3 High relevance — show source text

  2113 A .11.2.3 Multiple passageway. Concrete and masonry chimneys containing more than one passageway shall have the liners separated by a minimum 4-inch-thick (102 mm) concrete or solid masonry wall.

  2113 A .11.2.4 Termination height. Concrete and masonry chimneys for medium-heat appliances shall extend not less than 10 feet (3048 mm) higher than any portion of any building within 25 feet (7620 mm).

  2113 A .11.2.5 Clearance. A minimum clearance of 4 inches (102 mm) shall be provided between the exterior surfaces of a concrete or masonry chimney for medium-heat appliances and combustible material.

  2113 A .11.3 Concrete and masonry chimneys for high-heat appliances. Concrete and masonry chimneys for high-heat appliances shall comply with 2113.11.3.1 through 2113.11.3.4.

  2113 A .11.3.1 Construction. Chimneys for high-heat appliances shall be constructed with double walls of solid masonry units or of concrete, each wall to be not less than 8 inches (203 mm) thick with a minimum airspace of 2 inches (51 mm) between the walls.

  2113 A .11.3.2 Lining. The inside of the interior wall shall be lined with an approved high-duty refractory brick, not less than 4 [1] / 2 inches (114 mm) thick laid on the 4 [1] / 2 -inch bed (114 mm) in an approved high-duty refractory mortar. The lining shall start at the base of the chimney and extend continuously to the top.

  2113 A .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 A .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 A .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 A .13 Additional requirements.

• CMC § 2113.11.2.3 High relevance — show source text

  2113.11.2.3 Multiple passageway. Concrete and masonry chimneys containing more than one passageway shall have the liners separated by a minimum 4-inch-thick (102 mm) concrete or solid masonry wall.

  2113.11.2.4 Termination height. Concrete and masonry chimneys for medium-heat appliances shall extend not less than 10 feet (3048 mm) higher than any portion of any building within 25 feet (7620 mm).

  2113.11.2.5 Clearance. A minimum clearance of 4 inches (102 mm) shall be provided between the exterior surfaces of a concrete or masonry chimney for medium-heat appliances and combustible material.

  2113.11.3 Concrete and masonry chimneys for high-heat appliances. Concrete and masonry chimneys for high-heat appliances shall comply with 2113.11.3.1 through 2113.11.3.4.

  2025 CALIFORNIA BUILDING CODE 21-15

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  MASONRY

  2113.11.3.1 Construction. Chimneys for high-heat appliances shall be constructed with double walls of solid masonry units or of concrete, each wall to be not less than 8 inches (203 mm) thick with a minimum airspace of 2 inches (51 mm) between the walls.

  2113.11.3.2 Lining. The inside of the interior wall shall be lined with an approved high-duty refractory brick, not less than 4 [1] / 2 inches (114 mm) thick laid on the 4 [1] / 2 -inch bed (114 mm) in an approved high-duty refractory mortar. The lining shall start at the base of the chimney and extend continuously to the top.

  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.

• CMC § 11.3.3 High relevance — show source text

  2113 A .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 A .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 A .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 A .13 Additional requirements.

  2113 A .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 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).

  2025 CALIFORNIA BUILDING CODE 21A-15

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

  MASONRY

  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 .
• CMC § 2113.11.3.3 High 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.

  21-16 2025 CALIFORNIA BUILDING CODE

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  MASONRY

  FIGURE 2113.16—FLUE SIZES FOR MASONRY CHIMNEYS

• CMC § 802.5.4.2 High relevance — show source text

  Height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .802.5.4.2

  Inspection . . . . . . . . . . . . . . . . . . . . . .802.5.6, 802.5.7

  Installation . . . . . . . . . . . . . . . . . . . . . . . . . . .Chapter 8

  Location . . . . . . . . . . . . . . . . . . . . . . . . . .Table 402.4.1

  Masonry . . . . . . . . . . . . . . . . .802.5.3, 802.5.11, 802.6,

  Table 803.1.2(3), Table 803.1.2(4),

  Table 803.1.2(6), 803.1.6,

  803.1.9, 803.1.10, Table 803.2(3),

  Table 803.2(4), Table 803.2(6)-Table 803.2(9),

  803.2.19, 803.2.20, Appendix F

  574 2025 CALIFORNIA MECHANICAL CODE

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

  INDEX

  Metal . . . . . . . . . . . . . . . . . . . . . . . . .802.5.2, 802.10.11

  Serving appliances . . . . . . . . . . . . . . . . . . . . . .802.5.8

  Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .802.5.5

  Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .802.5.9

  Termination . . . . . . . . . . . . . . . . . . . . . . . . . . . .802.5.4

  Venting systems . . . . . . . . . . . . . . . .802.5.5, 802.6.2.4

  CHIMNEY CLASSIFICATIONS

  High-heat appliances-type . . . . . . . . . . . . . . . . . . 205.0

  Low-heat appliances-type. . . . . . . . . . . . . . . . . . . 205.0

  Medium-heat

  appliances-type. . . . . . . . . . . . . . . . . . . . . . . . . 205.0

  Residential appliances-type . . . . . . . . . . . . . . . . . . 205.0

  CHLORINATED POLYVINYL

  CHLORIDE (CPVC) PIPE OR TUBING

  Condensate waste piping . . . . . . . . . . . . . . . . . . . 310.3

  Ground source loop . . . . . . . . . . . . . . . . . . Table 1703.2

  Hangers and supports. . . . . . . . . . . . . . . . . Table 313.6

• CMC § 907.0 Medium relevance — show source text

  6, Section 907.0| |Category II, Category III, and
  Category IV appliances|As specified or furnished by
  manufacturers of listed appliances|Section 802.4.1, Section 802.4.3| |Incinerators|In accordance with NFPA 82|In accordance with NFPA 82| |Appliances that can be converted to use of solid fuel
  
  Unlisted combination gas- and oil-burning appliances
  
  Combination gas- and solid-fuel-burning appliances
  
  Appliances listed for use with chimneys only
  
  Unlisted appliances|Chimney|Section 802.5| |Listed combination gas- and oil-burning appliances|Type L vent|Section 802.6| |Listed combination gas- and oil-burning appliances|Chimney|Section 802.5| |Decorative appliances in vented fireplace|Chimney|Section 911.3| |Gas-fired toilets|Single-wall metal pipe|Section 802.7, Section 928.3| |Direct vent appliances|—|Section 802.2.6| |Appliances with integral vents|—|Section 802.2.7|

  other appliances listed for use with Type B vents shall be permitted. The liner shall be installed in accordance with the liner manufacturer’s installation instructions. A permanent identifying label shall be attached at the point where the connection is to be made to the liner. The label shall read: “This chimney liner is for appliances that burn gas only. Do not connect to solid-or liquid-fuel-burning appliances or incinerators.” [NFPA 54:12.6.1.3] 802.5.4 Termination. 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.

• CMC § 11.1.5 Medium relevance — show source text

  2113 A .11.1.5 Notice of usage. When a flue is relined with a material not complying with Section 2113 A .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 is for use only with (type or category of appliance) that burns (type of fuel). Do not connect other types of appliances.”

  2113 A .11.2 Concrete and masonry chimneys for medium-heat appliances. Concrete and masonry chimneys for medium-heat appliances shall comply with Sections 2113 A .11.2.1 through 2113 A .11.2.5.

  2113 A .11.2.1 Construction. Chimneys for medium-heat appliances shall be constructed of solid masonry units or of concrete with walls not less than 8 inches (203 mm) thick, or with stone masonry not less than 12 inches (305 mm) thick.

  2113 A .11.2.2 Lining. Concrete and masonry chimneys shall be lined with an approved medium-duty refractory brick not less than 4 [1] / 2 inches (114 mm) thick laid on the 4 [1] / 2 -inch bed (114 mm) in an approved medium-duty refractory mortar. The lining shall start 2 feet (610 mm) or more below the lowest chimney connector entrance. Chimneys terminating 25 feet (7620 mm) or less above a chimney connector entrance shall be lined to the top.

  2113 A .11.2.3 Multiple passageway. Concrete and masonry chimneys containing more than one passageway shall have the liners separated by a minimum 4-inch-thick (102 mm) concrete or solid masonry wall.

  2113 A .11.2.4 Termination height. Concrete and masonry chimneys for medium-heat appliances shall extend not less than 10 feet (3048 mm) higher than any portion of any building within 25 feet (7620 mm).

  2113 A .11.2.5 Clearance. A minimum clearance of 4 inches (102 mm) shall be provided between the exterior surfaces of a concrete or masonry chimney for medium-heat appliances and combustible material.

  2113 A .11.3 Concrete and masonry chimneys for high-heat appliances. Concrete and masonry chimneys for high-heat appliances shall comply with 2113.11.3.1 through 2113.11.3.4.

  2113 A .11.3.1 Construction. Chimneys for high-heat appliances shall be constructed with double walls of solid masonry units or of concrete, each wall to be not less than 8 inches (203 mm) thick with a minimum airspace of 2 inches (51 mm) between the walls.

• CMC § 16.2 Medium relevance — show source text

  2113 A .16.2 Determination of minimum area. The minimum net cross-sectional area of the flue shall be determined in accordance with Figure 2113 A .16. A flue size providing not less than the equivalent net cross-sectional area shall be used. Crosssectional areas of clay flue linings are as provided in Tables 2113 A .16(1) and 2113 A .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 A .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 A .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 A .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 A .20.

  Exceptions:

  1. Masonry chimneys equipped with a chimney lining system listed and labeled for use in chimneys in contact with combustibles in accordance with UL 1777, and installed in accordance with the manufacturer’s instructions, are permitted to have combustible material in contact with their exterior surfaces.

  2. Where masonry chimneys are constructed as part of masonry or concrete walls, combustible materials shall not be in contact with the masonry or concrete wall less than 12 inches (305 mm) from the inside surface of the nearest flue lining.

  3. Exposed combustible trim and the edges of sheathing materials, such as wood siding, are permitted to abut the masonry chimney sidewalls, in accordance with Figure 2113 A .19, provided that such combustible trim or sheathing is not less than 12 inches (305 mm) from the inside surface of the nearest flue lining. Combustible material and trim shall not overlap the corners of the chimney by more than 1 inch (25 mm).

  2025 CALIFORNIA BUILDING CODE 21A-17

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  MASONRY

  FIGURE 2113 A .19—ILLUSTRATION OF EXCEPTION THREE CHIMNEY CLEARANCE PROVISION

  MASONRY ABUTTING

  COMBUSTIBLE SHEATHING

  12" FROM FLUE LINING

• CMC § 2113.11.1.4 Medium relevance — show source text

  2113.11.1.4 Oil-fired appliances approved for use with 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 flue lining systems complying with Section 2113.11.1 and listed chimney liners complying with UL 641 (see Section 2113.11.1.5 for marking).

  2113.11.1.5 Notice of usage. When a flue is relined with a material not complying with Section 2113.11.1, 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 is for use only with (type or category of appliance) that burns (type of fuel). Do not connect other types of appliances.”

  2113.11.2 Concrete and masonry chimneys for medium-heat appliances. Concrete and masonry chimneys for medium-heat appliances shall comply with Sections 2113.11.2.1 through 2113.11.2.5.

  2113.11.2.1 Construction. Chimneys for medium-heat appliances shall be constructed of solid masonry units or of concrete with walls not less than 8 inches (203 mm) thick, or with stone masonry not less than 12 inches (305 mm) thick.

  2113.11.2.2 Lining. Concrete and masonry chimneys shall be lined with an approved medium-duty refractory brick not less than 4 [1] / 2 inches (114 mm) thick laid on the 4 [1] / 2 -inch bed (114 mm) in an approved medium-duty refractory mortar. The lining shall start 2 feet (610 mm) or more below the lowest chimney connector entrance. Chimneys terminating 25 feet (7620 mm) or less above a chimney connector entrance shall be lined to the top.

  2113.11.2.3 Multiple passageway. Concrete and masonry chimneys containing more than one passageway shall have the liners separated by a minimum 4-inch-thick (102 mm) concrete or solid masonry wall.

  2113.11.2.4 Termination height. Concrete and masonry chimneys for medium-heat appliances shall extend not less than 10 feet (3048 mm) higher than any portion of any building within 25 feet (7620 mm).

  2113.11.2.5 Clearance. A minimum clearance of 4 inches (102 mm) shall be provided between the exterior surfaces of a concrete or masonry chimney for medium-heat appliances and combustible material.

  2113.11.3 Concrete and masonry chimneys for high-heat appliances. Concrete and masonry chimneys for high-heat appliances shall comply with 2113.11.3.1 through 2113.11.3.4.

  2025 CALIFORNIA BUILDING CODE 21-15

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  MASONRY

  2113.11.3.1 Construction. Chimneys for high-heat appliances shall be constructed with double walls of solid masonry units or of concrete, each wall to be not less than 8 inches (203 mm) thick with a minimum airspace of 2 inches (51 mm) between the walls.

• CMC § 21-17 Medium 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.

• CMC § 304.8 Medium relevance — show source text

  )
  140
  110
  76
  70
  53
  32|269
  214
  168
  124
  82
  58
  37
  91|||||||||||||| |224
  187
  MIN. CROSS-SECTIONAL AREA (SQ. IN.)
  140
  110
  76
  70
  53
  32|269
  214
  168
  124
  82
  58
  37
  91|||||||||||||| |224
  187
  MIN. CROSS-SECTIONAL AREA (SQ. IN.)
  140
  110
  76
  70
  53
  32|269
  214
  168
  124
  82
  58
  37
  91|||||||||||||| |224
  187
  MIN. CROSS-SECTIONAL AREA (SQ. IN.)
  140
  110
  76
  70
  53
  32|269
  214
  168
  124
  82
  58
  37
  91|||||||||||||| |224
  187
  MIN. CROSS-SECTIONAL AREA (SQ. IN.)
  140
  110
  76
  70
  53
  32|269
  214
  168
  124
  82
  58
  37
  91||||||||||||||

  13 14 15 16 17 18 19 20 21 22 23 24 25

  HEIGHT, MEASURED FROM FLOOR OF COMBUSTION CHAMBER TO TOP OF FLUE (FT)

  For SI: 1 foot = 304.8 mm, 1 square inch = 645.16 mm [2] .

  R1003.16 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.

  R1003.17 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.

  R1003.18 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 fire blocking in accordance with Section R1003.19.

  Exceptions:

  1. Masonry chimneys equipped with a chimney lining system listed and labeled for use in chimneys in contact with combustibles in accordance with UL 1777 and installed in accordance with the manufacturer’s instructions are permitted to have combustible material in contact with their exterior surfaces.