My rental net income become negative number if I put mortgage interest in. Is this allowed?

As per our article https://turbotax.intuit.ca/tips/tips-for-renting-out-your-house-6375

"Unlike U.S. citizens, Canadians can’t normally deduct mortgage interest paid on their principal residence. The only exception is when you rent out part of your home. So if you have a mortgage, you can claim a portion of the interest as a tax deduction. For example, if you rent out 40 percent of your home, you can claim a tax deduction for 40 percent of your mortgage interest you paid in the year."

Hope this helps.

40% mortgage interest still make negative number. It increases my refund, is that allowed?