I sympathise with you!! A few months ago we had the same issue !!! Lots not just one though
We have solved it though it took a weekend and lots of sweat!!
We bought some extra strong small hole wire in rolls , we dug about a foot deep in the whole pen section by section, and covered the whole floor with wire, we used wire ties at every parallel join and brought the wire 1ft up the side of the pen and tied then nailed the edges along the wall of the pen, we try an backfilled all the soil, we had to drag the coop out so that we made sure we wired underneath that! Believe me that coop was heavy

, result? Not a single rats been inside for a month. On the movement activated camera we pick up rats roaming outside but they can’t get in!!! We had already ‘wired’ the walls and door thinking to avoid a fox attack, we hadn’t thought about rats!
And we have a winner! They identified one of the actual causes of the problem, paid the price in time and money, and won the fight.
The bad news? They missed a much faster and cheaper method of eliminating/controlling rodent populations. Someone already mentioned Howard E.'s posts on rodent control which are by far the best posts on this topic. Here they are with some of my comment at the end. Sadly this info gets posted dozens of times but the average poster refuses to do a forum search and get good advice on a topic that was solved long ago.'
Here is Howard E.'s past post:
"To the OP (and others like them), if you will spend the time, everything you need to know about rats and how to get ride of them will be found in the links below......
https://www.backyardchickens.com/threads/rat-control-the-video-series.1337456/
https://www.backyardchickens.com/threads/rat-control-101.1283827/
https://www.backyardchickens.com/threads/rat-proof-feeder-review.1180514/#post-18610285
This last one is a review of a rat proof chicken feeder built and sold by a BYC member, who is a staunch advocate for the plan of getting rid of rats by starving them out. Remove all sources of feed and they will be forced to move on or starve to death. If you are firmly against the use of poison bait blocks......for whatever reason.......then this is one of the best actions you can take. Bulk food in metal trash cans.....chicken feed in metal rat proof feeders. Can't get to the feed and birds do not spill and waste feed that the rats can survive on."
And the short version of Howard's post? Sanitation, exclusion, elimination.
Sanitation, bulk feed in metal cans or barrels with tight fitting lids, a treadle feeder, clean up the avenues of movement so the rodents have no cover to protect them from their natural predators. In my opinion and experience this is the quickest, surest, and cheapest way to solve a rodent problem.
Exclusion, plugging the holes and building a Fort Knox chicken coop and not leaving an opening for free range. Tough to do and expensive but it could work for rats. It did work for the OP of this thread.
Elimination, poison and traps. Problem is that rats are smart and will quickly learn to avoid both traps and poison bait. Were you to clean them out, the lack of sanitation would mean a new population of rodents would move right in. And there is risk and no end to the process, keeping fresh bait out. However, if you have done your sanitation using poison becomes effective as the rats are starving and will likely try the poison bait. Not needed though, they will leave in a few days as long as you are not feeding the rodents with a compost pile or other animal feed. Not all will leave, your area will have a natural carrying capacity for rodents, natural feed, but that natural ability to sustain rodents is quite small and the natural predators keep them in check and under cover as the rodents hustle to find this natural food.
Do a forum search on "rats chickens" and you will find most of the old wives tales exposed and read of long epic battles against the rodents. Sanitation, exclusion, or elimination all have associated costs but you are already paying for the feed and will recover the initial costs quickly with the first method.
Good luck and remember, it isn't just the stolen feed, disease and predators come with rodents.