We've had a fox through (when I forgot to lock it one night) but not a rat. I've also just had to replace the flap part of ours because one of the cats got fed up being locked in at night and headbutted it so hard that the flap cracked and went flying into the garden. Found a gaping hole in the morning. Do you mean an unlocked cat flap? I would think a rat could get through if a cat can if it's unlocked. But if you have one that is lockable one way, it may do the trick. Not very helpful, I know! Sorry!