Carpet at fireplace (C) Daniel FriedmanFireplace Hearth Size Specifications
Depth & width code for fireplace hearth & hearth support

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Fireplace hearth size, specifications, & support:

This article series provides information about masonry fireplaces, including inspection for damage/hazards (cracks and gaps that appear at masonry fireplaces due to chimney or fireplace settlement or movement), fireplace chimney sizing requirements, draft problems, chimney safety, creosote problems, inserts, and other topics.

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Fireplace Hearth Size Requirements: Hearth Dimensions

Fireplace burned floor (C) Daniel FriedmanOur photo (left) shows a burned wooden floor in front of a fireplace hearth.

Hearth dimensions: A fireplace hearth should extend at least 16" (M.I.A.) past the front edge of the fireplace and at least 8" beyond each side of the fireplace opening.

Where the fireplace opening is 6 sq. ft. or bigger the front extension needs to be increased to at least 20" and the side extensions to at least 12" beyond the fireplace front.

The hearth for a masonry fireplace needs to be made of a brick, concrete, stone, or other (approved, listed) non-combustible material. The hearth slab needs to be at least 4" in thickness, it has to be supported by noncombustible materials or able to carry its own weight.

The "cribbing" or wood forms used to support a poured concrete hearth should be removed after construction is completed. We often find this wood material left in place - where sparks falling through a crack or gap can start a fire.

Reader Question: hearth extension is 16" inches minimum not 18" inches

2016/07/13 Manku said:
These days hearth extension is 16" inches minimum not 18" inches as mentioned in the sketch, am i right ?


Gas log fireplace with narrow hearth (C) Daniel Friedman

Above: a masonry fireplace with no hearth extension in a home in San Miguel de Allende, Guanajuato, Mexico. This fireplace originally burned wood but has been converted to gas.

Thanks Manku. You're right - mostly. The required specifications for a masonry fireplace, including hearth dimensions depend on both what building codes apply (where you live, country, state or province) and the fireplace overall opening size as well as other features. The 18" hearth requirement to which you refer was a design recommendation in a fireplace sketch, not a code citation - apologies. The U.S. hearth depth dimensions are 16" or 20" as we explain in more detail here.

Please note that the hearth extension to front of a fireplace, given as 16" or 20" is a minimum dimension not a maximum. More is safer.

In the United States and referring to the ICC, a widely-used model building code, the code specifies a 16" hearth extension to front of the fireplace for fireplaces <6 sq.ft. of opening size, and 20" hearth extension to front of fireplace for fireplaces = or larger than 6 sq. ft.


R1001.10 hearth extension dimensions. hearth extensions shall extend at least 16 inches (406 mm)in front of and at least 8 inches (203 mm) beyond each side of the fireplace opening. where the fireplace opening is 6 square feet (0.6 m2) or larger, the hearth extension shall extend at least 20 inches (508 mm) in front of and at least 12 inches (305 mm) beyond each side of the fireplace opening. - ICC, Chapter 10, Chimneys and Fireplaces, Section R1001 Masonry Fireplaces, excerpted from the 2006 Virginia Residential Code.

Here's another quote from a typical residential building code:

There shall be a minimum distance of 36 inches from the back of the firebox to the end of the hearth extension. hearth extension shall extend at least 16 inches in front of, and 8 inches beyond each side of the fireplace opening. when the fireplace opening is 6 sq. ft. or larger, the hearth extension shall extend at least 20 inches in front of, and 12 inches beyond, each side.  - "TB 21, residential building code masonry, chimneys and fireplaces - areas frequently misunderstood." Allegheny county division of permits and land development services residential information sheet # 21 revised january 14, 2003 )

Remains of a fireplace built into the stone walls of Goodrich Castle, Ross on Wye, U.K. (C) Daniel Friedman

At left in the photo above you can see the remains of a fireplace built into the stone walls of Goodrich Castle, a Norman midieval castle north of the village of Goodrich, Ross on Wye, Herefordshire, U.K.

By the time the original wooden fortification was replaced with a stone keep and then expanded, this 13th century fireplace was very shallow, wide and tall, with a projecting hearth, now mostly fallen away. You can place the height of the floor relative to the fireplace hearth by noting the openings for floor beams in the opposing wall at the right in this photograph.

[Click to enlarge any image] Search for "Goodrich Castle" to see more cameo appearances of this historic old, interesting structure.

Adding Support Below a Settling Fireplace Hearth

Support added below a fireplace hearth (C) Daniel Friedman

You may find a temporary supporting column such as this Lally column which was placed below a sagging fireplace.

Some diagnosis of just what caused settling or movement in a hearth is critical.

A gap appearing between the hearth and the edge of the firebox might be due to inadequate hearth support - not such an ugly repair - or it might be due to settlement of the entire chimney and fire chamber away from the building - a major repair and a dangerous condition.



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