BrickWood Box Q’s

I am getting started on my box and I was wondering if anyone has built one?
First question is how come we don’t use castable on the bottom slab? We us it on the top (grilling) slab but not the oven (lower slab) slab?

Anyone do the castable slab in place?
That’s what I did on my pizza oven slab. Worked good but didn’t know if anyone has done it on the firebox

Any good general info on what to look out for or things people wish they would have done different

Thanks KOB

Any thoughts on this?
Pouring my pad this coming week and then setting blocks

I know that this is likely way too late but I built a BrickWood Box this summer and it turned out great. I need to submit pictures but I ended up going with a regular slab but I did pour it wider and longer than the directions so I would have more room around it on solid concrete. I followed the directions to a T except for the fireing up and ended up having cracks in both the concrete slab and the top oven chamber. It still works amazing and I couldn’t be happier. I used it as a smoker and rotisserie. I need to work on the smoking portion but that’s another topic. I honestly couldn’t be happier with both the BrickWood Box and Pizza that I built. Again, I know this is late but let me know if you have any questions.