Hello and welcome to BYC! 
Rats are nasty little things. Cork up every hole bigger than a 1/2 inch. Seal up every edge along the bottom of the coop either with hardware cloth or metal flashing, railroad ties are my favorite thing to use, deep gravel around the edges. Make sure nothing can dig under. Rats can climb so make sure there aren't any holes up high either. Some great links were posted above. Best wishes!

Rats are nasty little things. Cork up every hole bigger than a 1/2 inch. Seal up every edge along the bottom of the coop either with hardware cloth or metal flashing, railroad ties are my favorite thing to use, deep gravel around the edges. Make sure nothing can dig under. Rats can climb so make sure there aren't any holes up high either. Some great links were posted above. Best wishes!