Thread: Plywood under firebox?

Can someone please confirm that this is wrong? I took this picture by sticking my camera into the ash dump and pointing it up. That sure looks like plywood; should that have been removed?

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Originally Posted by Welmoed Sisson

Can someone please confirm that this is wrong? I took this picture by sticking my camera into the ash dump and pointing it up. That sure looks like plywood; should that have been removed?

Welmoed,

Your question has come up a few times in the past. I seem to recall Bob Harper saying the hearth needed to be 6" thick of masonry/concrete to separate wood framing from the fire. I could not find that post, but here is another with a similar issue. Generally, I see expanded metal lath or something similar holding up the hearth.

http://www.inspectionnews.net/home_i...t-plywood.html

Morning, Welmoed,
I am not demeaning your post.
A fireplace ash dump is installed on the inner hearth floor. Near the back of the fire box.
You push ashes and debris through the ash dump doors and the debris falls down the ash pit.
There well be a collection cleanout door at the bottom of the ash pit.


Please describe what the appliance is above.
Is that a CMU chimney or chase?
I suspect a chase.
I am making a guess, not the smartest move, but they installed a Zero Clearance gas/LP appliance on the second level bedroom.
I am certain that Zero Clearance will lift some members eyebrows.

Thanks, Robert. I like to get my terminology correct.

This was a chimney, not a chase; it was a masonry firebox with a masonry chimney and clay tile flue liner; no cap or spark screen. The firebox was full of ashes and I couldn't see the ash dump door. The chimney was filthy, but the ash pit looked like it had never been used. It did look like there was a cutout in the plywood for the ash dump. The door to the ash dump was in the garage in the lower level; the fireplace was on the main level in the living room.

If it were an insert, I wouldn't make a big deal out of it.

Originally Posted by Welmoed Sisson

Thanks, Robert. I like to get my terminology correct.

chimney parts.JPG

Originally Posted by Welmoed Sisson

This was a chimney, not a chase;

Not necessarily. I have seen many masonry chases, or at least that is what I call them.
1: I see no chimney foundation. Is there a footing?
2: A chimney with clay-tile flue liners. A flue liner is a secondary barrier in a chimney that protects the masonry from the acidic products of combustion, helps prevent flue gas from entering the house, and reduces the size of an oversize flue.

Originally Posted by Welmoed Sisson

it was a masonry firebox with a masonry chimney and clay tile flue liner; no cap or spark screen. The firebox was full of ashes and I couldn't see the ash dump door. The chimney was filthy, but the ash pit looked like it had never been used.

Are you calling the CMU interior an ash pit? Look at the voids in the masonry. Smoking smoldering embers could escape into the wall assembly.

Originally Posted by Welmoed Sisson

It did look like there was a cutout in the plywood for the ash dump. The door to the ash dump was in the garage in the lower level; the fireplace was on the main level in the living room.

That plywood might have been used as the bottom of the form for the hearth. Should be removed if used as an ash pit.

Originally Posted by Welmoed Sisson

If it were an insert, I wouldn't make a big deal out of it.

That would be a mistake IMO.

Robert,

I believe she put her camera through the cleanout door into the bottom of the ash pit, aimed it upwards at the underside of the hearth and took the pic. I think that plywood is directly under the hearth and the rectangle is the ash dump.

Welmoed,

I suggest searching past posts of Bob Harper's for specific information. I know I have seen posts by him specifically addressing this type of condition. Did you follow the link I provided?

Right, pics have been posted here and elsewhere of that same thing. The brick masons left the form in under the hearth. It is wrong. Pics have also been posted of the plywood or planks burnt or charred from the heat.
The good thing is there are probably no other combustibles under there, but it is a fault, you bet.

Neither the IRC nor NFPA 211 specify a thickness of solid masonry units or reinforced concrete that is deemed sufficient to a combustible form under the hearth. The hearth extension carries a 4" clearance to combustibles below per NFPA 211-11.3.1 2015 ed.

11.3.1.2 states that "Wooden forms used during the construction of the hearth and hearth extension shall be removed when construction is completed."

This is a view of the underside of the hearth taken from the ash cleanout below looking up. You can usually spot combustible forms by inserting a telescoping mirror down into the ash pit, where provided with the mirror angled to provide a view of the underside and shine a light. The ash pit is also an excellent viewpoint where there is sufficient lighting.

If you breach the walls of the ash pit chase and cut out these plywood forms you will have to grout the resulting gap created by the removed plywood.

HTH,

The top of the ash pit was plywood, which continued through masonry to the band joist and sill plate of the house. Just as we were leaving the house my son noticed a thin thread of smoke coming up through a tiny crack in the mortar of the hearth and being drawn into the front of the fireplace, and thence up the chimney. No smoke smell. I knew there was an ash pit, and concluded there must be some smoldering going on in there... since any reasonable pit would expect hot embers (we let the fire go out and the ashes cool, but ashes can keep a hot coal for a loooong time, and some small amount of hot coals should be expected through the ash dump in normal operation) this should not really be a problem. A little smoke, warm ashes, smoke comes up through the dump and up the flue, so no worries.

But, I went outside and peered in through the cleanout, just in case. I saw some flames. Up at the top. (?!?). Crawled under the house. The plywood had caught on fire, burned through to the band joist (doubled 2x12) and now the band joist, the sill plate, and a portion of subfloor and floor joists was just starting up.

In ten minutes it would have been a massive fire. A garden hose knocked it back until the fire department arrived.

Yes, this is a hazard.