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Hi Paul, I wouldn't cut the SM ribs. I would cut pieces to fit inbetween the ribs then inside cut one to fit the studs and screw the two together. No weather leak problems this way. I would also use screws that they use for decks and use metal joist hangers as without side walls there's going to be more up lift on the roof for the wind to tear off. I would also put concrete footers in for the outside columns and bolt with a u-channel fixed to the concrete footers(Pier type). Your roof won't have enough weight to keep it on the ground without anchors. Also use roofing screws with neophrene washers so they don't tear threw the SM roof decking. Since each screw cost about $.25 to $.35 eh, I've also used regular SM screws with a 3/16" flat washers under the head to keep the roofing on. Use SM screws that have a hex head and about #10x 1-1/2" depending on what rib style of decking you go with. Put the screws on the high rib so water won't leak. 3" minmum lapp the roofing with 6" being the best. If you have a heavy snow load then go with 2x6". In any event I wouldn't span more than 24" on 2x4"
with 1x4 stringers every 2ft to cross screw the roofing too. 26ga Sm decking. The 28ga and 30ga won't last very long although it's the cheapest. You really have to watch buying cheap metal roofing as the galvanzied with rust off in a short time. Drive around and see who's roof has no rust then ask which brand it is. If they don't, know ask if you can look as they have a mfg stamp on them then buy the same. The cheap stuff will rust in 3yrs or sooner. In that 50ft span you might want to throw in a truss perlin made out of 2x4 and 1/2" plywood about 10" deep every 8ft or so. You might be able to free span most of the 12.5ft by using a header truss. T_Bone
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