I was going to put it on concrete blocks, but from what you say, it will probably need to be higher? So like 2 blocks high? My dad's dog is a good 'ratter', so as long as she can get under it, it should be okay.
Oh, well, if you have a good rat terrier or similar, then clearance under there is somewhat less of an issue -- at least as long as the dog can get under there they won't *live* there. You still want to make it as high as you can (without creating structural or tipping-over problems!) so that rats feel exposed and out-in-the-open under there, rather than feeling concealed enough to sit and gnaw a while.
And by 'cover the underside of the floor with hardwarecloth', do you mean on the outside, underneath? We also have tin (?) that is used for siding/roofing, is that what you mean on the 'wide steel or aluminum flashing'? How would I put that on, underneath as well?
There are lots of ways of doing it; I can tell you what I've seen done that's worked reasonably well, but by no means do I know enough about rats to have a Very Best Solution to recommend. (Other than a concrete slab. Yes, big scary 'city rats' occasionally enlarge holes/cracks in concrete by chewing, but this is not something you usually have to worry about in the normal world
)
One thing I've seen done that's worked well in a tackroom was to take up the flooring and put hardwarecloth down and then replace the flooring, so that the hardwarecloth was sandwiched between the joists and the plywood. It doesn't give you as good a joint between plywood and joists, obviously, but that plus a flashing-covered ratproof sill on the doorway succeeded in keeping rats out of a tackroom that had had an ongoing problem. Or you can put it on the undersides of the joists, but if you are going to want a dog going under there to chase rats that would significantly decrease his headspace so I'm not sure whether it would be the most appropriate for your particular situation.
I've seen it work to tack wide steel flashing (aluminum would probably work too) all around the bottom chewable portions of a wooden grain bin, and around the chewable portions where the lid closes. You could do this with a floor, if you had sufficient amounts of wide flashing lying around, in the same way as the hardwarecloth described above. If you were retrofitting an existing coop, and it was high enough for you to get under, you could probably run flashing along the strips of exposed floor (the underside) between joists, preferably bending the flashing to go a few inches down the sides of the joists too.
I've also seen rat damage covered up, and further rat damage mostly prevented, by putting flashing around the bottom edges of walls where rats were chewing their way in.
I do not think you could substitute metal roofing for the flashing in the above scenarios, because it is ribbed. however there may be a way to adapt it to your needs, if you think about it.
The other two things that really helps are to a) clear ALL WEEDS, BRUSH AND TRASH (and I really mean *all*) away from the coop and other buildings for a distance of ten feet or more. Even just a *little* bit of cover really attracts the rats. And b) no food in the chicken run -- keep their feeder indoors, and if you feed them kitchen/garden scraps, feed them in the coop unless you are positive the chickens will clean it all up by evening.
Good luck, rats really suck,
Pat
