Chapter 10 of the International Residential Code regulates two types of fireplaces. Those that are constructed on-site using masonry and other approved materials, and those that are factory-built.
While the hearth extension of the factory-built fireplace relies on the listing and labeling of the fireplace itself, a masonry fireplace relies on the prescriptive requirements outlined in Chapter 10 of the IRC, specifically Section R1001.
Before we get into the requirements of the hearth extension, let us properly define these types of fireplaces based on the definitions in the code.
Factory-built fireplace – A listed and labeled fireplace and chimney system composed of factory-made components, and assembled in the field in accordance with manufacturer’s instructions and the conditions of the listing.
Since manufacturers can have their hearth extension requirements vary slightly, Section R1004.2 requires the hearth extension for these types of fireplaces to be in accordance with the listing of the factory-built fireplace and shall comply with UL 1618.
Masonry fireplace – A field-constructed fireplace composed of solid masonry units, bricks, stones or concrete.
While the code uses the term “masonry fireplace”, we can see based on the definition in the code that it also refers to fireplaces built using any of the noncombustible materials mentioned above.
Therefore these masonry fireplaces are subject to Section R1001 of the IRC which is what we will discuss below.
What is a Fireplace Hearth?
A fireplace assembly consists of a hearth, fire chamber, and chimney. The hearth is considered to be the floor within the fireplace and the floor projection directly in front of the fireplace opening is known as the hearth extension.
The purpose of the hearth extension is to protect wood, carpet and other combustible materials from sparks, hot embers or ashes that may fall out of the fireplace.
Fireplace Hearth Material
Both the hearth and hearth extension shall be constructed of concrete or masonry. We can see this in the code section below:
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.
Now that we know what a fireplace hearth is, how far past the fireplace opening must the hearth extend?
Let’s discuss this below.
Hearth Extensions for Fireplaces
The length and width of a hearth extension depends on the size of the fireplace opening. Section 1001.10 specifies the dimensions based on if the opening of the fireplace is less than 6 square feet or 6 square feet or larger. We can see this in the code section below:
R1001.10 Hearth extension dimensions. Hearth extensions shall extend not less than 16 inches in front of and not less than 8 inches beyond each side of the fireplace opening. Where the fireplace opening is 6 square feet or larger, the hearth extension shall extend not less than 20 inches in front of and not less than 12 inches beyond each side of the fireplace opening.
Basically the larger the fireplace opening is, the further the hearth extension must extend out. The code recognizes a 6 square foot opening to be the trigger for a larger extension.
Fireplace Opening Less than 6 Square Feet
If the fireplace opening is less than 6 square feet, the hearth extension must extend at least 16 inches in front of the opening and at least 8 inches beyond each side of the opening.
Here is a graphic to best visualize this.
Fireplace Opening 6 Square Feet or Larger
If the fireplace opening is 6 square feet or larger, the hearth extension must extend at least 20 inches in front of the opening and at least 12 inches beyond each side of the opening.
Here is a graphic to best visualize this.
Final Words
The hearth is an important feature of any fireplace to ensure it extends past the fireplace opening to a sufficient amount that will prevent burning embers from making contact with combustible surfaces . Having proper understanding of what size a fireplace hearth extension needs to be is of utmost importance to any code professional.
For more information regarding fireplace hearths and their extensions, be sure to check out Section R1001.9 of the 2018 International Residential Code.
* Reference Source – 2018 International Building Code – [Buy on Amazon]
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