Three slabs oven base

I finished the base and the three slabs, everything was leveled once we installed it. But once again I had to put-off continuing building the top oven because my husband had surgery. Now I notice that the three slabs shifted, and they are not level. I do not know how to proceed. Any ideas? Or it would be okay to continue as it is?

It’s hard to tell from the pictures how much your slabs actually shifted. However I’m guessing you can hide it with some mortar. I would definitely fill all the gaps with mortar.

My slabs also shifted a little after I had the arch complete. Water was leeching into the oven through the cracks so I just filled them with mortar in a tube and that took care of it.

If the gaps continue to grow then you have a problem. Can you tell if the slabs are stable now or are they still shifting?

Thank you.
I can’t tell if they are still shifting, I will take some measurements and closer pictures.

Hoping your husband is recovering well from his surgery!

You are actually in a better position than Ken @kgondoly, because this happened before you built your oven barrel. I suspect that the mortar under the frontward side of the front slab sunk into open spaces of the concrete blocks or simply oozed out after you placed it and before it was set. I doubt it will move any more.

You should definitely patch the gaps on top and the outsides (including underneath). And, you must seal the inside “void” before you pour your Portland cement/Perlite mix into it, or else that too will ooze down and through the gaps.

Do not use mortar. You are better off using an acrylic concrete patching compound like Quikrete. This is easy to apply and can be worked up to ¼-inch into the gap. It offers a simple water cleanup before curing, and is weather-resistant once dry.

As for the “heave” itself, the reason I say you are in a better position is that you can correct the unevenness with your brick hearth frame. (I erred earlier in saying this: with your first course of firebrick. Just pay extra attention to your level and allow your heat-resist mortar to make a “bed” that levels that first course on the tops of the brick. (If you do this correctly, you will notice that the mortar underneath the first course looks uneven.) ) You won’t see the outside of the barrel at all once your oven is insulated and stucco’ed, and you might see it in the mortar inside, but the slab is not visible inside. If it bothers you at all, plan your veneer layer to cover to the edge of the slab, and veneer down the sides as well.

In the realm of things that could go wrong, Lia, this is in the category of Very Minor Issues. Best of luck continuing your build!

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Thank you so much!!!


Glad to help!

A couple of refinements:

  • Look underneath the slab from both inside and outside, just to verify there is mortar supporting it wherever it is resting on the concrete blocks. If there is a large gap, you can correct it by filling it with a simple wooden wedge driven into the spot.
  • My advice above did not account for the brick frame you will build on top of the slab. That’s actually where you are going to make your leveling correction. This will make it double important to veneer over the sides and brick frame rather than leaving them exposed, because from the outside the slab heave will be obvious. Veneer covers a multitude of faults!
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There is no mortar between the concrete blocks and slabs. Because the slabs are sooo heavy, I thought I did not need to use mortar. MY MISTAKE ! Maybe I should get some help to remove the slabs and use mortar to support them. I’m not going to give up :slight_smile:

Ahhh, okay! In that case, we’ll file away everything above for another occasion. (Don’t worry, nothing is ever wasted in these message boards!)

In this case, you are still at the right stage to remedy this.

The mortar helps with leveling and keeping the slabs in place and tightly together. So, yes, you want a couple of strong people (4 would be better because of what you need to do) to help out.

Some advice:

  • Mix a slightly “stiff” batch of mortar. Have a screed board (2x4), your 4 foot level, and a hose ready.
  • If you have the muscle power available, remove all three slabs. You’ll need to do this in order to get an accurate level.
  • Lay in the mortar for the middle slab per instructions. Use the screed board and the level to make sure the mortar beds are dead level on both sides. Meanwhile, have your helpers spray the bottom of the middle slab with a mist of water. Then, they should walk it over from the front and lay it gently into position.
  • Use the bottom of the middle slab as your guide for the mortar bed for the rear and front slabs. Check your leveling by butting the box level’s end against the center slab and its bottom even with the bottom of the slab, then out across the mortar bed. Your goal is to allow the level-line of the middle slab to continue out to the ends. Your helpers should mist the bottom of each of the two remaining slabs and then bring them in flush with the middle slab.
  • Mist the two joints between the slabs and seal with a thin coat of mortar. If the slabs are tightly together, you won’t be able to work mortar into the joints and that’s fine.

I’m glad you let me know about the missing mortar. You will end up with a dead-level hearth, which you will appreciate when you cook something like Pork Saltimbocca.

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thank you and YUM! :yum:


2 posts were split to a new topic: Is this mortar mix suitable as an ingredient for high temperature mortar mix?