[dieseldawg142]

the biggest prob with screwing to sheathings is you have no holding power in the wind. plywood and osb will NOT keep your metal roof down in a wind storm. screw will rip right out of it like nothing.
i don't think you will find a roof metal manufactor anywhere that will tell you to do this. metal roofs are waaaaay more suspectable to wind and wind uplift/shear than any other kind of roof. this is why you strap onto your trusses, these boards are now what you will screw into.....they also give your roof a "breathing" room. metal will out-condensate ANY other kind of roof, this is why underlay is also MANDATORY on tin roofs, unless it's your UN-HEATED outhouse...(and on a hot summer day it still will...)
and despite what may be posted, the BIGGEST and by a far margin, the most common thing for any roof failure (especially metal) is lack of knowledge/workmanship. i do 2-300 hundred roofs every yr, times 38 1/2 yrs, from half a square public toilets to thousands of squares malls, to 800 ft hi-rises, to foundations hundreds of feet underground, to jobs where you are hanging by your 1/2" lifeline all day hundreds of feet up. i can honestly tell you i have seen it all in this trade.....4th gen roofer in my family. i have 2 TQ tickets in my trade, Master Sheet Metal accredited,RCABC certified as well as many tickets and certifications from Manufactors, such as the Soprema Platinum warranty, they are confident enough in my ability to install their product, they give the building owner the Platinum Warranty--25 yrs.
i'm not going to go all religious here, all i'm saying is if you don't feel comfortable doing it, dont do it. if you want to, i stand by what i earlier suggested-CHECK your local building codes, shop around and read as much info as you can. start with the manufactors specs first, they KNOW better than anyone or anyone's opinion on how they want their product installed.......