Additional stringers fastened to the end framing and supported with wood in the middle run the length of the coop and serve as roosts. Laying hens usually prefer to roost if given the option. Meat birds or broilers typically don't roost. Pullets and cockerels begin roosting at about three to four months of age.
The collar ties, seen here and mentioned in the previous step also serve as additional roost space. Generally, you want to aim for 6 to 10 inches of roosting space per bird. Roosts should be at least two feet off the ground. For high roosts, make sure hens have a way to get up to them (that can be via a lower roost pole).
Roosts can be made out of recycled and repurposed materials (as can the entire coop, of course). Twigs and branches work especially well, as do old ladders. For movable coops, though, make sure that the roosts are fastened to the frame of the tractor securely.