Good morning Paul,
This seems most unusual for a Lender to reject an offer with a known roof replacement needed. So this leads me to believe that the inspection report and two roof replacement quotes weren't provide to the Seller's Lender. Do you know if your Agent provided the inspection report and roof quotes to the Listing Agent? However, if you simply submitted your offer on your own conclusion that the home needs a new roof then yes, it's unlikely the Lender will budge. You must have your inspection completed.
I've had this scenario happen several times in which the Seller's Lender reduced their approval amount once an inspection report and roof quotes were provided to them as they understand that the home will not sell for the originally approved price that did not take into consideration the need for a new roof.
The best to you and get the report and quotes submitted as this may be your answer.
Short Sale Specialist