JeninMN
The 2x4 flat side up is perfect especially for cold weather climate.
How old are your birds? If they are young as they age they generally become less aggressive with the flying off the top roost and use the ladder.
I would consider a Buff Orp. a heavy breed.
My highest roost is about 6 feet as well and the lowest is 30 inches from the ground with a total of 5 roost boards. I actually used a "form" I bought at Lowes for human stairs and this is what I use for roosts. I nailed one to each side of the coop wall and then nailed my 2x4's accross that.
I have Buff Orp, Silver and Golden laced wynodottes, Blk Aust. plus a number of other breeds I would consider "light" weight breeds.
All my heavies use the roost bar in front of me to get to the next lowest bar and then, of course, jump down from the lowest one.
My "light" weights all seem to just leap from where they slept.
With my system bumble foot is a rare occurance with my chickens. The occasion it does show I figure was going to happen anyway with or without high roosts.
Make sure to keep all sharp objects out of the coop and run. Bumble foot is actually a staph infection and enters the pad of the foot through small cuts and abrasions.