I can only add I'm lucky that the manufacture of my shop used a very heavy grade of mineral felt an stapled it down, what looks like every 1/2" .. mind for all the rain an a week of snow on it, it look a little green .. mossy ..
How cost efficient is it for a firm to come and tar/felt your size of roof, you'd have no down time, and other than a drip of hot tar, you could continue working ..
I've found in our other line, it's best to buy pre-cut print mounts off a firm, than for me to cut them and have odd waste pieces from a sheet.
Fibreglass was always used on slate or any roof to stop them from slipping off as nails failed, but for houses with it, I understand there was concern with fighting a fire in emergency .. but sounds good for a shop roof ... Oh the options.
Cheers David