My partner had a similar problem with a bird enclosure and a local company recommended she use a fairly heavy mesh that worked well and solved the problem. The following Garden Zone 403010 1/2" x 1" Mesh 16-Gauge Galvanized Wire Fence available on Amazon looks pretty similar. It was fairly tedious but I left a little overlap on the inside and tacked the wire to the inside of the enclosure using 5 minute epoxy so the rats couldn't simply push it out of the way.
In the case of chickens maybe you could do something similar and leave straw over the top so they don't notice the difference. Regarding the mesh size the following source states this that seems likely:
The smallest diameter hole a small rat might fit through is an important consideration when rat-proofing an outdoor structure against wild rats, and when choosing a cage which will house baby rats. Generally, wire mesh with 1" x 1" holes is considered too large a spacing for young rats, while 1" x 1/2" and smaller dimensions are narrow enough to keep even the smallest rats inside. For rat-proofing an outdoor enclosure the recommendation is usually 1/2" x 1/2" mesh or smaller.
So you might like to consider 1/2" x 1/2" mesh if available in your area to error on the side of caution, but the larger size worked well in her particular case.