Continuing the discussion from Student Discussion: The 2021 International Residential Code® (IRC®) Exam Study Course:
Continuing the discussion from Student Discussion: The 2021 International Residential Code® (IRC®) Exam Study Course: What construction materials are permitted for hearths and hearth extensions? What is the minimum thickness of a hearth? A hearth extension? What are the minimum dimensions of a hearth extension?
Per IRC 2021 Section R1001.9, hearths and hearth extensions must be constructed of masonry or other approved noncombustible material, such as concrete, brick, tile, or stone. The hearth slab itself must be at least 4 inches thick, while the hearth extension only needs to be 2 inches thick, unless the hearth extension is less than 8 inches above the floor, in which case it must be at least 4 inches thick.
For dimensions, hearth extensions must extend at least 16 inches from the front of the fireplace opening and 8 inches beyond each side of the opening; however, if the fireplace opening is 6 square feet or larger, those minimums increase to 20 inches in front and 12 inches on each side (R1001.10).