Over our many years of designing, building and Supply - we have a 'basic formula' that appears to on the whole survives the British weather well. 

There are lots of ideas on the Internet and whilst ours is just another, we feel that by employing standard industrial products - rather than, random ingredients mixed together ie mud, straw lime etc - you can obtain a much much better result.

Lets break the oven down into the following layers, from the inside out:-

Cast Dome - minimum thickness 50mm, ideally 70mm of DC 1400
Insulation Layer - 50mm of Fibre, ideally comprising of 2 x 25mm 
Securing Mesh -  A layer of chicken wire to secure the fibre around the dome 
Final Layer -  Sand and cement render

After this layer you can decorate as you see fit.....

"Cant I just use clay?"

Something to bear in mind is that clay that does not reach a high enough temperature to become ceramic, will very easily return to its plastic (squishy) state, meaning that too much moisture in the air will mean a collapsed pizza oven.

"How many bags of Castable do I need?"

Int Diameter  Thickness
50mm 70mm
600mm 3 5
700mm 4 7
800mm 5 9
900mm 6 11
1000mm 7 13

All amounts are approximate and will vary on proportions of your dome - these figures are based on a perfectly spherical dome