Can I sue my roommate for unpaid rent?

Yes. You can sue your roommate for the unpaid rent even if you didn't have a written agreement as to how to divide the rent among yourselves.

Yes, you can definitely sue her for her unpaid portion. Actually, if both of your names are on the lease, the landlord is usually entitled to get the rent from either one of you without regard to how you agreed to split it.

Yes, you can sue for unpaid rent in small claims court. In January 2012, the limit for small claims actions was raised from $7,500 to $10,000 for most claims, and you can only file two claims over $2,500 per calendar year. Filing fees should be about $75. You can expect a hearing to be scheduled in about 30-60 days. Although a written agreement as to how the rent was to be divided would be helpful, it is not necessary. You can prove the arrangement through witness testimony or other documents like canceled checks. Evidentiary rules in small claims court are less stringent than in superior court. It will often come down to credibility of the parties and witnesses. You also need to think about how you will collect the judgment if you win. Good luck.

Absolutely. However, understand without a written agreement you are going to have to convince the judge there was an agreement b/t you regarding splitting the rent. One other way is to show that initially there was a pattern of conduct that he was splitting it and then due to circumstances you started paying it with the understanding that he was going to repay you. Small claims court is the best place b/c if you were to file this in superior court you would have to deal with some statute of frauds issues regarding any interest in real property must be in writing. However, the fact that you are both on the lease is good evidence that the agreement was to split it.