The Home Inspector Logo


The Home Inspector

Like boot camp for homeowners.

The Health Inspector: Fireplace hearth extension rules should not be boring topic

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

Subscribe to Reuben’s Home Inspection Blog


The Home Inspector: How good is the $1,500 Corentium Pro radon monitor?

The folks at Airthings recently sent me one of their new NRPP approved Corentium Pro radon monitors to review, which currently retails for $1,499. This company had a booth at ASHI's InspectionWorld conference in January of this year and was previously called Corentium, but they re-branded about a month ago, apparently leaving the Corentium name to be specific to their radon monitors. It sounds like they're planning to get into other types of air quality testing, but I haven't heard any specifics yet.


Corentium Pro

The Corentium Pro is a sleek-looking  radon monitor that comes with a handy tripod mount on the bottom, obviously giving a much more professional appearance than the utilitarian upside-down 5-gallon buckets that my company currently uses for radon testing. It looks like a new, modern device... unlike most other radon monitors available today. It also comes with a USB cable and a nice padded carrying case.


This device is powered by three AA batteries, which are supposed to be good for 18 months. I absolutely love this feature. The radon monitors that we currently use all need to be plugged in. This means that we need to keep an extension cord with every radon monitor.  We also need to make sure that we're not plugging our monitors into switched outlets (ask me how I know). The power cords for our existing monitors also get damaged and occasionally left on site, so we're always having to purchase replacement power cords. All of that hassle is eliminated with the Corentium Pro. The batteries are supposed to be replaced annually, ideally at the same time that the unit is calibrated.


Like any other electronic radon monitor, these units must be calibrated annually, in accordance with NRPP standards. Calibration is done through Bowser-Morner in Dayton, OH.


The Corentium Pro is designed to interface with either an iPhone or Android. This is probably the way that it should be, but it increases the potential for bugs. I dealt with a few bugs just trying to get my Galaxy S5 phone work with this device, and was told to update my app three days after I had installed it.  That did the trick, and I was able to use the device. Connecting to the monitor using the phone app was intuitive and simple. I didn't need to read any instructions.


The reporting options are quite limited using the phone app for Android; you get what you get, which includes a no-frills hour-by-hour printout of the radon levels, but no graphs and no information about things like temperature and atmospheric pressure. That data can be included in the reports for iOS users, and the company plans to make this functionality available to Android users in the very near future, but until that happens I can't recommend the Corentium Pro to Android users. Using the PC based interface, the options for reporting seem to be limitless. In fact, it was really too much information for me to try to learn on a device that I was only testing out. My impression is that you would be able to get a report that plots data exactly the way you want it, but you need to invest the time to learn how to do it. Check out this short video clip they sent me, which helps to understand how detailed the PC interface is:

Tamper Resistance

One thing that seems to be lacking with the mobile phone reporting is a good way to determine if the device has been tampered with. Once the 'mobile' report has been generated, the report tells you whether or not the radon monitor has been moved, which could indicate tampering, but it doesn't give any specific information about the movements, which makes the information almost useless. If a child or pet simply bumps the monitor once during the testing, that's not critical information that would make me suspect tampering.

On the other hand, if the monitor was moved a few hours after I set it up and moved again a few hours before the end of the test, I would be extremely suspicious. The mobile report doesn't give that information; it simply tells the user whether or not the device was moved, period. It should really tell users each time the device was moved. If it could also say how far it was moved, now that would be definitive, but I'm not aware of any devices that do that.

The report that is generated through a PC interface does have the ability to show when the device was moved.


We tested this device alongside a recently calibrated Sun Nuclear 1027 radon monitor, and the hour-by-hour readings were nearly identical.


This is an easy-to-use, feature packed device that does everything you need and more. The fact that it's powered only by batteries is also a huge selling feature. The only big downside is the price, which might be too high for radon professionals to justify. I'd be willing to pay about 20% more than what I currently pay to get the additional features that the Corentium Pro offers, but this device costs nearly twice as much as what I'm currently using, which is the bare-bones Sun Nuclear 1027. The Android functionality is also currently too limited, but I'm sure that a software update will fix that in the near future.

Author: Reuben SaltzmanStructure Tech Home Inspections

Subscribe to Reuben's Home Inspection Blog