My BR turkeys roost in the chicken coop about 4 feet up with the chickens. But when outside, they prefer to go higher -- I'm sure 8 feet would be fine. Because how would you build one higher than that anyways?
My turkey decided that she'd rather roost in one of the trees in the wooded part of our yard. She was roosting up on the deck railing that is about 18 feet off the ground, but decided she liked the cover of the trees instead. Every morning, just before sun up she comes up to the deck and starts calling for her chicken flock to come out and play.