It is a pretty small shed. So, I am guessing I will just vent the shed as a whole. I was planning on cement boards (or some sort of heat resisting layers...) for the wood floor. Any particular brand, thickness works better than others?
Is the shed made of sheet metal? If not, what materials are used for the walls and ceiling, and how far your kiln will be from those surfaces?
If the shed is small, the air there will become very hot during firing, so you need to provide a good ventilation. Something like open doors with or without exhaust fan, etc. Just make sure that there is a way for fresh cool air to freely enter the space if your exhaust fan sucks hot air out of it.
I must say that if your kiln rests on a kiln stand, and there is a good air movement, igniting wood floor will be next to impossible, in my personal opinion. If you don't have a stand, you can use a couple of concrete blocks.
However, to be safe and follow official guidelines, you must rest your kiln on a fire-safe surface. Building a large and thick concrete slab in your shed would be a bit ridiculous, so you can use a sheet or two of HardieBacker, HardieBacker covered with aluminum foil or galvanized metal to reflect heat or something similar ( http://www.homedepot...0023/100170507# ) Any available thickness will work for fire protection, so base your choice on structural considerations/convenience. What worries me more is a chance to trip over the edge of the board, so think of it when planning how to lay it.
You can also make a pad using concrete blocks or bricks. But whatever you do, make sure that the kiln rests on a very stable foundation and there is absolutely no chance of rocking and tipping.
Again, I, personally, don't think the board or pad are necessary at all: the kiln will be well away from the floor, the heat rises, you will ventilate the space, so... But many (incl. your fire department and insurance company) will definitely disagree with me, and it's always better to be safe than sorry. Being anal retentive, I'd also use something non-combustible if I had a wood floor. This is despite the fact that I don't think it's really necessary.
Here is a couple of links I found for you: http://community.cer...a-wooden-floor/