I'm sure this is pretty 101 for most, but I've just never done large bookcases before. I'm going to build some 7ft tall bookcases in the shop/garage and assemble them in place. The room they're going in is carpeted. Should I just set the on top of the carpet or what? Oh..three cases, one at 90* to the other two, in a corner. They will be joined and trimmed out, like built-ins, I guess. Thanks for the help!