I run 10 GHash/S and have been worried about fire as well. I have melted many many things but thank god these things are designed not to burn.
Floor is tile and wall is concrete so nothing much can rally happen.
Mine is 8GH/s and it's a constant worry. I have the cost of the equipment added into the house insurance but I need to call them back to make sure they will cover such a setup running in the house.
The only thing I can think of is the CO extinguishers. Those FM200 units like they have in data centers and business server rooms are very expensive. I think I was quoted at least 8000 euros for 2metres x 2metres.