You might be able to get away with sistering the rafters from the inside. The bowed rafters can even be unbowed at that same time. I have TONS of experience with doing this. Lets see some pics of the attic. As for professional roofers wanting to push rafter work off their plate - SHAMEFUL. Sometimes when doing a reroof, the total picture of whats going on will not be obvious untill the tearoff is well underway. This is especially true for flat roofs that dont have attics or easy access to inspect beforehand. Since the roofer wont want to deal with bad rafters, we need to make arrangements ahead of time with a carpenter to be on call and ready to change horses in midstream with the roofer in the event rafter repair is needed. I'd rather have the a roofer that understands and can do the whole roof job - including rafters.