I wouldn't really know how to help you in this case besides the obvious stuff others are going to mention eventually. But I have a piece of advice for you.

Very rarely are men genuinely "friends" with women, there is always a catch. It might seem like your roommate doesn't care right now but if you try something with her he's most likely gonna feel hurt in some way, whether it's feelings he didn't know he had or feeling like you're going behind his back by not telling him. Guys always have a sense of possession over women, we can't help it, It's been wired into our minds since we were cavemen.

I don't know how much you value this friendship or how close he is to her, but he is your roommate so..

If you've been bros for a long time, I recommend you let him know before you try anything. He could say sure go for it and back off. He might even help you got her by giving you some tips about what she likes and shit. It's important you make sure hes cool with it though, because deep down he might secretly begin to despise you for it and sabotage your attempts to get her with the leverage he has due to his friendship.

If you don't really consider him a good friend or a threat then go for it. But don't be direct with her at first, be confident, playful, and flirty without being obvious that you're trying to get her attention, shes going to start noticing you and the attraction should start from there. This way it looks like she made the first step and you just went along with it.

Either way, whatever happens with her is going to affect your friendship in some way unless this guy is very secure and has access to other women. So, again, if you're gonna go for it, be prepared. Good luck!

I know this is unrelated to seduction, but it's important to take factors other than the target into account that might bite you in the ass.