I have used car decking, or tong and groove as many sawmills call it as both the floor of a loft and the ceiling of the room below in log homes.The 2" thick (really 1 1/2") works well and can span up to 4' between supports although I try not to go that far. The only problem is that it is usually made from soft
wood, pine or fir that can scratch or dent when firnachure is moved on it without care.