My roomie has shingles under his armpit, how can i prevent to get herpes zoster from him? (i had varicella when i was a kid)

You're in luck...: Shingles comes from within. A person who had varicella in childhood should have immunity to varicella, and should have varicella virus remaining inside some of his or her cells' DNA. Such a person should not be able to "catch" the virus from anyone else. However, the virus that is living in such person's DNA can form new copies of the virus, which travel to the skin surface to cause "shingles".

You can't get it: A person might get chickenpox from exposure to debris from weeping sores of shingles.( If they never had chickenpox) You cannot catch shingles (Zoster) from a person with shingles.