Rats are extremely intolerant of either heat or cold, and are also extremely intolerant of dampness and/or very high humidity.
Besides the risk of them becoming infected by parasites, viruses, etc., from wild species that may contact them, you have to consider how you are going to keep them dry, and maintain them at tolerable temperatures.
Building a shed to house them is your best bet, and you need to make it vermin-proof. You should set up a heating system for the winter, and a cooling system for the summer. Personally, I much prefer to keep them indoors. As you've already been told: if their cages are cleaned regularly, and kept dry, they're almost completely odorless.
>>Hello all, I'm about to recieve a group of rats that I'd like to breed for my growing collection of snakes and had a question. My wife would prefer I not keep them in the house (because of the odor) and was wondering if it was possible to build a hutch (similar to a rabbit hutch) and keep them outdoors. They would not be in a separate building, just the hutch (which would obviously require measures to protect them from the elements.) Does anybody feel this is possible (or advisable) or would I be best to not even try this idea. Any help (and tips) for such a project would be greatly appreciated. Thank you, Scott Sullivan.
>>My website (still under construction)
-----
tricia