Well, it’s possible you went a little long there.
Along the back, though, you’ll have the following:
- About ⅜ inch of mortar
- 4 ½ inch width of firebrick
- Another layer of mortar
- 2 ½ inch more of firebrick if laid on their sides
- About 3 inches of insulation
- About 1 inch of stucco
I added mortar and the second layer of firebrick because you mentioned 2 layers of brick across the back (the oven to design uses just one). I would not use a full 9 inch back wall of firebrick, but a half-thickness (on sides) wouldn’t hurt.
If you add that layer, you’ll have almost 12 inches of back wall projecting out, which will more than cover the firebrick. Without it, you subtract about 3 inches to come up with 9 inches projecting. Either one will cover the hearth in the rear.
The two possible issues that crop up are:
- Do you have enough room past the hearthbrick for the structure to come out all the way and leave you a little margin for error?
- A small risk that heat conducted over many hours will reach the stucco and in a period of many years perhaps degrade it, maybe. (Obviously I don’t think that’s likely, but it should be mentioned.)
As you’ve started to do in the photos, just “dummy it up” first, using a couple of bricks as placeholders for the stucco and insulation because they’re handy sizes, and satisfy yourself that this will all work before you start mixing up mortar.
I’m really impressed by the quality of your work, Dave, and like that you signed the corner of your hearth. Also, I’m pretty sure no one will ever have a need to see it because this oven is going to outlast all of us.