He might be right about the ventilation IF your space between the finished ceiling and roof deck is stuffed solid with insulation. If there is airflow, you should be able to accomodate with eave and ridge vents.
I have a finished attic, and I’ve had more than one roofer tell me that most likely in this case ridge venting and soffit venting will not help. Is this true? and how would i ever know whether the space above the ceiling is packed with insulation (I didn’t finish the attic). There are crawl spaces where I can directly access the roof deck.
The soffit venting is no small expense, but my understanding is that my Certainteed warranty will be void without proper ventilation. also, that ridge venting is not very effective without the soffit venting. should I just forgo each of these?