also, what breed of chickens will you have? Hard featherd birds roost's need to be from 5-6 foot tall, but birds that have trouble flying such as silkies only need roost as tall as 3-4 feet. the reason for high rost is because if the roost are the same height or smaller than the nest, the chicken will roost and poop in the nest, also it makes the birds feel safe from predators.
