If a new roof is really "needed" and your credit score is an issue, then you take any loan you can qualify for. If you have sufficient equity in your home now, it shouldn't be an issue to get a loan for at least 80% of that equity in the form of a HELOC. But you can expect the interest rate to be up there in the clouds somewhere.
Generally, the current mortgage holder on your home has a vested interest in your property - especially if you still owe quite a bit on the mortgage and the current roof is in really bad shape. So that's probably your best bet. (But not best deal necessarily.)
