Hearth Extension Size and Thickness (IRC R1001.10, R1001.9.2)
How far a masonry fireplace's hearth extension must reach and how thick it must be, checked against what was built.
Example
You enter
- Fireplace opening width (in) 36
- Fireplace opening height (in) 30
- Measured extension in front of the opening (in, blank to skip) 18
- Measured extension beyond each side (in, the smaller side; blank to skip) 12
- Measured extension thickness (in, blank to skip) 2
- Firebox opening bottom above the extension (in) 0
You get
- Opening area 7.50 sq ft (6 sq ft or more)
- Req front (in) 20
- Req side (in) 12
- Footprint width (in) 60
- Req thickness (in) 2
Checked against published worked examples from: ICC.
Details, formula, and sources
ScopeIRC 2024 R1001.10 sizes it by the opening it faces: under 6 sq ft, at least 16 in in front and 8 in past each side; 6 sq ft or more, 20 in and 12 in. A 36 in wide opening is 7.5 sq ft at 30 in tall and needs the larger hearth, 60 by 20 in at least, so an extension poured 18 in deep fails by 2 in; the same opening 22 in tall is 5.5 sq ft and needs only 16 and 8. R1001.9.2 makes the extension at least 2 in thick, or 3/8 in of brick, concrete, stone, tile or other approved noncombustible material where the bottom of the firebox opening is raised 8 in or more above it. The numbers are the same in IRC 2021. The hearth under the firebox, its support, clearance to combustibles and factory-built fireplaces are separate checks.
Formula or tableopening area = width x height / 144 (sq ft); area < 6 sq ft: front >= 16 in, each side >= 8 in; area >= 6 sq ft: front >= 20 in, each side >= 12 in; footprint = (width + 2 x side) by front; thickness >= 2 in, or >= 3/8 in where the firebox opening bottom is raised >= 8 in above the extension.
Source and editionIRC 2024 (International Residential Code). Sections R1001.10 and R1001.9.2 (the same sections and values in IRC 2021), both read at codes.iccsafe.org on 2026-10-19.
Free accessFree to read at codes.iccsafe.org.
What governsEstimate. AHJ and licensed structural engineer govern.
API field namesopening_width_in, opening_height_in, front_in, side_in, thickness_in, firebox_raise_in, opening_area_sqft, req_front_in, req_side_in, footprint_width_in, req_thickness_in
- Rectangular opening area = width x height; for an arched opening keep the true width and enter the height that gives its actual areastated assumption
- Masonry fireplace factory-built fireplaces follow their listing (R1004)IRC R1004