on other thing i woyld suggest is rather than pop rivets use tek screws, you can get meany flavors of tek there are stiching teks desinged for fastening tin to tin, as well as tin to wood, and tin to steel not just for cladding side sheets but also for roofing.
personaly if it was my shed and i had the room and the funds to do so, id move the back wall back, it's one (not very big frame), maybe 6 z purlins say 6 side rails. then 2 roof sheets and 2 side sheets per meter you go back oh and gutter and concrete
personaly if it was my shed and i had the room and the funds to do so, id move the back wall back, it's one (not very big frame), maybe 6 z purlins say 6 side rails. then 2 roof sheets and 2 side sheets per meter you go back oh and gutter and concrete