The old roost debate. You’ll find some people have pretty strong feelings on either side of this one. The reason you see some people use tree branches or round things as roosts is that they work, even in cold climates. The reason you see some people use 2x4’s with the narrow side up or 2x4’s or 2x6’s with the wide side up is that they work in all kinds of climates. They all work.
Regardless of what they are roosting on, when a chicken squats down in cold weather on the roost, they fluff up their feathers to trap air inside to provide better insulation. Their feet are covered by those feathers, whether they are on a flat surface or wrapped all the way around a smaller tree branch.
If you elect to go with sawn lumber I suggest you round off the corners a bit. Sandpaper works great. It’s not so much to make those more comfortable to the chickens, it’s to remove splinters.
In my opinion with this issue, Bill Murray got it right in Meatballs. “It just doesn’t matter!” “It just doesn’t matter!” “It just doesn’t matter!”
But did I mention some people feel strongly about this, either way.