I can agree that this is complicated and you are playing in a dangerous field, because of the danger to lose both of your friends. But how about you do propose to this girl and express your emotions for her. My guess actually is that she is expecting this; and if she is and you don't do anything, then you could again lose friendship with her. So if you do propose:
1) She might not accept a relationship, so you'll feel a little stupid for a day but it will pass.
2) She will accept, so both of you will be happy.
About your roomate? I think that as a real friend he will respect your true desires. He might feel a little strange in the beginning, but on the long term he will respect you more, and you probably won't lose your friendship whatever happens.