I have a heavy breed (Orpington) and a lighter breed (Barnevelder) and my main roost is at 4 feet, with a step-up roost at about 2.5 feet. They only sleep on the taller roost, but they benefit highly from the lower roost as a jump-off point to get to the taller roost, as well as to help them maneuver and get into position. They like to arrange and rearrange themselves before bed, and if they only had one roost, they'd have to either step on top of each other to change places, or hop all the way down to the floor and all the way back up. Jumping from the floor up to a roost that already has chickens on it results in the chickens already on it being knocked off, and chickens stepping on others once on the roost if they want to change places causes drama. With the lower roost though, they'll just step off the top one to the lower one, walk over to the spot they want, and step back up in that exact spot, without falling off or knocking anybody else off, and without drama, fighting and tension. I have a camera in my coop and have watched them arrange themselves for bed. They do this a lot - step down, walk over, step back up. Until they're happy with their arrangement. I've heard that there can be a lot of drama while they get ready for bed, and I guess if they don't have a convenient way to arrange themselves, it could happen, but mine are very peaceful at bedtime and don't squabble, because they can rearrange themselves easily. So I recommend an auxiliary lower roost for that reason.
Here's what the roosts look like. The third one at the very bottom is their "baby roost" from when they were chicks. I left it in there because they like walking on it, but it's not really necessary.
.They were going to hurt themselves eventually jumping down like that. I keep my nest boxes at ground level so there's no issues there. They seem just as happy with the lower roosts.
