Sinking oven floor

[RobG]

Hello. This is my first post. After browsing some, I did not see a post like this, so here goes.

I built a 42" ID oven following the design offered by FB. A contractor friend who built one for himself about 8 years earlier using the FB design helped me. We completed the project without problem during the Fall of 2020. About 3 months after construction, I noticed the bricks beginning to sag a bit in the center of the oven floor. Now, about 9 months after construction, the middle of the oven floor has sagged about an inch and one brick has dropped well below the others. This all suggests to me that there is a void under the cement board and that the board has broken. But we filled the circle under the cement board up to the top with vermiculite. Also, the cement board was the same used on my friend's oven and his is fine after about 8 years (1/2 inch I think). I will attach some pictures.

I am wondering what to do. One option might be to fill the spot where the brick dropped with a new brick (possibly using a split brick since the brick dropped about half the thickness), grind the surface smooth and hope for the best since the floor dip doesn't really interfere with cooking. If the problem gets worse, or if this forum gives me the necessary courage, I could try something more aggressive (pull floor bricks and rebuild somehow, or float the existing floor with high-temp mortar and lay in a thin floor of soapstone, or something else). What do you all think? One concern I have is confined space entry. Should I blow air in while I work in there?

Thank you.

Rob

[Gulf]

Hi Rob, Welcome to the forum.

Though you and your contractor may have built to FB dimensions, the under floor method y'all used is not an FB practice. Cement board will eventually degrade under oven temps. It also sounds like y'all used loose fill vermiculite to fill the void underneath the cement board. Is that correct? We recommend a 5 to 1 (vermiculte to portland) mix, referred to as vermicrete, to support the oven.

[MarkJerling]

Hello. This is my first post. After browsing some, I did not see a post like this, so here goes.

I built a 42" ID oven following the design offered by FB. A contractor friend who built one for himself about 8 years earlier using the FB design helped me. We completed the project without problem during the Fall of 2020. About 3 months after construction, I noticed the bricks beginning to sag a bit in the center of the oven floor. Now, about 9 months after construction, the middle of the oven floor has sagged about an inch and one brick has dropped well below the others. This all suggests to me that there is a void under the cement board and that the board has broken. But we filled the circle under the cement board up to the top with vermiculite. Also, the cement board was the same used on my friend's oven and his is fine after about 8 years (1/2 inch I think). I will attach some pictures.

I am wondering what to do. One option might be to fill the spot where the brick dropped with a new brick (possibly using a split brick since the brick dropped about half the thickness), grind the surface smooth and hope for the best since the floor dip doesn't really interfere with cooking. If the problem gets worse, or if this forum gives me the necessary courage, I could try something more aggressive (pull floor bricks and rebuild somehow, or float the existing floor with high-temp mortar and lay in a thin floor of soapstone, or something else). What do you all think? One concern I have is confined space entry. Should I blow air in while I work in there?

Thank you.

Rob

Hi Rob

What does your whole floor "sandwich" consist of, from the slab, working up?
I wonder if you can't remove the central floor bricks, have a look what's created the void and then fill in level with sand and replace the bricks?
Sand is a very good insulator. (Which is why, when you walk on the beach, the sand just under the surface is cool, even if the top is hot.) My floor bricks lay on a thick layer of sand and the whole thing works well. I think that could be an easy, quick fix for you without any need for mortar etc.

Kind regards,
Mark

[UtahBeehiver]

I second what Gulf said. Cement board is not a recommended floor sublayer under the floor and we need to confirm how the vermiculite (loose, no portland or high ratios) was installed. As Gulf said, 5 parts vermiculite to 1 part portland is the recommended ratio for "vcrete" ratio under the brick floor. Before a solution can be determine the root cause of the the floor setting must be determine and the correction of the problem done or the floor will continue to droop.

[RobG]

Thank you all for the quick responses. I went back to the plans and now see that we made a major mistake as you all indicate.

As requested, here is a description of the construction. After we built the cement hearth, we used bender board to form a circle (40" ID) and built a cement ring wall (about 12" thick and 4" tall) around the bender board. The wall was build by packing cement against the outside of the bender board and under the leveled cement board placed on top of the bender board. The ring wall ID (40") was smaller than Dome ID at the floor (42") so the ring would sit under and support the dome wall. After the cement had set for a day, we removed the bender board, filled the circle with loose vermiculite, placed the cement board on top, and built the dome. One of the pictures in the original post shows the construction elements described to this point. After the brick dome was complete, we trimmed off the cement board corners and poured a layer of cement around the ring wall all the way out to the edge of the table to a height above the pizza oven floor. The rebar in this upper cement layer was doweled into the hearth layer beneath. So, from bottom to top under the dome center, the material layering is: cement hearth, loose vermiculite, cement board, fire brick. Under the dome wall, the layering is: cement hearth, cement ring wall, cement board, fire brick, outer dome layers (insulation and stucco). At the perimeter of the table, the layers are: cement hearth, upper cement layer, tile.

I am thinking that the floor needs to be pulled and rebuilt. My initial thoughts about the steps are:
1) number all of the floor bricks with a sharpie and take pictures to create a map for relaying the bricks,
2) punch out the fallen brick (see picture from original post) down into the loose vermiculite to create a hole so that the bricks can be grabbed for removal,
3) remove floor bricks working out from the hole created by punching out the brick,
4) remove the cement board that sits over the vermiculite,
5) vacuum out the vermiculite,
6) fill the circle under the floor with the proper vermiculite/cement material stopping short of filling to the intended floor bottom height (will be hard to level the material to an exact height in the confined space),
7) rebuild the brick floor following the numbering map created in step 1 and using sand to level the bricks,
8) fire the oven a few times to drive out moisture from the new vermiculite/cement fill and also allow for any brick settling, and
9) grind any high point in the brick to create a smooth floor.

I am a complete newbie, so guidance is greatly appreciated.

Thank you.

Rob

[MarkJerling]

It sounds as if the cement board is flexing and settling down over the loose vermiculite. Your thinking is sound, Rob. What you're proposing will be the best long-term fix.
Kind regards,
Mark

[Gulf]

If you leave a little reveal inside the dome when you cut the cement board, you will have a ledge to place a custom cut screed board. That will allow you to get a uniform thickness for your vermicrete. That will help a bunch in getting the floor brick level. I would use a dark grease pencil to mark the floor brick. Most are non toxic and the wax will burn out in subsequent firings. I'm not sure about magic markers. You should only have to mark the cuts. The whole splits should be uniform.

The only critical error here that can't be fixed is that the oven dome is not separated from the hearth slab by insulation. That creates a big heat loss.