If you are the sole owner of the house, any rent you receive from the roommate is considered income. The only exception is if you rent the room less than 15 days a year. If you and the roommate jointly own the house, the roommate's share of the mortgage payments is not considered income.

See IRS Publication 17, www.irs.gov/publications/p17/ch09.html, where it says: "You generally must include in your gross income all amounts you receive as rent. Rental income is any payment you receive for the use or occupation of property. In addition to amounts you receive as normal rent payments, there are other amounts that may be rental income."

The rental income is reported on Schedule E. You may be able to offset some of the rental income with expenses associated with the rental and depreciation.