I am not familiar with your exact scenario but in general yes the roof would need to be replaced unless a roofing specialist was willing to state that the whole roof has 3-5 yrs of life left. If the roof is bad and at the end of its life and the seller is not willing to replace the roof you can always look at a renovation loan for the property which allows you to close As-Is and have money set aside in escrow for the roof to be replaced after closing!
Please contact me if you would like to discuss your situation further.
Mandy Fritsche, Prospect Mortgage, Mortgage Loan Officer, NMLS# 557211
262-327-5700 Cell, 877-868-9198 Fax, firstname.lastname@example.org