Fireplace Hearth Requirements

Combustible Materials

• According to code R1001.11, all combustible materials such as wood joists, beams and studs cannot have a clearance less than 2 inches from the sides and front faces of a masonry fireplace and not less than 4 inches from the back faces of the fireplace. You should not fill the air space, unless to provide fire-blocking in compliance with Section R1001.12. (Exemptions: if a masonry fireplace is integrated into masonry or concrete walls, the walls cannot touch combustible materials less than 12 inches from the inside surface of the closest hearth lining.) The hearth's side walls and extension may abut exposed combustible trim edges and sheathing materials, such as wood siding, drywall and flooring, as long as they are a minimum of 12 inches from the inside surface of the hearth or firebox lining. Exposed combustible materials must be located more than 6 inches from the hearth opening and can be located directly on the masonry surrounding the hearth opening. Combustible materials cannot project more than 1/8 inch for each 1 inch distance from the hearth opening if they are located within 12 inches from the opening.

Material Composition

• As stated in code R1101.9, the hearth and hearth extension must be constructed of concrete or masonry, stabilized to carry its own weight and all imposed loads, and held together with noncombustible materials. All combustible materials must be removed from the underside of the hearth and hearth extension after it is constructed.

Thickness

• The hearth thickness must be a minimum of 4 inches according to code R1001.9.1. Code R1001.9.2 requires the hearth extension to be a minimum of 2 inches thick.