I have tried turning up the oven to 450-500 C but the pizza burns on the bottom right away.
im surprised here, so we just keep the oven at 300 C to insure we are serving decent pizzas for customers.
any ideas why the bottom of the pies are burning at 400? logically I can assume the floor is too hot? but doesn't make sense when woodfired ovens are at 400 - 500 C easy and not burning the bottoms..
Different materials have different thermal conductivity. The more conductive the deck, the faster it will burn the bottom, all other things equal. The higher the temperature of the oven, the lower the conductivity you need in the floor. Unfortunately there isn't a good one-size-fits-all material. If the conductivity is right at 500F, it will be too high at 900F.
Even in wood fired ovens, the conductivity varies quite a bit. The Neapolitan wood fired ovens, which are specifically designed to run at very high temperatures, typically have floors made from biscotto di sorrento which has very low conductivity. Some wood fire ovens have floors made from fire brick that is more conductive than the biscotto and makes it more difficult to bake at high temps. One member here built an oven with a fire brick floor, decided it was too conductive, and put another floor on top of the firebrick with lower conductivity.
It sounds like your deck is made from something too conductive for high temperatures. One thing you could try at high temperature is to slide a pizza screen under the pizza after the bottom gets close to being done. This should stop it from burning while the top of the pizza finished baking.