Chickens tend to roost on the highest perch they can find. If you want that board to stay clean, offer them a higher roost in another location where you don't mind the poop falling.
I have a 2x4 in my run for my birds to roost on during the day. They go up there to preen in the sun or to just get away from each other sometimes. I keep a wide putty knife in the run to scrape it down.
It's pretty much a given that birds will roost and poop so you just have to give them a place to do it that is more acceptable for you and at least ( if not more) desirable than where they are doing it now.