Have you considered that maybe your ventilation is the issue in the first place? It might be the reason Behind your replacement of the metal parts in the first place. We had the same issue with my house as the ventilation was very bad and throughout the years with the mold and humidity ,it was inevitable to change the whole roof. As you know you learn by mistakes so the next time I installed my roof I chose one of the best exhaust fans. It's cheaper to pay for a nice ventilation system than changing the roof every 10 years due to bad ventilation.