Yes the irony is laughable! Incidentally, I saw more collapsed roofs on large barns and outbuildings last winter than I’ve seen in years. Even metal roofs.
No surprise, metal roofing does not add strength to the assembly.
Nor does it add the extreme weight, compared to metal, of a standard composition roof.
The difference in weight between metal and composition roofing is pretty inconsequential.
Trusses are designed based on live / dead loads.
Typical dead load is 10 pounds per square foot.
Live loads based on the environment, so to speak.
The thinking years ago is metal roofing could be installed on purlins.
Eliminating plywood and felt paper does add up when compared to a comp roof. Still inconsequential in terms of a properly engineered truss.
On a home, plywood with ice and water shield should be used if you want to avoid leaking light fixtures. Especially if using AG panels.
