I've got 7/12 steel on my shop that I put on myself. With washed clean soft rubber soles right after a rain, you can JUST hold your position on the roof if you don't move. Any other conditions you are always between slowly sliding and quickly sliding.
My sisters barn was sided with steel in the 1950's. We pick up all the nails every year and go over it with screws. The first screws I put into it were 20 years ago, haven't moved.
To the guy who wants hand torqued, you're nuts. I've used drills but the torque limiter think doesn't work well. Cordless impact by far is a better tool for reliable compression of the washer.
We have snow here, I installed everything on the top of the rib. It leaks less when sitting in snow, it gives the screws some room to lean as the 36 foot long panels grow and shrink with temp changes.
On the flip side, you have to be really careful not to overdrive and cause the panels to get out of square. You can steer a roof about 1/4" per 36" sheet without meaning too if you're not careful. Top of rib does make stripping unlikely to as it will kink the rib first.
Re green purlins, you have to use short screws in green wood. Or they will loosen as it dries. Same as drywall, long screws = more nail pops.
