Fireplace hearth extension rules

This blog post is about fireplace hearth extension rules, but first, a quick story to explain why I'm blogging about such a boring topic.

I've said it before and I'll say it again; one learns from teaching. We've constantly been adding new home inspectors to our company since 2009, which means that I've not only had the pleasure of training a lot of fine individuals, but I've had the opportunity to learn a lot of new information. Training others forces one to research topics and to look up references to prove the information being taught. In some cases, it makes me take a closer look at something that I've always gotten wrong.

While doing a home inspection with one of the newest inspectors on our team, Matt, we came across a wood burning fireplace with a tiled hearth extension. Matt instantly recognized this as a defect, noting the fact that the hearth extension should be a minimum of 2" thick, and presumably consist of concrete. In this case, the hearth extension consisted of tile on top of wood. The tiles were loose and the wood below was badly charred, as you can clearly see in the photo below.

Scorched hearth extension

Bad news, right? This would have been an acceptable installation if the material below the tile was concrete, but that obviously wasn't the case. So where does the requirement for a full 2" hearth extension actually come from?

I turned to the actual Minnesota Building Code requirements for fireplace hearth extensions, and that's where I found my answer. Section R1001.9.2 of the 2015 Minnesota Residential Code says the following:

R1001.9.2 Hearth extension thickness. 
The minimum thickness of hearth extensions shall be 2 inches (51 mm).

Exception: When the bottom of the firebox opening is raised at least 8 inches (203 mm) above the top of the hearth extension, a hearth extension of not less than 3/8-inch-thick (10 mm) brick, concrete, stone, tile or otherapproved noncombustible material is permitted.

According to the exception, this fireplace would have been fine if the firebox opening was at least 8" above the floor.  At that point, it's acceptable to have a simple tiled hearth extension. The whole purpose of the hearth extension is to make sure that embers or logs that fall out of the fireplace don't start the floor on fire.

Also, the hearth extension must extend at least 16" from the front of the fireplace and 8" on the sides for smaller fireplaces. When the opening of the fireplace is at least 6 square feet, the hearth extension needs to extend at least 20" from the front of the fireplace, and 12" on the sides. This is illustrated in the diagram below, courtesy of the fine folks at Code Check.

hearth extension requirements

Also, just a little bit of history on the matter: my oldest code book is a 1967 edition of the UBC, which required a hearth extension of 18" in front and 8" on the sides, regardless of the size of the fireplace opening.

1967 UBC Section 3704

That requirement changed in the 1976 edition of the UBC to the numbers that we have today.

1967 UBC Section 3707

Does any of this history matter from a home inspection perspective? Absolutely not. We're not code enforcement officials. It's just trivia. Included below are a few photos showing different fireplace hearths, along with my commentary on what makes them acceptable or not.


Proper hearth extension

Tiled hearth extension short

Author: Reuben SaltzmanStructure Tech Home Inspections

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Reuben is a second-generation home inspector with a passion for his work. He grew up remodeling homes and learning about carpentry since he was old enough to hold a hammer. Reuben grew up thinking he was going to be a school teacher because he enjoyed teaching others so much. In a sense, that’s a lot of what home inspections are about, so Reuben truly does what he loves. Sharlene has worked with Structure Tech since 2000 and Reuben has been contributing to her blog since 2008.

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