Adjusting the base Size

I’d like to expand the base width a bit so that it is flush with the insulated slab to provide more wood storage. Do you foresee any slab support problems with the expanded footprint?

It’s an interesting and tempting idea. You are right, though, to be thinking about what it would do regarding your upper, insulated slab.

The “U” walls are designed to sit directly under the oven, which weighs about 1,000 pounds when completed. The idea there is to transfer that load directly to the base slab, which in turn diffuses it and spreads it across the ground.

Offsetting it puts more horizontal load on the insulated slab, which at least theoretically means a possibility of cracking the slab. In practice that’s not too likely to happen, because of the rebar lattice embedded in the slab, and especially if you use 4,000 or 5,000 psi concrete.

As designed, the insulated slab rests on the U wall, held just by mortar.

I do have a thought that would give you some peace of mind, and should add less than $50 to your costs:


  • Cement board (HardieBacker) 4x8
  • Standard mortar mix
  • Rebar to make a loose matrix

Build the U wall per instructions. When you embed rebar vertically in the concrete block voids, leave about 6 inches above the top of the wall for the 4 corner rebars.

Trim the cement board to the outer dimensions of the U wall. Drill 4 holes for the corner rebars to pass through. Mix some mortar and spread a 2 inch layer on top of the blocks. Lay the board on top of the wall (catching the holes) and level before the mortar sets. Remove any mortar that squeezed out from the joints.

Construct your rebar matrix, tying at the cross points. This does not have to be as dense as the one in the base slab.

Mix mortar and spread a 2 inch layer on top of the cement board, then place the matrix. Use a hammer to bend the 4 corner rebars in toward the center of the board (diagonally).

When you are ready to place the insulated slabs, go back to step 46 of the instructions for general guidance. (Generous coat of mortar, place one slab at a time, be sure it is level and flush to the cement board.) Place all 3 slabs, and once they are flush, seal the seams with a thin layer of mortar.

This is certainly a bit of extra work, but you will have accomplished two important things here:

  • The oven load will transfer properly to the walls
  • The walls are now physically tied to the slab

I hope this is helpful, and will ease your mind about supporting your oven properly. I’d be especially interested in pictures of your construction if you do this modification.

Good luck and let us know how it’s going!

Thanks. I’ll incorporate your suggestion into my build…I’ll keep you informed on how it turns out.

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