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Constructing a NON-COMBUSTIBLE hearth extension


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As far as making a non-combustible hearth extension, what makes it non combustible? If framing it with wood, will placing concrete backer board below the tile surface make it non combustible? Im trying to understand this.

Again; frame it with wood and then place plywood subbase with cement backer board and then tile. Is that non-combustible?

I found this on the web regarding a pad for a stove, but I figure this is the same as an extension in front of a pre-fab Fp.

What say you guys?

Quote:Technical Note: Conductivity:

Cement backer board is an ideal material for a hearth pad because of

its low “kâ€

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Hi,

I don't think you have to go to that extent for a hearth extension. the whole purpose of a hearth extension is to prevent sparks that jump out of a woodstove insert from igniting a surface. They sell them at fireplace/woodstore dealers here and they aren't anything more than a piece of plywood with tile bonded to the face and a piece of trim at the edges.

OT - OF!!!

M.

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Originally posted by hausdok

Hi,

I don't think you have to go to that extent for a hearth extension. the whole purpose of a hearth extension is to prevent sparks that jump out of a woodstove insert from igniting a surface. They sell them at fireplace/woodstore dealers here and they aren't anything more than a piece of plywood with tile bonded to the face and a piece of trim at the edges.

OT - OF!!!

M.

No, no, no. NO. The purpose of a hearth extension is to prevent radiant firebox heat from lowering the kindling temperature of combustible materials in front of and around the hearth, including combustible plywood that might be under tile just in front of the hearth (under the hearth extension), and any wooden trim around and too close to the hearth (there are specific guidelines for such). That the purpose of a hearth extension is to prevent sparks & embers from setting stuff on fire is a commmon misconception (though it does that also).

One could greatly benefit from taking Dale Feb's fireplace inspection course for home inspector's.

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Noncombustible material doesn't burn, support combustion, or release flammable vapors. Materials that have passed ASTM E 136 are considered noncombustible. I know that "Hardibacker" board has passed and is listed as noncombustible.

I look for a thickness of 2" solid masonry, unless the firebox is at least 8" above the hearth extension, then the minimum thickness is 3/8".

I don't know if the rules are different for prefab. I'm mostly looking at real fireplaces, not the sheet metal containers that I'd be afraid to use as an ashtray.

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Originally posted by Jerry Simon

No, no, no. NO. The purpose of a hearth extension is to prevent radiant firebox heat from lowering the kindling temperature of combustible materials in front of and around the hearth, including combustible plywood that might be under tile just in front of the hearth (under the hearth extension), and any wooden trim around and too close to the hearth (there are specific guidelines for such). That the purpose of a hearth extension is to prevent sparks & embers from setting stuff on fire is a commmon misconception (though it does that also).

One could greatly benefit from taking Dale Feb's fireplace inspection course for home inspector's.

Ok, but back to the original post and question. . . does a plywood substrate with concrete backer and tile suffice as an adequate hearth?

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I went over a number of different manu. specs. for heatilator & majestic, two of the most common prefab units out here and here is what I found.

Gas Fireplace:

Both companies gas fireplaces only recommend and do not require a non-combustible hearth extension.

Wood Fireplace:

These require a non-combustible hearth extension. Below are non-combustible materials and the required thickness that meets the specs.(min. R value of 1.16) for the fireplaces non-combustible hearth extension.

Hearth Extension Insulation Alternatives, R Value = 1.16

Hearth & Home HX3, HX4 - 1/2 inch

USG Micore 300 - 1/2 inch

USG Durockâ„¢ Cement Board - 2-1/4 inches

Cement Mortor - 5-7/8 inches

Common Brick - 5-7/8 inches

Ceramic Tile - 14-5/8 inches

Marble - 16-5/8 in. to 23-3/8 in.

These are the materials that can separate combustible material, lets say the wood flooring or wood framed hearth extenstion, from the fireplace hearth.

As you can see it would take over 14 inches of ceramic tile applied over a combustible surface to meet the manu specs. - thats a lot of tile!

http://www.heatilator.com/products/fireplaces/index.asp

http://www.vermontcastings.com/indes.cfm

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Originally posted by Martin Lehman

I went over a number of different manu. specs. for heatilator & majestic, two of the most common prefab units out here and here is what I found.

Gas Fireplace:

Both companies gas fireplaces only recommend and do not require a non-combustible hearth extension.

Wood Fireplace:

These require a non-combustible hearth extension. Below are non-combustible materials and the required thickness that meets the specs.(min. R value of 1.16) for the fireplaces non-combustible hearth extension.

Hearth Extension Insulation Alternatives, R Value = 1.16

Hearth & Home HX3, HX4 - 1/2 inch

USG Micore 300 - 1/2 inch

USG Durockâ„¢ Cement Board - 2-1/4 inches

Cement Mortor - 5-7/8 inches

Common Brick - 5-7/8 inches

Ceramic Tile - 14-5/8 inches

Marble - 16-5/8 in. to 23-3/8 in.

These are the materials that can separate combustible material, lets say the wood flooring or wood framed hearth extenstion, from the fireplace hearth.

As you can see it would take over 14 inches of ceramic tile applied over a combustible surface to meet the manu specs. - thats a lot of tile!

http://www.heatilator.com/products/fireplaces/index.asp

http://www.vermontcastings.com/indes.cfm

I've been nosing around those sites and I can't find the information you posted. Where on those sites is it?

I question the facts in that table.

- Jim Katen, Oregon

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  • 2 years later...

I have been looking at this question also for my own place. NFPA211 calls for 2 inches of masonry for non-rated stoves if the legs are 6"+ tall. Also have seen refs to R 2 for hearth pad. The USG Micore looks to me like a good solution to get thickness and R value. USG says that Micore160 has R1.27, and M300 is R 1.03. Thus ~R2.00 for the 3/4" M160 stuff. Else several layers of 1/2" Durock at R .26. A good solid 3/4 ply base, quickset, suitable number of Micore and Durock or Hardiebacker layers bonded with quickset with tile surface seems like it might work. I did not forget about checking loads on the floor joists. Just to be safe, planning here to add a couple of concrete deck piers and 2x4 cross beams under the 24" oc floor joists in question. It is an old house. Thanks for the forum. Did not realize that brick and tile were so conductive to heat. Whew. Best from WA State.

As far as making a non-combustible hearth extension, what makes it non combustible? If framing it with wood, will placing concrete backer board below the tile surface make it non combustible? Im trying to understand this.

Again; frame it with wood and then place plywood subbase with cement backer board and then tile. Is that non-combustible?

I found this on the web regarding a pad for a stove, but I figure this is the same as an extension in front of a pre-fab Fp.

What say you guys?

Quote: Quote:Technical Note: Conductivity:

Cement backer board is an ideal material for a hearth pad because of

its low “kâ€

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  • 2 months later...

Each factory built fireplace has its own requirements. Most but not all gas fireplaces do not require non-combustible HX but all woodburners do. This HX protection is comprised of both ember protection and radiant heat protection. This is why those instructions list the various acceptable substitutes. This is based upon testing. Some larger fireplaces require thicker insulation and larger non-comb board. Not all cementitious backer boards have passed ASTM E-136. For instance, Durock has passed it but Wonderboard crumbles apart.

Rather than a framed HX, why not just use cinder block? Regardless, you will need to ensure the spark strip extends fully across the fireplace opening half under the Fp and half under the HX. Failure to do this is a major source of fires.

BTW, NFPA 211 is a national std. but only enacted as an enforceable code in a few areas. With factory built appliances, you go by the listed instructions first then compare with the code then apply the more stringent requirement.

I recommend all H.I.s attend one of Dale's HI one day courses. I've taught it several times now and always have good discussions with the HIs.

HTH,

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