Roofing can be of many materials, and certainly scavenged and repurposed roofing of any sort would be a great choice. We used painted galvanized steel roofing. Other materials include greenhouse plastic or even a tarp.
Whatever material you use, make sure the roof covers at least part of the chicken tractor. For the box-type tractors where the chickens never venture outside the box itself, leaving part of the roof open to the sun (covered in chicken wire) will make for happier chickens.
You can see that we left a little extra on the length of the roof cap, just to provide additional rain protection.