You roll when you lie down and move from your front to your back (or the other way around). If you keep rolling you can travel quite some distance. You roll a ball or something else round when you push or pull it along the ground and it moves by turning over and over without losing contact with the ground.
A roost is a place where birds go to sit, or the act of birds sitting down to roost. The chickens in the picture are roosting.
