Once they get a little older (mine were about 10 weeks when they started) chickens instinctively sleep on the highest thing they can find. Sleeping at night is called roosting and a roost is what they sleep on. If you have open rafters at the top of your coop, they will find their way up there.
I use tree branches as roosts. Many people use 2x4's with the corners rounded off a bit. You need at least 8" of roost space per chicken. I prefer 12" myself. They don't take up that much room and will normally sleep squeezed together, but they need the extra room to get up on the roosts with their wings spread and some room to sort themselves out and determine who gets to sleep besides whom. Those higher in the pecking order get the best places.
Since chickens poop a lot at night, you do not want them to roost in the nests so the roosts need to be higher than the nest boxes. Many people put droppings boards under the roosts to keep a lot of this poop out of the litter and make it easier to remove from the coop. You do need to clean these regularly or rake the area under the roosts or it will build up quickly.
The roosts need to be clearly higher than anything else in the coop yet you want them to be as low as possible. Mine are about 4 feet high. Some fly straight up to them, but most hop on the nesting box top then hop over to the roost. In the morning, they all fly down without using the nesting box as a step. One reason you want them to be as low as possible yet higher than anything else is that the chickens can hurt themselves coming down in the morning, especially the heavier breeds of chickens. It is unusual but it can happen. Also, the higher the roosts are, the more horizontal area it takes for them to jump/fly down. They are pretty clumsy fliers and can hit walls, feeders, waterers, nest boxes, whatever, on the way down if they don't have enough room.
I suggest you consider making your roosts removable. They can get in the way when you are cleaning the coop or working in there. I drilled holes in the ends of my roosts where they rested on the support and holes in the support, them use 5" nails as pins to hold the roosts in place. This has come in handy a few times.
Yeah, a roost is where you want your chickens to spend the night. If you provide a roost that is higher than the nest boxes, it will keep them from sleeping and pooping in the nest box. Since chickens do not grip branches well while sleeping, the consensus appears to be to have a 2x4 with the 4"side flat for average roosting material. Adding a droppings board under the roost will make cleaning the coop much easer...just scrape the droppings board every day or so and the main coop can go a looong time if using deep litter
Hi PTG! They are getting big now; the senior hen has a serious red comb and wattles and is definitely in charge. The other three are coming along nicely and are almost fully feathered.
I just had to go break up a roost order discussion out there a few minutes ago...there were really raising a rucus, and when I turned off the coop light, it sounded like the Waltons out there saying goodnight! They sang to themselves for about ten minutes and then quieted right down. I have a 40 watt bulb in the light fixture and I put it on for about an hour after sundown since it's still getting dark pretty early. It won't be long till it's still twilight at 10 PM and I won't need the light till fall.
They are up on the roost every night now and the poop board seems to be working pretty well. It only takes about ten minuets to clean in the morning and they are all facing the same direction most of the time. If they let one go off the front of the roost it all falls in a neat row that isn't too hard to retrieve and transfer to the compost pile. Piece of cake really, and the chickens are a lot easier to clean up after than my ferrets are...