Thread: Hearth extension

The IRC and 211 say the minimum "thickness of a hearth extension is 2" but is there a reference that specifically states that the thickness defined is "solid" non combust ?

Originally Posted by Marc M

... minimum "thickness of a hearth extension is 2" ... states that the thickness defined is "solid" ...

Marc,

Not fully understanding your question ... if the the minimum thickness required is 2" and the whatever material is hollow inside, then the thickness of the material is only the thickness of the top and bottom around the hollow space inside ... not sure if I clearly stated what I am thinking in my mind?

If you have a steel 2" by 6" square tube, with a wall thickness of 1/8", then the thickness of steel is only 1/4" (1/8" + 1/8"), not 2" or 6" - I think I described what I am thinking a lot better there.

Hi Marc,

By your info, I see you are in 'so. county', so you know that CA always had their 'own' Bldg. Code versions/variations separate of the IRC (kept it interesting over the years).

This is a link I had for SD county Bldg. Dept. fireplace/chimney requirements that was updated March of this year, specific for our 'Seismic Zone' and keep with the current 2013 CBC:

http://www.sandiegocounty.gov/pds/docs/pds180.pdf

Scroll down, your 'hearth' & 'hearth extension' thickness info/code, as well as all other requirements are on page 4 of 5.

It has more info than you'd ever want for any masonry structures with all the dimensions, proper terms and pertinent code sections for just about anything you'd want to reference. I've always had the understanding that any brick/stone/concrete extensions had a 2" min. thickness (that would include the 'mortar bed' you can see in older home crawls), but they have had all kinds of 'man-made' composite, fire-resistant extensions they've been using in remodels/new construction that are supposedly 'listed' and can be used (best to get the documentation).

Hope this helps, but if not I'm sure Bob will clue us in.

Steve L.

Originally Posted by Steve Lottatore

I've always had the understanding that any brick/stone/concrete extensions had a 2" min. thickness (that would include the 'mortar bed' you can see in older home crawls) ...

Steve,

I didn't see that part of including the mortar bed in there, did I miss it?

Most mortar beds are laid using a notched trowel, the mortar does not flatten out and become "solid", there are grooves, gaps, and air spaces.

Hi Marc & Jerry

I'm out and on iPhone, so hopefully this works.

The one 'exception' for CBC 2111.9.2 - 'Hearth Extension' reads:

Exception: When the bottom of the firebox opening is raised at least 8” (203 mm) above the top of the hearth extension, a hearth extension of not less than 3/8”-thick (9.5 mm) brick, stone, or tile or other approved non-combustible material is permitted.

Jerry, as here on the left coast there is as many, if not more homes built with a concrete 'slab-on-grade' with min. 4"-5" thickness, than with any kind of wood floor structure. I don't think there was anything to 'miss', but I am of the understanding that the 'intent' was always that the hearth extension is to be the 2" min. thickness of 'non-combustible material'. If that ends up being all/solid concrete, solid brick, or thinner brick/stone paver or ceramic tiles (non-combustible), over the 'compliant' min. thickness concrete slab, you were good to go. (as long as there were no non-compliant, 'combustible materials' of any kind within that top, exposed 2" thick extension)

Like I said, I'm sure Bob will kick in and tell me if I'm off the mark here.

Steve L.

Originally Posted by Steve Lottatore

I've always had the understanding that any brick/stone/concrete extensions had a 2" min. thickness (that would include the 'mortar bed' you can see in older home crawls),

Originally Posted by Steve Lottatore

Jerry, as here on the left coast there is as many, if not more homes built with a concrete 'slab-on-grade' with min. 4"-5" thickness, than with any kind of wood floor structure. I don't think there was anything to 'miss', but I am of the understanding that the 'intent' was always that the hearth extension is to be the 2" min. thickness of 'non-combustible material'. If that ends up being all/solid concrete, solid brick, or thinner brick/stone paver or ceramic tiles (non-combustible), over the 'compliant' min. thickness concrete slab, you were good to go. (as long as there were no non-compliant, 'combustible materials' of any kind within that top, exposed 2" thick extension)

I fully agree with you when on slabs, what confused me was this part: " ... in older home crawls ... "

The crawlspace part is what messed with my mind on thickness.

Jerry,

Sorry, wasn't looking to confuse anybody. I was just referring to an older home that I or Marc might see with raised foundation/wood floor structure, instead of the usual slab-on-grade. There should still be the 'mortar bed' or 'crib' portion for the hearth extension, visible and properly supported in the crawl space, whether the fireplace room is original or has been 'upgraded'.

I do see 'remodel' variations of improper/unsafe flooring in front of fireplaces quite a bit; worst case scenario with the original bricks/stone plus the 'mortar bed' sections completely removed, with (combustible) wood sub-flooring filling in the extension space so (thin) tile or (combustible) wood flooring can be installed throughout the remodeled room; and then worse again by having a 'continuous flooring pattern' with no distinguishable hearth extension in front of the fireplace opening.

Marc, wanted to ask you was there a specific hearth extension condition you saw that was suspect for being modified, and what is the 'not solid' condition you might be speaking about?

Steve L.

Hey Steve thanks for the response. I get what it says, thanks.
I was at a house the other day and came across a built up hearth extension and was letting the client know that it needs to be 2" of "solid" material in thickness. And the agent asked me to prove it. So here I am. I didnt see anywhere that said the words "solid" so I assumed it was presumed or implied. Fun fun...

Originally Posted by Steve Lottatore

Jerry,

Sorry, wasn't looking to confuse anybody. I was just referring to an older home that I or Marc might see with raised foundation/wood floor structure, instead of the usual slab-on-grade. There should still be the 'mortar bed' or 'crib' portion for the hearth extension, visible and properly supported in the crawl space, whether the fireplace room is original or has been 'upgraded'.

I do see 'remodel' variations of improper/unsafe flooring in front of fireplaces quite a bit; worst case scenario with the original bricks/stone plus the 'mortar bed' sections completely removed, with (combustible) wood sub-flooring filling in the extension space so (thin) tile or (combustible) wood flooring can be installed throughout the remodeled room; and then worse again by having a 'continuous flooring pattern' with no distinguishable hearth extension in front of the fireplace opening.

Marc, wanted to ask you was there a specific hearth extension condition you saw that was suspect for being modified, and what is the 'not solid' condition you might be speaking about?

Steve L.

A hearth extension must be integral to the structural support for the fireplace itself. That means it must be corbelled brick our cast concrete haunches. You cannot construct a chimney and fireplace with a big retangular opening in the combustible floor then form up, dump in some mud and finished noncombustible face material and call it a proper hearth extension. It must be capable of supporting itself and all live loads placed on it but those loads must be transferred to the chimney/ foundation. The form method I described is what we call a "floating hearth extension". Oh, sure, it may seem solid when first constructed but over time, it will flex and move resulting in a gap opening between the Hx and the hearth proper. If you have combustible forms below, you'll have fire trucks in the driveway one night. The only way I know to rectify this without demolishing and rebuilding it from scratch is what I've done: bore into the masonry and dowel in rebar using anchoring cement. Form a cage from one side looping around the HX and back to the other. I would use a minimum of four anchor points into the mass of masonry. Then removeable forms are placed such that the slab will have a 4:1 ratio of HX projection out from from hearth versus thickness of slab. This ensures sufficient vertical contact against the original structure or footprint. Without it, such as a 2" continuous slab, you have no inherent stability as pressure at the outer edge of the hx would place enormous compression loading on the bottom junction of the new HX slab to where it presses against the foundation, which would probably fail and open that same gap over time. I pour a high psi glass fiber reinforced concrete. Screed it off at the the height required for thickness and finished material height and let it cure for about 2 weeks depending upon weather, temp, ventilation rates and indoor Rh%. Keep it damp for a strong cure. Then, strip the forms from underneath and lay your finished noncombustible Hx material.

NFPA211 requires a 4" clearance to combustibles maintained below the HX. If you have a lousy 2" thick slab then the closest combustible to the face of the hx is 6".

As for the HX being solid, there are some things the code just doesn't have the time to account for and this is one of them. If you were to allow a less than solid 2" mass then some weenie would surely get two thin sheets of sheetmetal, form a hollow box and call it a legal HX because it is non-combustible and 2" min. Yes, solid masonry of whatever material but self supporting, too. Quarry tile laid on a mud bed poured onto a form using left over plastic lath and floor board forms just doesn't cut it. Mortar has insufficient strength to be self supporting--it must lie on concrete or other properly supported masonry.

If a Realtor wants you to confirm a min. 2" thick continuous slab, first, I'd ask them why. It's only one of a hundred details that could be verified so why that one? How to do it? That's a Level 3 inspection. You can drill through the Hx monitoring the dust for wood chips. If unsure, you could weigh a sample of the drill dust, wave a flame over it to observe any combustion, then re-weigh it but this is tricky because the mortar will calcine and crumble around 800F. Also, each test hole is just one point and no guarantee the whole slab is made that way. They could hire a company to come X-ray it...

HTH.

Excellent Bob, thanks for your time. I think the agent was attempting to avoid removal and replacement of the HX because they just complete a rehab / flip with new tile etc... . Oh yea, not to mention that the agent has never had an inspector call that out before me. You know how that goes..

Originally Posted by Bob Harper

A hearth extension must be integral to the structural support for the fireplace itself. That means it must be corbelled brick our cast concrete haunches. You cannot construct a chimney and fireplace with a big retangular opening in the combustible floor then form up, dump in some mud and finished noncombustible face material and call it a proper hearth extension. It must be capable of supporting itself and all live loads placed on it but those loads must be transferred to the chimney/ foundation. The form method I described is what we call a "floating hearth extension". Oh, sure, it may seem solid when first constructed but over time, it will flex and move resulting in a gap opening between the Hx and the hearth proper. If you have combustible forms below, you'll have fire trucks in the driveway one night. The only way I know to rectify this without demolishing and rebuilding it from scratch is what I've done: bore into the masonry and dowel in rebar using anchoring cement. Form a cage from one side looping around the HX and back to the other. I would use a minimum of four anchor points into the mass of masonry. Then removeable forms are placed such that the slab will have a 4:1 ratio of HX projection out from from hearth versus thickness of slab. This ensures sufficient vertical contact against the original structure or footprint. Without it, such as a 2" continuous slab, you have no inherent stability as pressure at the outer edge of the hx would place enormous compression loading on the bottom junction of the new HX slab to where it presses against the foundation, which would probably fail and open that same gap over time. I pour a high psi glass fiber reinforced concrete. Screed it off at the the height required for thickness and finished material height and let it cure for about 2 weeks depending upon weather, temp, ventilation rates and indoor Rh%. Keep it damp for a strong cure. Then, strip the forms from underneath and lay your finished noncombustible Hx material.

NFPA211 requires a 4" clearance to combustibles maintained below the HX. If you have a lousy 2" thick slab then the closest combustible to the face of the hx is 6".

As for the HX being solid, there are some things the code just doesn't have the time to account for and this is one of them. If you were to allow a less than solid 2" mass then some weenie would surely get two thin sheets of sheetmetal, form a hollow box and call it a legal HX because it is non-combustible and 2" min. Yes, solid masonry of whatever material but self supporting, too. Quarry tile laid on a mud bed poured onto a form using left over plastic lath and floor board forms just doesn't cut it. Mortar has insufficient strength to be self supporting--it must lie on concrete or other properly supported masonry.

If a Realtor wants you to confirm a min. 2" thick continuous slab, first, I'd ask them why. It's only one of a hundred details that could be verified so why that one? How to do it? That's a Level 3 inspection. You can drill through the Hx monitoring the dust for wood chips. If unsure, you could weigh a sample of the drill dust, wave a flame over it to observe any combustion, then re-weigh it but this is tricky because the mortar will calcine and crumble around 800F. Also, each test hole is just one point and no guarantee the whole slab is made that way. They could hire a company to come X-ray it...

HTH.