To begin the construction of the pizza oven foundation you first need to establish a solid flat surface. If building directly on soil this will simply mean removing the grass with a spade to expose the soil. If you are to build on an existing structure you will need to ensure that the weight of the pizza oven foundation and the pizza oven itself will not cause any problems. As the materials used may be extremely heavy it is likely that if an oven were to be built on a wood platform that cracks or other failures may occur.
Once a suitable surface has been prepared you may begin construction of the foundation. Material used may include brick, concrete besser blocks, poured concrete, rock or wood such as recycled railway sleepers. The materials may be laid directly on bare soil and an initial concrete foundation is not necessarily required if the ground is solid or the oven is temporary. Construct a four walled box as the foundation up to the height you would like the hearth of the pizza oven to sit. Ideally you want the hearth to be high enough that viewing within the oven does not require extensive bending, however not so high that placing of pizzas or cleaning is difficult. Once the box is complete you may create a surface as either a floating concrete level, or fill in the center of the box with soil, sand or another readily available material up to about 20cm from the surface. Upon this newly created cube you will want to place a small concrete foundation to provide a solid surface for the sand and then the fire bricks. You can either create your own concrete from sand, cement and aggregate (small rocks) or purchase premixed concrete in bags. Fill up the cube with concrete 5cm from the surface. You want to leave 5cm spare to insert builders sand to provide a completely flat surface for the fire bricks. The sand is critical to create a flat surface and to provide insulation to prevent heat from dissipating below the oven. If the inside of the foundation is to remain empty or used for storage, a floating concrete table is an option. This is accomplished by laying cement sheet across the void in the middle of the box, installing reinforcement steel oven the cement sheeting, and then pouring concrete to create a level surface. A temporary wooden dam will be required to contain the concrete.
The foundation lifts the oven off of the ground and provides a level area of concrete for the hearth and dome of the oven to be built. Generally a level of insulation is placed directly on the concrete foundation, upon which the fire brick hearth is built. The foundation and insulation are necessary to create the level surface for the fire bricks and to prevent heat from escaping below the oven.
The size of the foundation desired will be dependent upon the size of the oven built. Generally you should aim for a 8" - 12" (20cm - 30cm) gap around the base of the outside of the oven wall and the edge of the foundation side. This gap is aesthetically pleasing and useful as useable area while using the oven. It's important to take into account the expected size of the finished oven including the thickness of the inner oven wall, the insulation layer, and the final cladding layer. A common mistake for builders is to make the oven foundation too small for the final oven. The below table gives examples of ovens sizes and suggested foundation dimensions.
The bellow tables provide estimates regarding the amount of materials that will be required to create the oven foundation. The purpose of the foundation being to raise the oven off of the ground. In some cases with mud or cob ovens however the oven is build directly on the dirt ground. For an example if building a 90cm by 100cm foundation 3 feet off the ground out of builder's bricks, approximately 168 bricks will be required. You should confirm first with your own measurements before ordering supplies.
Standard hardwood used for construction or old railway sleepers are also options for building the foundation of the oven. Extra care should be taken however to determine that the material utilised and the fixtures used to hold the structure together are of an adequate quality so as to resist the heavy weight of the oven. Due to the extreme temperatures and hazard of fire from the oven extra precautions may need to be taken to ensure sufficient insulation is placed to protect the wood from overheating. This will be particularly important below the hearth.