OSB Beneath a Fireplace

[Mike Lamb]

I took this pic from the ash pit of OSB left beneath a wood burning masonry fireplace hearth. The black stains could be from water or from scorching.

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[Tom Raymond]

I say water. There are drip stains on the concrete below the black stains.

[kurt]

Looks like water to me too.

How dangerous does the brethren think this condition is? I know what the fireplace people think. I'm just curious if anyone thinks this is a real hazard, or if it's another thing to put in the report.

[Jim Katen]

In my area, its more common to find forms under there than to not find them.

As far as I can tell, they rarely if ever cause a problem. Of course it's wrong . . .

[Jerry Simon]

In Mikes picture, you can see the cut-out for the ash-dump flapper assembly that's put in the bottom of the hearth. Over time, the wood dries-out from the fires, and the wood undergoes pyrolysis, and the wood's kindling temp lowers. A spark or an ember falling down the chute could touch the edge of that cut-out area and readily ignite that wood.

And, since that same wood form typically runs/extends into the basement or crawl area, under the hearth extension, the entire house can burn down. I've seen it. Guy teaching a *fire* class in St. Louis about 15 years ago took a group of us to a nearby house to see the remains, and explained how they knew the wood form was the culprit.

I also saw scorching like Mike mentioned, but much more wide-spread and definitely heat/fire related scorching; scorching had extended to the wood under the hearth extension. Wasn't caused by the ash-dump chute location, though; the concrete hearth had cracked (like most all concrete cracks), and an ember had fallen though crack and atop wood form. After looking at this for a while, it was obvious how the wood got scorched.

I always call it out, and I use NFPA 211, Section 8-3.1 as my code reference. Same reads: "Masonry fireplaces shall have hearth extensions of brick, concrete, stone, tile, or other approved noncombustible material properly supported and with no combustible material against the underside thereof. Wooden forms used during the construction of hearth and hearth extension shall be removed when the construction is completed."

[kurt]

Where's the cut out showing the ash dump flapper? The cut out in the middle doesn't look like that; it looks like a piece of scrap that was used for the form.

I'm not disagreeing with your assessment. I think you're right. I'm just curious about that cutout now.

Inasmuch as every house has this same condition, I was just wondering how dangerous it really is.

[Jerry Simon]

Where's the cut out showing the ash dump flapper? The cut out in the middle doesn't look like that; it looks like a piece of scrap that was used for the form.

To me it looks exactly like that (right size, right location); it is a flapper-thingy cut-out IMHO, but I think it was covered-over with firebrick. Odd. . .

Inasmuch as every house has this same condition, I was just wondering how dangerous it really is.

Not every house; I see about a 50-50 mix, though your mileage can vary. Dangerous enough, though, to include in my reports. . .

[Eric B]

I was just wondering how dangerous it really is.

The five CSIA sweeps I know would make an issue with it - these are not guys I'd want to debate with. Also consider, when fireplaces are used for many hours at a time, like during extended power outages, heat from the fireplace can radiate out quite far from the firebox. The most lenient acceptance I have heard is that the wood form can stay in if the ash pit damper is permanently sealed closed and the form is no closer than 8 inches to the floor of the firebox - if closer than 8 inches and then the form should be removed - which is doable but expensive.

[kurt]

I would never debate that it's not dangerous, and I've spent enough time being harangued by fireplace guys about all the issues with fireplace safety to understand not to ask them questions like "how dangerous is it really?" You ask that question of a fireplace geek and they go berserk and look at you like you're totally insane.

That's why I asked in here, where some semblance of reasonableness still exists.

Personally, I like my fires outside where I can play with them.

[Marc]

.....

Personally, I like my fires outside where I can play with them.

I've never experienced a more relaxing activity than solitude with a campfire in my backyard. Its magic. I can feel the tension drifting away.

Marc

[Hearthman]

There are three basic ways to allow combustible forms to remain under a hearth or hearth extension:

1- If a PE writes a letter stating to a reasonable degree of engineering certainty it is not a hazard

2- If the AHJ signs off on it

3- You install an insert that is approved for installation directly on a combustible floor. This may be gas or pellet depending upon the listing but never a wood insert.

Anything else is an unprofessional opinion that won't hold up in court. Now, if you can determine combustibles are pyrolyzed then they should be removed 100% regardless of the above or the fireplace condemned. Period. Pyrolyzed wood has a much lower ignition temperature and has already proven sufficient heat gets down there. Once a material has undergone degredation by heat it is not the same material it once was.

[gtblum]

Drift;

Pyrolysis was added to my vocabulary a few years back when what I thought was my woodstove backdrafting, turned out to be smoke from the end of a piece of wood lit like a cigar. I had stacked a cord of wood inside, about three feet from the stove in anticipation of not having to go outside some night while being buried in lake effect. It had been there for at least a month.

That stroke of genius could have been very costly had I left the house, or gone to bed early. I don't do that anymore.

[Eric B]

There are three basic ways to allow combustible forms to remain under a hearth or hearth extension:

2- If the AHJ signs off on it

I would never refer my clients to the AHJ. Much of what I write up in my reports was originally approved by them.