The main factor that may weaken the stability of the roof structure is dampness. On its negative impact it is particularly exposed rafter framing, which deform under impact of water. Therefore is important roof protection against getting in  water into its internal structure.

Typically, investors during the construction of the roof attach great importance both for building materials, as well as careful execution. Unfortunately, later very often they forget about the survey, and any minor defects  commission outsource cheaper and often non-professional roofing companies. This is a big mistake,

View topic commercial tin roof on older double wide I also think you should remove the old shingles and repair any water damaged decking. The metal roof as you describe should weigh less than b per sq ft, but it is. Ford Aerostar Accessories & Parts

A very important element that must be considered when the roof is projected, is ventilation, allowing drainage of excess dampness from under the roofing.