If you have any large breeds of chickens, my experience says to NOT use the wide size of the 2x4. Round dowel, round/thick branch, narrow side of 2x4 all are fine.
Here is my thread on the very bad (still not completely healed) breast blister on my nearly 11 pound (11 month old) Black Jersey Giant. The breast blister was caused by the size/shape/weight of the BJG rubbing on the wide roosts. The thread details it all, with pics. Later, we had a conversation with the very experienced owner of the hatchery where we got the BJG. He said, without a doubt the wide roost caused the breast blister.
https://www.backyardchickens.com/th...ed-thing-on-roos-chest.1292202/#post-20988160
If you have regular size to bantam sized breeds - I don't think roost width will be very impactful - certainly it is unlikely to cause major problems. My recommendation of a round or narrower roost is for the very large breeds of chicken. My BJG appears to be a good representation of the breed, and standards say a full grown BJG rooster will be a minimum of 13 pounds. At 11 months he was nearly 11lbs and they grow for about 18 months, so I'll guess he will make the weight by 18 months. We turned all but the bottom roost to the narrow side of the 2x4. His breast blister is healing, partly helped by not having any friction on his keel bone.
Here is my thread on the very bad (still not completely healed) breast blister on my nearly 11 pound (11 month old) Black Jersey Giant. The breast blister was caused by the size/shape/weight of the BJG rubbing on the wide roosts. The thread details it all, with pics. Later, we had a conversation with the very experienced owner of the hatchery where we got the BJG. He said, without a doubt the wide roost caused the breast blister.
https://www.backyardchickens.com/th...ed-thing-on-roos-chest.1292202/#post-20988160
If you have regular size to bantam sized breeds - I don't think roost width will be very impactful - certainly it is unlikely to cause major problems. My recommendation of a round or narrower roost is for the very large breeds of chicken. My BJG appears to be a good representation of the breed, and standards say a full grown BJG rooster will be a minimum of 13 pounds. At 11 months he was nearly 11lbs and they grow for about 18 months, so I'll guess he will make the weight by 18 months. We turned all but the bottom roost to the narrow side of the 2x4. His breast blister is healing, partly helped by not having any friction on his keel bone.