Rent-a-Room Allowance and/or income-less-expenses calculation?

Can I claim Rent-a-Room Allowance on my client's own home and income-less-expenses on the property she rents out? If the rental property makes a loss, can I offset that against the Rent-a-Room figure? I've searched on the HMRC website and can't see this particular scenario explained. If I do have to do the calculation for both properties on the income-less-expenses basis, how can I find out the interest amount paid during the year on the repayment mortgage for her home? The statement just shows total paid for the year, with no breakdown between interest and capital. I would be grateful if someone could give me some advice, please, or at least point me in the right direction! Thank you so much.