Mine are at 30", simply because that was the size of scraps I had around at the time. No ladder, but about 6' of horizontal space for them to flap down. If they didn't have this space I would provide a ladder or a couple of jumping down shelves or something. It doesn't happen real often, but chickens definitely do injure themselves coming off high roosts, especially when they don't have a lot of space by the roosts. I believe it happens more often to large roos. I really think they can get down from a hay loft without difficulty if there is space in the barn to soften their landing by flying down. Baby chicks can, certainly.
Mine are all at the same height because I read they will compete for the highest roost, which goes to those at the top of the pecking order. I didn't want to encourage this, but now doubt it makes much difference, as they're going to have a pecking order anyway.
There are a couple of places in my coop away from the roosts where they can sleep up off the ground, besides the nests. All are more or less the same height. Sometimes one or two will choose one of these spots for a while. When my last group of chicks got near full size, they were hanging around together, off from the group, and would sleep in one of these spots. One of the many advantages of extra space in the coop.
Mine are all at the same height because I read they will compete for the highest roost, which goes to those at the top of the pecking order. I didn't want to encourage this, but now doubt it makes much difference, as they're going to have a pecking order anyway.
There are a couple of places in my coop away from the roosts where they can sleep up off the ground, besides the nests. All are more or less the same height. Sometimes one or two will choose one of these spots for a while. When my last group of chicks got near full size, they were hanging around together, off from the group, and would sleep in one of these spots. One of the many advantages of extra space in the coop.