Andrew... I stand corrected. I had no idea that that an architect employed by the government would allow anything less than top grade copper to be used. The thinnest copper sheathing lasts at least 60 years even with acid rain conditions. As far as poor trades, it's pretty hard to mess that up unless that attachment is made with materials that will cause a galvanic reaction... in which case, the sheathing would likely fail much earlier. The usual grade for buildings such as the subject should last two hundred years minimum.