I also use 2x4s with the 4" side up inside my coop. I have tree branches outside in the run with various circumferences - some of the chickens use the outside branches at times and some don't.
Here's my inside roosts...
Ignore the goats... they were just making sure the roosts were sturdy enough for the chickens to be on
Since this picture we've put a 2x4 across the ceiling at about a 7 or 8 foot height and that is the one that everybody wants to roost on.