CRC · California Residential Code

What are the required hearth, hearth-extension and firebox dimensions for a masonry fireplace?

For a homeowner: the California Residential Code requires a solid noncombustible hearth slab at least 4 inches thick, a hearth‑extension that projects 16 inches in front and 8 inches to each side for ordinary openings (or 20 inches front and 12 inches each side for openings 6 sq ft or larger), and a masonry firebox at least 20 inches deep with the throat at least 8 inches above the opening; these rules and their exceptions are summarized in **§ R1001.1**, Table **R1001.1**, and the cited subsections.

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Last reviewed: July 6, 2026

What the code requires — 2–4 sentences

Masonry fireplaces must have a 4‑inch minimum hearth thickness, a 2‑inch minimum hearth extension thickness (with a permitted thin‑brick exception), and hearth‑extension plan dimensions that depend on the size of the fireplace opening. The firebox must have a minimum depth of 20 inches, a throat at least 8 inches above the fireplace opening and a throat opening depth of at least 4 inches. These prescriptive requirements are summarized in § R1001.1 and Table R1001.1 and are implemented in the following specific sections of the CRC (see § R1001.9, § R1001.10, § R1001.6) .

The single most important rule: provide a 4" thick hearth and the proper hearth‑extension footprint (16"×8" minimum in front/sides for small openings; 20"×12" for openings ≥ 6 sq ft) and make the firebox at least 20" deep with the throat ≥ 8" above the opening. See § R1001.9, § R1001.10, § R1001.6.

Requirements in detail

Key defined elements (first mention bolded)

  • Hearth — the noncombustible slab immediately in front of the firebox (minimum thickness requirement in § R1001.9.1) .
  • Hearth extension — the noncombustible floor protection extending in front of and to the sides of the fireplace opening (dimensions in § R1001.10) .
  • Firebox — the chamber where the fuel is burned; minimum depth, throat location and throat opening depth are in § R1001.6 (and summarized in Table R1001.1) .

Decision‑relevant dimensions (summary table)

Item Requirement (inch / mm) When it applies / note Code Reference
Hearth slab thickness 4 in (102 mm) Minimum thickness for masonry hearth slab § R1001.9.1 ; Table R1001.1
Hearth extension thickness 2 in (51 mm) (minimum) Standard minimum; thinner facing permitted if bottom of opening is raised ≥ 8 in (203 mm) (see exception) § R1001.9.2 ; Table R1001.1
Hearth extension — front projection 16 in (406 mm) or 20 in (508 mm) 16 in front for openings < 6 sq ft; 20 in front for openings ≥ 6 sq ft § R1001.10 ; Table R1001.1
Hearth extension — side projection 8 in (203 mm) or 12 in (305 mm) 8 in each side for openings < 6 sq ft; 12 in each side for openings ≥ 6 sq ft § R1001.10 ; Table R1001.1
Firebox minimum depth 20 in (508 mm) Minimum depth for concrete or masonry firebox (Rumford exception applies) § R1001.6 ; Table R1001.1 (firebox notes)
Throat location (top of opening → throat) 8 in (203 mm) minimum Throat shall be located not less than 8" above lintel/opening § R1001.7 and § R1001.6
Throat opening depth 4 in (102 mm) minimum Minimum depth of throat opening § R1001.6
Firebox wall thickness 10 in solid masonry; 8 in if lined with 2 in firebrick Minimum back and sidewall thickness (firebrick lining reduces required masonry thickness) Table R1001.1 and § R1001.6 (firebox walls)

Notes: Table R1001.1 provides a compact summary of these items; see § R1001.1 and the referenced subsections for mandatory text and exceptions .

Exceptions & special cases

  • Thin hearth‑extension facing permitted: If the bottom of the firebox opening is raised ≥ 8 in (203 mm) above the hearth-extension top, the code permits a hearth extension of 3/8‑inch thick brick, tile or other approved noncombustible facing instead of the standard 2‑inch thickness § R1001.9.2 (Exception) .
  • Large openings: The hearth‑extension footprint increases when the fireplace opening is 6 sq ft or larger — that threshold changes front projection from 16 in → 20 in and side projections from 8 in → 12 in § R1001.10 and Table R1001.1 .
  • Rumford fireplaces: A Rumford design may be shallower — minimum depth 12 in (305 mm) — but must meet the special throat and proportional area requirements stated in § R1001.6 (exception text) .

Common mistakes

  • Applying hearth‑extension dimensions to the hearth slab thickness: the slab thickness (4 in) is separate from the extension thickness (2 in) — both are required and specified in different subsections § R1001.9.1 and § R1001.9.2 .
  • Using the 16 in / 8 in rule regardless of opening size: the code raises the required projections to 20 in / 12 in once the fireplace opening is ≥ 6 sq ft — check opening area before sizing the extension § R1001.10 and Table R1001.1 .
  • Forgetting throat height: the throat must be at least 8 in above the fireplace opening (damper location) — omitting this impairs draft and violates § R1001.7 / § R1001.6 .
  • Substituting combustible support under hearth: hearth and hearth extensions must be supported by noncombustible materials and not leave combustible material against the underside — see § R1001.9 .

Worked example — concrete scenario

Scenario: You are designing a masonry fireplace whose finished fireplace opening is 30 inches wide and 24 inches high.

  1. Opening area = 30 in × 24 in = 720 in² = 5.0 ft² (720 / 144 = 5.0). Because the opening is less than 6 sq ft, use the smaller hearth‑extension dimensions. (Threshold: 6 sq ft) .
  2. Hearth slab thickness: provide 4 in (102 mm) thickness (minimum) per § R1001.9.1 .
  3. Hearth extension thickness: provide 2 in (51 mm) minimum (unless you raise the bottom of the opening ≥ 8 in) per § R1001.9.2 .
  4. Hearth extension plan size: extend 16 in (406 mm) in front of the opening and 8 in (203 mm) beyond each side of the opening (because opening < 6 sq ft) — total front projection from the face of the firebox = 16 in; total side clearance = 8 in each side — § R1001.10 .
    • So with a 30 in opening width you would provide side protection extending to 30 + 8 + 8 = 46 in total protected width.
    • Depth in front of the opening = 16 in measured from the plane of the opening.
  5. Firebox depth: design the firebox with a minimum depth of 20 in (508 mm) measured from the inside front of the firebox back to the rear — per § R1001.6 .
  6. Throat: place the throat/damper assembly not less than 8 in (203 mm) above the top of the fireplace opening and provide a throat opening depth ≥ 4 in (102 mm) per § R1001.7 and § R1001.6 .

Related provisions

  • § R1001.1 — General (scope; see Table R1001.1 summary) .
  • § R1001.6 — Firebox dimensions (minimum depth, throat and throat opening) .
  • § R1001.7 — Lintel and throat (throat location and damper requirements) .
  • § R1001.8 — Smoke chamber dimensions and construction (affects throat/smoke‑handling) .
  • § R1001.11 — Fireplace clearance to combustible framing and trim (airspace requirements) .
  • Table R1001.1 — Compact summary of hearth, hearth‑extension, firebox and related masonry requirements (use as quick reference) .

Code references

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

  • CRC § 8.1 High relevance — show source text

    R1001.8.1 Smoke chamber dimensions. The inside height of the smoke chamber from the fireplace throat to the beginning of the flue shall not be greater than the inside width of the fireplace opening. The inside surface of the smoke chamber shall not be inclined more than 45 degrees (0.79 rad) from vertical where prefabricated smoke chamber linings are used or where the smoke chamber walls are rolled or sloped rather than corbeled. Where the inside surface of the smoke chamber is formed by corbeled masonry, the walls shall not be corbeled more than 30 degrees (0.52 rad) from vertical.

    R1001.9 Hearth and hearth extension. Masonry fireplace hearths and hearth extensions shall be constructed of concrete or masonry, supported by noncombustible materials, and reinforced to carry their own weight and all imposed loads. Combustible material shall not remain against the underside of hearths and hearth extensions after construction.

    R1001.9.1 Hearth thickness. The minimum thickness of fireplace hearths shall be 4 inches (102 mm).

    R1001.9.2 Hearth extension thickness. The minimum thickness of hearth extensions shall be 2 inches (51 mm).

    Exception: Where the bottom of the firebox opening is raised not less than 8 inches (203 mm) above the top of the hearth extension, a hearth extension of not less than [3] / 8 -inch-thick (10 mm) brick, concrete, stone, tile or other approved noncombustible material is permitted.

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    CHIMNEYS AND FIREPLACES

    R1001.10 Hearth extension dimensions. Hearth extensions shall extend not less than 16 inches (406 mm) in front of and not less than 8 inches (203 mm) beyond each side of the fireplace opening. Where the fireplace opening is 6 square feet (0.6 m [2] ) or larger, the hearth extension shall extend not less than 20 inches (508 mm) in front of and not less than 12 inches (305 mm) beyond each side of the fireplace opening.

    R1001.11 Fireplace clearance. Wood beams, joists, studs and other combustible material shall have a clearance of not less than 2 inches (51 mm) from the front faces and sides of masonry fireplaces and not less than 4 inches (102 mm) from the back faces of masonry fireplaces. The airspace shall not be filled, except for noncombustible material or to provide fireblocking in accordance with Section R1001.12.

    Exceptions:

    1. Masonry fireplaces listed and labeled for use in contact with combustibles in accordance with UL 127 and installed in accordance with the manufacturer’s instructions are permitted to have combustible material in contact with their exterior surfaces.

    2. Where masonry fireplaces are part of masonry or concrete walls, combustible materials shall not be in contact with the masonry or concrete walls less than 12 inches (306 mm) from the inside surface of the nearest firebox lining.

    3. Exposed combustible trim and the edges of sheathing materials such as wood siding, flooring and gypsum board shall be permitted to abut the masonry fireplace sidewalls and hearth extension in accordance with Figure R1001.11, provided that such combustible trim or sheathing is not less than 8 inches (203 mm) from the inside surface of the nearest firebox

  • CRC § 15.9 High relevance — show source text

    2111 A .9 Smoke chamber walls. Smoke chamber walls shall be constructed of solid masonry units, hollow masonry units grouted solid, stone or concrete. The total minimum thickness of front, back and sidewalls shall be 8 inches (203 mm) of solid masonry. The inside surface shall be parged smooth with refractory mortar conforming to ASTM C199. Where a lining of firebrick not less than 2 inches (51 mm) thick, or a lining of vitrified clay not less than [5] / 8 inch (15.9 mm) thick, is provided, the total minimum thickness of front, back and sidewalls shall be 6 inches (152 mm) of solid masonry, including the lining. Firebrick shall conform to ASTM C1261 and shall be laid with refractory mortar conforming to ASTM C199. Vitrified clay linings shall conform to ASTM C315.

    2111 A .9.1 Smoke chamber dimensions. The inside height of the smoke chamber from the fireplace throat to the beginning of the flue shall be not greater than the inside width of the fireplace opening. The inside surface of the smoke chamber shall not be inclined more than 45 degrees (0.76 rad) from vertical where prefabricated smoke chamber linings are used or where the smoke

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    MASONRY

    chamber walls are rolled or sloped rather than corbeled. Where the inside surface of the smoke chamber is formed by corbeled masonry, the walls shall not be corbeled more than 30 degrees (0.52 rad) from vertical.

    2111 A .10 Hearth and hearth extension. Masonry fireplace hearths and hearth extensions shall be constructed of concrete or masonry, supported by noncombustible materials, and reinforced to carry their own weight and all imposed loads. Combustible material shall not remain against the underside of hearths or hearth extensions after construction.

    2111 A .10.1 Hearth thickness. The minimum thickness of fireplace hearths shall be 4 inches (102 mm).

    2111 A .10.2 Hearth extension thickness. The minimum thickness of hearth extensions shall be 2 inches (51 mm).

    Exception: Where the bottom of the firebox opening is raised not less than 8 inches (203 mm) above the top of the hearth extension, a hearth extension of not less than [3] / 8 -inch-thick (9.5 mm) brick, concrete, stone, tile or other approved noncombustible material is permitted.

    2111 A .11 Hearth extension dimensions. Hearth extensions shall extend not less than 16 inches (406 mm) in front of, and not less than 8 inches (203 mm) beyond, each side of the fireplace opening. Where the fireplace opening is 6 square feet (0.557 m [2] ) or larger, the hearth extension shall extend not less than 20 inches (508 mm) in front of, and not less than 12 inches (305 mm) beyond, each side of the fireplace opening.

  • CRC § 2111.10.1 High relevance — show source text

    2111.10.1 Hearth thickness. The minimum thickness of fireplace hearths shall be 4 inches (102 mm).

    2111.10.2 Hearth extension thickness. The minimum thickness of hearth extensions shall be 2 inches (51 mm).

    Exception: Where the bottom of the firebox opening is raised not less than 8 inches (203 mm) above the top of the hearth extension, a hearth extension of not less than [3] / 8 -inch-thick (9.5 mm) brick, concrete, stone, tile or other approved noncombustible material is permitted.

    2111.11 Hearth extension dimensions. Hearth extensions shall extend not less than 16 inches (406 mm) in front of, and not less than 8 inches (203 mm) beyond, each side of the fireplace opening. Where the fireplace opening is 6 square feet (0.557 m [2] ) or larger, the hearth extension shall extend not less than 20 inches (508 mm) in front of, and not less than 12 inches (305 mm) beyond, each side of the fireplace opening.

    2111.12 Fireplace clearance. Any portion of a masonry fireplace located in the interior of a building or within the exterior wall of a building shall have a clearance to combustibles of not less than 2 inches (51 mm) from the front faces and sides of masonry fireplaces and not less than 4 inches (102 mm) from the back faces of masonry fireplaces. The airspace shall not be filled, except to provide fireblocking in accordance with Section 2111.13.

    Exceptions:

    1. Masonry fireplaces listed and labeled for use in contact with combustibles in accordance with UL 127 and installed in accordance with the manufacturer’s instructions are permitted to have combustible material in contact with their exterior surfaces.

    2. Where masonry fireplaces are constructed as part of masonry or concrete walls, combustible materials shall not be in contact with the masonry or concrete walls less than 12 inches (306 mm) from the inside surface of the nearest firebox lining.

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    1. Exposed combustible trim and the edges of sheathing materials, such as wood siding, flooring and drywall, are permitted to abut the masonry fireplace sidewalls and hearth extension, in accordance with Figure 2111.12, provided that such combustible trim or sheathing is not less than 12 inches (306 mm) from the inside surface of the nearest firebox lining.
    2. Exposed combustible mantels or trim is permitted to be placed directly on the masonry fireplace front surrounding the fireplace opening, provided that such combustible materials shall not be placed within 6 inches (153 mm) of a fireplace opening. Combustible material directly above and within 12 inches (305 mm) of the fireplace opening shall not project more than [1] / 8 inch (3.2 mm) for each 1-inch (25 mm) distance from such opening. Combustible materials located along the sides of the fireplace opening that project more than 1 [1] / 2 inches (38 mm) from the face of the fireplace shall have an additional clearance equal to the projection.

    FIGURE 2111.12—ILLUSTRATION OF EXCEPTION TO FIREPLACE CLEARANCE PROVISION

    EDGE ABUTTING MASONRY

    2 ″ CLEARANCE (AIRSPACE) TO COMBUSTIBLE FRAMING

  • 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 § 10.1 High relevance — show source text

    2111 A .10 Hearth and hearth extension. Masonry fireplace hearths and hearth extensions shall be constructed of concrete or masonry, supported by noncombustible materials, and reinforced to carry their own weight and all imposed loads. Combustible material shall not remain against the underside of hearths or hearth extensions after construction.

    2111 A .10.1 Hearth thickness. The minimum thickness of fireplace hearths shall be 4 inches (102 mm).

    2111 A .10.2 Hearth extension thickness. The minimum thickness of hearth extensions shall be 2 inches (51 mm).

    Exception: Where the bottom of the firebox opening is raised not less than 8 inches (203 mm) above the top of the hearth extension, a hearth extension of not less than [3] / 8 -inch-thick (9.5 mm) brick, concrete, stone, tile or other approved noncombustible material is permitted.

    2111 A .11 Hearth extension dimensions. Hearth extensions shall extend not less than 16 inches (406 mm) in front of, and not less than 8 inches (203 mm) beyond, each side of the fireplace opening. Where the fireplace opening is 6 square feet (0.557 m [2] ) or larger, the hearth extension shall extend not less than 20 inches (508 mm) in front of, and not less than 12 inches (305 mm) beyond, each side of the fireplace opening.

    2111 A .12 Fireplace clearance. Any portion of a masonry fireplace located in the interior of a building or within the exterior wall of a building shall have a clearance to combustibles of not less than 2 inches (51 mm) from the front faces and sides of masonry fireplaces and not less than 4 inches (102 mm) from the back faces of masonry fireplaces. The airspace shall not be filled, except to provide fireblocking in accordance with Section 2111 A .13.

    Exceptions:

    1. Masonry fireplaces listed and labeled for use in contact with combustibles in accordance with UL 127 and installed in accordance with the manufacturer’s instructions are permitted to have combustible material in contact with their exterior surfaces.

    2. Where masonry fireplaces are constructed as part of masonry or concrete walls, combustible materials shall not be in contact with the masonry or concrete walls less than 12 inches (306 mm) from the inside surface of the nearest firebox lining.

    3. Exposed combustible trim and the edges of sheathing materials, such as wood siding, flooring and drywall, are permitted to abut the masonry fireplace sidewalls and hearth extension, in accordance with Figure 2111 A .12, provided that such combustible trim or sheathing is not less than 12 inches (306 mm) from the inside surface of the nearest firebox lining.

    4. Exposed combustible mantels or trim is permitted to be placed directly on the masonry fireplace front surrounding the fireplace opening, provided that such combustible materials shall not be placed within 6 inches (153 mm) of a fireplace opening. Combustible material directly above and within 12 inches (305 mm) of the fireplace opening shall not project more than [1] / 8 inch (3.2 mm) for each 1-inch (25 mm) distance from such opening.

  • CRC § 1001.11 High relevance — show source text

    3||||X|||||||||||||||||||| |R1001.4||||X|||||||||||||||||||| |Table R1001.1, Footnote (b)||||X|||||||||||||||||||| |R1001.11|||X||||||||||||||||||||| |Figure 1001.11|||X||||||||||||||||||||| |R1001.12|||X||||||||||||||||||||| |R1003.3||||X|||||||||||||||||||| |R1003.4||||X|||||||||||||||||||| |R1003.9.1|||X||||||||||||||||||||| |R1003.9.2|||X||||||||||||||||||||| |R1003.11.3||||X|||||||||||||||||||| |R1003.14||||X|||||||||||||||||||| |R1003.18|||X||||||||||||||||||||| |Figure R1003.18|||X||||||||||||||||||||| |R1003.19|||X||||||||||||||||||||| |R1004.1.1||||X|||||||||||||||||||| |R1004.4||||†||||||||||||||||||||

    The state agency does not adopt sections identified with the following symbol: †

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    10-2 2025 CALIFORNIA RESIDENTIAL CODE

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    10 CHIMNEYS AND FIREPLACES

    User notes:

    About this chapter: Chapter 10 contains requirements for the construction, seismic reinforcing and anchorage of masonry chimneys and fireplaces; and establishes standards for the use and installation of factory-built chimneys, fireplaces and masonry heaters. Chimneys and fireplaces constructed of masonry rely on prescriptive requirements for the details of their construction; factory-built versions rely on the listing and labeling method of approval.

    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.
  • CRC § 2111.9 High relevance — show source text

    2111.9 Smoke chamber walls. Smoke chamber walls shall be constructed of solid masonry units, hollow masonry units grouted solid, stone or concrete. The total minimum thickness of front, back and sidewalls shall be 8 inches (203 mm) of solid masonry. The inside surface shall be parged smooth with refractory mortar conforming to ASTM C199. Where a lining of firebrick not less than 2 inches (51 mm) thick, or a lining of vitrified clay not less than [5] / 8 inch (15.9 mm) thick, is provided, the total minimum thickness of front, back and sidewalls shall be 6 inches (152 mm) of solid masonry, including the lining. Firebrick shall conform to ASTM C1261 and shall be laid with refractory mortar conforming to ASTM C199. Vitrified clay linings shall conform to ASTM C315.

    2111.9.1 Smoke chamber dimensions. The inside height of the smoke chamber from the fireplace throat to the beginning of the flue shall be not greater than the inside width of the fireplace opening. The inside surface of the smoke chamber shall not be inclined more than 45 degrees (0.76 rad) from vertical where prefabricated smoke chamber linings are used or where the smoke chamber walls are rolled or sloped rather than corbeled. Where the inside surface of the smoke chamber is formed by corbeled masonry, the walls shall not be corbeled more than 30 degrees (0.52 rad) from vertical.

    2111.10 Hearth and hearth extension. Masonry fireplace hearths and hearth extensions shall be constructed of concrete or masonry, supported by noncombustible materials, and reinforced to carry their own weight and all imposed loads. Combustible material shall not remain against the underside of hearths or hearth extensions after construction.

    2111.10.1 Hearth thickness. The minimum thickness of fireplace hearths shall be 4 inches (102 mm).

    2111.10.2 Hearth extension thickness. The minimum thickness of hearth extensions shall be 2 inches (51 mm).

    Exception: Where the bottom of the firebox opening is raised not less than 8 inches (203 mm) above the top of the hearth extension, a hearth extension of not less than [3] / 8 -inch-thick (9.5 mm) brick, concrete, stone, tile or other approved noncombustible material is permitted.

    2111.11 Hearth extension dimensions. Hearth extensions shall extend not less than 16 inches (406 mm) in front of, and not less than 8 inches (203 mm) beyond, each side of the fireplace opening. Where the fireplace opening is 6 square feet (0.557 m [2] ) or larger, the hearth extension shall extend not less than 20 inches (508 mm) in front of, and not less than 12 inches (305 mm) beyond, each side of the fireplace opening.

    2111.12 Fireplace clearance. Any portion of a masonry fireplace located in the interior of a building or within the exterior wall of a building shall have a clearance to combustibles of not less than 2 inches (51 mm) from the front faces and sides of masonry fireplaces and not less than 4 inches (102 mm) from the back faces of masonry fireplaces. The airspace shall not be filled, except to provide fireblocking in accordance with Section 2111.13.

  • CRC § 25.4 High relevance — show source text

    For SI: 1 inch = 25.4 mm.

    masonry, that is designed to absorb and store heat from a solid-fuel fire built in the firebox by routing the exhaust gases through

    e fireplace opening is 6 square feet (0.6 m2) or larger, such combustible tri
    masonry fireplace sidewalls and hearth extension provided that such co
    inches (305 mm) from the inside surface of the nearest firebox lining.
    ustible mantels or trim is permitted to be placed directly on the masonry f
    ng providing such combustible materials are not placed within 6 inches (1
    aterial within 12 inches (306 mm) of the fireplace opening shall not projec
    mm) distance from such an opening.
    FIGURE R1001.11—CLEARANCE FROM COMBUSTIBLES
    MASONRY
    COMBUSTIBLE SHEATHING
    EDGE ABUTTING MASONRY
    12 IN. FROM FIREBOX
    FRAME WALL
    2 IN. CLEARANCE (AIRSPA
    TO COMBUSTIBLE FRAMIN
    WOOD MANTEL
    cking. Fireplace fireblocking shall comply with the provisions of Section R
    ories. Listed and labeled fireplace accessories shall be installed in accorda
    r’s instructions. Fireplace accessories shall comply with UL 907.
    SECTION R1002—MASONRY HEATERS
    onry heater is a heating appliance constructed of concrete or solid maso    		 Col2 Col3 Col4 Col5 Col6 Col7 Col8 Col9 Col10 Col11 Col12 Col13 Col14 Col15 Col16 Col17 Col18 Col19 Col20 Col21 Col22 Col23 Col24 Col25 Col26
    cking. Firepl
    ories. Listed
    r’s instructio
    onry heater

    WOO
    cking. Firepl
    ories. Listed
    r’s instructio
    onry heater

    WOO
    cking. Firepl
    **ories.
  • CRC § 7.1 High relevance — show source text

    R1001.7 Lintel and throat. Masonry over a fireplace opening shall be supported by a lintel of noncombustible material. The minimum required bearing length on each end of the fireplace opening shall be 4 inches (102 mm). The fireplace throat or damper shall be located not less than 8 inches (203 mm) above the lintel.

    R1001.7.1 Damper. Masonry fireplaces shall be equipped with a ferrous metal damper located not less than 8 inches (203 mm) above the top of the fireplace opening. Dampers shall be installed in the fireplace or the chimney venting the fireplace, and shall be operable from the room containing the fireplace.

    R1001.8 Smoke chamber. Smoke chamber walls shall be constructed of solid masonry units, hollow masonry units grouted solid, stone or concrete. The total minimum thickness of front, back and side walls shall be 8 inches (203 mm) of solid masonry. The inside surface shall be parged smooth with refractory mortar conforming to ASTM C199. Where a lining of firebrick not less than 2 inches (51 mm) thick, or a lining of vitrified clay not less than [5] / 8 inch (16 mm) thick, is provided, the total minimum thickness of front, back and side walls shall be 6 inches (152 mm) of solid masonry, including the lining. Firebrick shall conform to ASTM C1261 and shall be laid with medium-duty refractory mortar conforming to ASTM C199. Vitrified clay linings shall conform to ASTM C315.

    R1001.8.1 Smoke chamber dimensions. The inside height of the smoke chamber from the fireplace throat to the beginning of the flue shall not be greater than the inside width of the fireplace opening. The inside surface of the smoke chamber shall not be inclined more than 45 degrees (0.79 rad) from vertical where prefabricated smoke chamber linings are used or where the smoke chamber walls are rolled or sloped rather than corbeled. Where the inside surface of the smoke chamber is formed by corbeled masonry, the walls shall not be corbeled more than 30 degrees (0.52 rad) from vertical.

    R1001.9 Hearth and hearth extension. Masonry fireplace hearths and hearth extensions shall be constructed of concrete or masonry, supported by noncombustible materials, and reinforced to carry their own weight and all imposed loads. Combustible material shall not remain against the underside of hearths and hearth extensions after construction.

    R1001.9.1 Hearth thickness. The minimum thickness of fireplace hearths shall be 4 inches (102 mm).

    R1001.9.2 Hearth extension thickness. The minimum thickness of hearth extensions shall be 2 inches (51 mm).

    Exception: Where the bottom of the firebox opening is raised not less than 8 inches (203 mm) above the top of the hearth extension, a hearth extension of not less than [3] / 8 -inch-thick (10 mm) brick, concrete, stone, tile or other approved noncombustible material is permitted.

    2025 CALIFORNIA RESIDENTIAL CODE 10-5

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

    CHIMNEYS AND FIREPLACES

  • CRC § 0.6 High relevance — show source text

    R1001.10 Hearth extension dimensions. Hearth extensions shall extend not less than 16 inches (406 mm) in front of and not less than 8 inches (203 mm) beyond each side of the fireplace opening. Where the fireplace opening is 6 square feet (0.6 m [2] ) or larger, the hearth extension shall extend not less than 20 inches (508 mm) in front of and not less than 12 inches (305 mm) beyond each side of the fireplace opening.

    R1001.11 Fireplace clearance. Wood beams, joists, studs and other combustible material shall have a clearance of not less than 2 inches (51 mm) from the front faces and sides of masonry fireplaces and not less than 4 inches (102 mm) from the back faces of masonry fireplaces. The airspace shall not be filled, except for noncombustible material or to provide fireblocking in accordance with Section R1001.12.

    Exceptions:

    1. Masonry fireplaces listed and labeled for use in contact with combustibles in accordance with UL 127 and installed in accordance with the manufacturer’s instructions are permitted to have combustible material in contact with their exterior surfaces.

    2. Where masonry fireplaces are part of masonry or concrete walls, combustible materials shall not be in contact with the masonry or concrete walls less than 12 inches (306 mm) from the inside surface of the nearest firebox lining.

    3. Exposed combustible trim and the edges of sheathing materials such as wood siding, flooring and gypsum board shall be permitted to abut the masonry fireplace sidewalls and hearth extension in accordance with Figure R1001.11, provided that such combustible trim or sheathing is not less than 8 inches (203 mm) from the inside surface of the nearest firebox

    For SI: 1 inch = 25.4 mm.

    masonry, that is designed to absorb and store heat from a solid-fuel fire built in the firebox by routing the exhaust gases through

    |e fireplace opening is 6 square feet (0.6 m2) or larger, such combustible tri
    masonry fireplace sidewalls and hearth extension provided that such co
    inches (305 mm) from the inside surface of the nearest firebox lining.
    ustible mantels or trim is permitted to be placed directly on the masonry f
    ng providing such combustible materials are not placed within 6 inches (1
    aterial within 12 inches (306 mm) of the fireplace opening shall not projec
    mm) distance from such an opening.
    FIGURE R1001.11—CLEARANCE FROM COMBUSTIBLES
    MASONRY
    COMBUSTIBLE SHEATHING
    EDGE ABUTTING MASONRY
    12 IN. FROM FIREBOX
    FRAME WALL
    2 IN. CLEARANCE (AIRSPA
    TO COMBUSTIBLE FRAMIN
    WOOD MANTEL
    cking. Fireplace fireblocking shall comply with the provisions of Section R
    ories. Listed and labeled fireplace accessories shall be installed in accorda
    r’s instructions.

  • CRC § 6.4 Medium relevance — show source text

    2111 A .6 Firebox walls. Masonry fireboxes shall be constructed of solid masonry units, hollow masonry units grouted solid, stone or concrete. Where a lining of firebrick not less than 2 inches (51 mm) in thickness or other approved lining is provided, the minimum thickness of back and sidewalls shall each be 8 inches (203 mm) of solid masonry, including the lining. The width of joints between firebricks shall be not greater than [1] / 4 inch (6.4 mm). Where a lining is not provided, the total minimum thickness of back and sidewalls shall be 10 inches (254 mm) of solid masonry. Firebrick shall conform to ASTM C27 or ASTM C1261 and shall be laid with medium-duty refractory mortar conforming to ASTM C199.

    2111 A .6.1 Steel fireplace units. Steel fireplace units are permitted to be installed with solid masonry to form a masonry fireplace provided that they are installed according to either the requirements of their listing or the requirements of this section. Steel fireplace units incorporating a steel firebox lining shall be constructed with steel not less than [1] / 4 inch (6.4 mm) in thickness, and an aircirculating chamber that is ducted to the interior of the building. The firebox lining shall be encased with solid masonry to provide a total thickness at the back and sides of not less than 8 inches (203 mm), of which not less than 4 inches (102 mm) shall be of solid masonry or concrete. Circulating air ducts employed with steel fireplace units shall be constructed of metal or masonry.

    2111 A .7 Firebox dimensions. The firebox of a concrete or masonry fireplace shall have a minimum depth of 20 inches (508 mm). The throat shall be not less than 8 inches (203 mm) above the fireplace opening. The throat opening shall be not less than 4 inches (102 mm) in depth. The cross-sectional area of the passageway above the firebox, including the throat, damper and smoke chamber, shall be not less than the cross-sectional area of the flue.

    Exception: Rumford fireplaces shall be permitted provided that the depth of the fireplace is not less than 12 inches (305 mm) and not less than one-third of the width of the fireplace opening, and the throat is not less than 12 inches (305 mm) above the lintel, and not less than [1] / 20 the cross-sectional area of the fireplace opening.

    2111 A .8 Lintel and throat. Masonry over a fireplace opening shall be supported by a lintel of noncombustible material. The minimum required bearing length on each end of the fireplace opening shall be 4 inches (102 mm). The fireplace throat or damper shall be located not less than 8 inches (203 mm) above the top of the fireplace opening.

    2111 A .8.1 Damper. Masonry fireplaces shall be equipped with a ferrous metal damper located not less than 8 inches (203 mm) above the top of the fireplace opening. Dampers shall be installed in the fireplace or at the top of the flue venting the fireplace, and shall be operable from the room containing the fireplace. Damper controls shall be permitted to be located in the fireplace.

  • CRC § 2111.7 Medium relevance — show source text

    The firebox lining shall be encased with solid masonry to provide a total thickness at the back and sides of not less than 8 inches (203 mm), of which not less than 4 inches (102 mm) shall be of solid masonry or concrete. Circulating air ducts employed with steel fireplace units shall be constructed of metal or masonry.

    2111.7 Firebox dimensions. The firebox of a concrete or masonry fireplace shall have a minimum depth of 20 inches (508 mm). The throat shall be not less than 8 inches (203 mm) above the fireplace opening. The throat opening shall be not less than 4 inches (102 mm) in depth. The cross-sectional area of the passageway above the firebox, including the throat, damper and smoke chamber, shall be not less than the cross-sectional area of the flue.

    Exception: Rumford fireplaces shall be permitted provided that the depth of the fireplace is not less than 12 inches (305 mm) and not less than one-third of the width of the fireplace opening, and the throat is not less than 12 inches (305 mm) above the lintel, and not less than [1] / 20 the cross-sectional area of the fireplace opening.

    2111.8 Lintel and throat. Masonry over a fireplace opening shall be supported by a lintel of noncombustible material. The minimum required bearing length on each end of the fireplace opening shall be 4 inches (102 mm). The fireplace throat or damper shall be located not less than 8 inches (203 mm) above the top of the fireplace opening.

    2111.8.1 Damper. Masonry fireplaces shall be equipped with a ferrous metal damper located not less than 8 inches (203 mm) above the top of the fireplace opening. Dampers shall be installed in the fireplace or at the top of the flue venting the fireplace, and shall be operable from the room containing the fireplace. Damper controls shall be permitted to be located in the fireplace.

    2111.9 Smoke chamber walls. Smoke chamber walls shall be constructed of solid masonry units, hollow masonry units grouted solid, stone or concrete. The total minimum thickness of front, back and sidewalls shall be 8 inches (203 mm) of solid masonry. The inside surface shall be parged smooth with refractory mortar conforming to ASTM C199. Where a lining of firebrick not less than 2 inches (51 mm) thick, or a lining of vitrified clay not less than [5] / 8 inch (15.9 mm) thick, is provided, the total minimum thickness of front, back and sidewalls shall be 6 inches (152 mm) of solid masonry, including the lining. Firebrick shall conform to ASTM C1261 and shall be laid with refractory mortar conforming to ASTM C199. Vitrified clay linings shall conform to ASTM C315.

    2111.9.1 Smoke chamber dimensions. The inside height of the smoke chamber from the fireplace throat to the beginning of the flue shall be not greater than the inside width of the fireplace opening. The inside surface of the smoke chamber shall not be inclined more than 45 degrees (0.76 rad) from vertical where prefabricated smoke chamber linings are used or where the smoke chamber walls are rolled or sloped rather than corbeled. Where the inside surface of the smoke chamber is formed by corbeled masonry, the walls shall not be corbeled more than 30 degrees (0.52 rad) from vertical.

Frequently asked questions

How do I tell whether to use the 16/8 or 20/12 hearth‑extension rule?

Calculate the fireplace opening area. If the opening area is 6 sq ft (≈0.557 m²) or larger, use 20 in front / 12 in sides; if smaller, use 16 in front / 8 in sides — see § R1001.10 and Table R1001.1 .

Can I use a thin tile or brick hearth extension instead of 2 inches thick?

Only if the bottom of the firebox opening is raised at least 8 in (203 mm) above the hearth extension top; then a 3/8‑inch thick approved facing is permitted per the exception to § R1001.9.2 .

What is the minimum firebox depth for a Rumford fireplace?

A permitted Rumford fireplace may be not less than 12 in (305 mm) deep (instead of 20 in) but must meet special throat height and area proportion requirements in the exception to § R1001.6 .

Where do I find a quick summary of these masonry requirements?

Table R1001.1 in the CRC summarizes hearth slab thickness, hearth‑extension dimensions, firebox wall thickness and other key masonry fireplace requirements — see § R1001.1 and Table R1001.1 .

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Masonry Fireplaces

California Residential Code