We built our coops with 1/2 inch wire floors due to a rat problem.  We added several inches of dirt on top of that. It worked great until I had to clean it!  No way to sterilize, standing water with chicken poop, yuck. So we added 16 inch square stepping stones (used red ones for country look) from wall to wall, that way there are spaces in between for drainage when cleaning.  I later discovered that throwing cheap hay all over the floor makes cleaning even easier. Bottom line is, with wire and concrete the rats have not gotten in. But it's not just the floor you have to worry about, rats can get in thru the roof as well, so we even have wire under the roof.  I can't figure out how to add a photo to this post so if you wish to see one just drop a note.
An added suggestion is to order the TomCat Rat Bait Station as featured in the Verminators t.v. show.  My husband ordered it online, don't know the site but you can do a search. It is a large black box with rat tunnel inside, poison cannot be reached by chickens or other pets. We put two of them inside the goat pen where the coops are, and we weighted them down with logs. We went from about 30 rats to 1-2 and holding.