The Hearth Slab...
The block walls proved to be more of a challenge than I originally expected. The back and side walls went up three courses before two pieces of angle iron were installed to form a lintle for the front wall of the final course. The primary purpose of the block walls, apart from storage, is to raise the floor of the oven to a comfortable height.

 

The cores on the corners were filled to the bottom with concrete and rebar to add stability. Notches were cut in the top course to accept the 15mm rebar that holds up the hearth. Then rest of the cores were only filled (only the top course of bricks). This was done to add strength so the blocks won't colapse when the oven is built. The area under the hearth is for wood storage (and not the fire as many people expect) and ash collection.
Once the walls were completed the framing for the hearth slab and ash slot was constructed. The frame keeps the hearth away from the wall and allows for expansion. Under the hearh slab is a 2 inch layer of a vermiculite/cement mixture. This layer provides insulation for the hearth slab. The slab and insulating layer were poured the same day. The ash slot was also added at this time (although we almost forget to put the space in place.