I used to worry about things like racks and half racks and floor load limits, but consider the following: Think about how heavy a rack is. Probably about 500lbs or so for most of our stuff (I'm not talking maxed-out S/390s or VAX 9000 installations), and there's probably one or two in a room. For the area of the rack, the load/sqft is similar to a 250lb person, which the floor is hopefully specced for. A small piece of 3/4" plywood underneath to keep the carpet happy is nice, bigger one if you're going to be pulling the rack in and out for connections to the rear. n.b. I do put my heavy stuff near the bearing walls so things are more straightforward.