Base slab too short, help please

I am making the mattone barile oven and I realized when I started laying out cinder blocks that I measured the length of the slab at 58 inches instead of 68. If I build the cinder block wall the same size and push it all the way to the back of the slab will the slab still have enough support? Should I shorten the wall by half a block and would it have enough strength to support the heath slab?

Hi Van and welcome to the BrickWood forum!

Okay, so what you’re saying is that your base slab is 55 inches wide by 58 inches deep (front to back); almost a square. In an ideal build you want to have a little bit of clearance all around so that the load is balanced on the slab itself and not pushing down on the edges. You’re still going to have an inch away from the front and rear edges. (And no, I wouldn’t push it all the way back. I’d center it on the slab front to back.)

I definitely would not shorten the wall. If your rebar grid was built according to the dimensions, and your footings (if used) are spaced properly, you’d only be compounding the error by shortening the wall. Also, you would take support away from the back wall of the oven.

Here’s a way to think of it:

  • the base slab supports the concrete block wall. You still have slab fully under it, although it will be 5 inches short front and back.
  • the concrete block wall supports the oven. The three “legs” of the wall need to be under the three contact surfaces of the oven—the two ends of the arch barrel and the back wall. This is the critical structure because it’s supporting a very heavy load. If you shift the walls underneath, you will change the lines of force from vertical to outward as the hearth slab transfers load from the oven barrel to the concrete walls.

There are large margins of error built into the design, just for people like us who are not building a structure like this five days a week. Stay with the plan for the concrete block wall, and center it front to back as per design, and you should be fine.

Be sure to come back and let us know how things are progressing!

Thank you for the reply! So, because of the rebar placement, the bricks only fit if the cinder blocks are almost flush with the back wall of the slab and have 2 inches of clearancein the front. Do you think it will still support the full load or is there another way I can fix it?

Yes, I do. The important thing is to keep the blocks under the oven structure. It’s not “on the nose” spec, but it will do the job. The rebar doesn’t support the wall base—it helps keep the slab from cracking, so you do not have to center on it. The important thing is to get your blocks away from the edge, even if it’s just an inch front and back.

And while I have your attention…this all holds true if you built the base slab from 4,000 psi or 5,000 psi concrete mix, which is also part of the spec. Since your whole oven is going to weigh under 4,000 pounds when it’s done (it will up in the 1,000 + pound range), the slab can support many times that weight as long as it is distributed over the area of the slab. Something else you can do—and many builders have done this—is to fill all the voids in your concrete block with the high strength concrete, rather than just the ones with the vertical rebar. Do that, and you’ll have a wall that would require a wrecking ball to dismantle.

So, yes: keep the vertical line of the concrete block wall under the footprint of the oven above it. Do that, and you’ll be fine.

@BrickWood helped another builder with a variation of this question. This is the link if you want to check it out, but the bottom line is the same: line up oven and concrete block walls.

Hope this helps, Van!

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Perfect, then I should be okay! Thanks again!