The 3-slab process (our design) gives oven builders with little to zero construction experience the ability to make a perfect oven base slab w/out the hassle (or dangers) of making a pour-in-place single monolithic slab. Making a monolithic slab frame can be difficult for some - plus, you are going to put 800+ lbs of wet concrete in that frame. Better hope you got it braced correctly or your are going to have a big, big mess if it falls, breaks or begins to lean… And if it does break, you have a big mess that is going to turn in to a rock-hard big mess in no time.
Our simple-to-build 3-step base slab frames (that are all identical in size) allow you to build your base slab on a nice flat surface with little-to-zero possibility of failure. In addition, building the void for the insulating vermiculite / perlite layer is super-easy by gluing Styrofoam into each frame that you will remove once the 3 slabs have cured. Inserting this void into a single monolithic slab is kinda difficult…
To answer the 2nd part of your question - you MUST have that insulation layer in the slab! If you place your firebrick directly on the concrete slab, the slab will absorb the heat from those firebrick like a sponge! In addition, you don’t want the concrete slab to get too hot as concrete is unable to handle high-heat for extended bake times. The vermiculite is an inexpensive way to insulate the slab from that high-heat AND keep your hearth firebrick super-hot as you bake the perfect pizza!
DIY Pizza Oven Installation Instructions