You can treat the amount received from your roommate as shared expenses and not taxable income.
If you want to consider the amount received as rent, then it would be a not-for-profit rental which is covered in IRS Publication 523 on page 16.
Otherwise any amounts received from your room-mate is shared expenses and not reported on your federal tax return.
You can still deduct the mortgage interest and property taxes as itemized deductions on Schedule A of the tax return as the owner of the property.
