You have a great idea there. We have built a few of them. The best thing to do is not to skimp at all. Buy good quality and it will last a lifetime. You will be glad you did.
There are a lot of good suggestions here so far. And a drain is imperative. However the drain needs to encompass more than just the floor. It must also encompass the roof of the safe.
IMO you are building your vault way too large. Unles it is meant to be a "secure room". However I am not that paranoid. We use ours for our firearms and valuables only, so it can be much smaller in size. By making it smaller you can afford better materials.
There are two things you need to protect your valuables from in a fire. That is heat damage and almot importantly-water damage. That is why we put a "drain lip" on the top of our vault-so the firemans water will flow away from the vault-and you want to elevate the vault because the fireman will put a bunch of water in your home during a fire. Whether you like it or not.
Also you need to make sure that you build your vault on an ouside wall. The fire does not burn as hot on the exterior walls-it burns considerably hotter in the center of the home. And for a longer time. Usually a lot of vaults can take the heat, but only for a limited time, so building on the outside wall will lower that "hot burn" time. Tom.
