Our upside-down method of building a hearth slab was developed after attempting our very first pizza oven many moons ago. We searched the internet for quality instructions on how to build a monolithic hearth slab, but all we found were flimsy, haphazard and sometimes dangerous directions. Most of them required 2x4 supports, tie-downs to hold the frame together and a little prayer to keep it all together until it dried. Once it was dry, you then had to apply your insulation layer and allow for even more drying time.
While the construction process was not only difficult, it was time and labor intensive… and what exactly would happen if a 2x4 support slipped or shifted? We would have 800+ lbs of wet concrete to clean-up (and quickly too!)
So instead of building one giant hearth slab frame, we designed 3 smaller, but identically sized frames that could be built on the ground with very little cutting / nailing skills required. Once the three wood frames were constructed the concrete mix could simply be poured and packed. The frames didn’t need any tie-downs or 2x4 supports to stay sturdy and/or elevated! Just a simple piece of plywood, some studs and a few nails.
We then went one step further and incorporated foam panels that can be glued into the wood frame which provides the proper open area in the hearth slab for the 2" Insulation Mix layer.