Sorry, now I know what you are asking..... ducks do not need roosts. They are low laying and sleep on the ground (most of the time unless in the wild). That's a BIG expensive building. How many ducks are you planning to keep?
i was just courious how many it could hold it will be a while away i would keep ducks and maybe 10 years later chickens it would be easier for me to just get that i would first get mini appleyard i am not good at building things lol but how many would it hold the ducks would just ignore the roosts
12x40= 480sqft so if they were not going to get to free range during the day you could hold 5 pair (11 birds or so) rule of thumb 100sqft per pair if that is permanent pen. But if they get out during the day you could put as many as 24 in there just for the night.