Well, irrespective of whether you can believe it
, rats CAN get into places through 1/2" holes. Not great big huge trophy-size rats, but young rats. All they need is a hole they can fit their skull through -- and the skull of a young rat is not real big...
IMO there are two different situations though -- one where you KNOW you have an existing rat population around, and another where you just don't want a rat population to *occur*. If you KNOW you have rat problems in the area already, I would go whole-hog on the ratproofing thing, preferably a poured slab for both coop and run and extra ratproofing measures all round. However if you have no particular reason to believe there are rats in the area, you just want to *keep* it that way, you can leave a certain number of things as "options for future retrofitting" if rats should ever show up. The biggest thing, if you want to be proactive, is to make it so they can't tunnel into your coop and run, since that is usually hard to fix later on.
Mice are far more common than rats, though, so if you
really don't want any rodents attracted to the chickens, you'll need 1/4" hardwarecloth everywhere, plus extremely tight carpentry. Eventually some mice will still probably find their way in but at least you'll have made 'em work for it
Good luck, have fun,
Pat