Roost and poop board details - suggestions please!

[Hopeful Peacock]

I am designing the roost part of my coop and have some questions on details.

First this is what I am going to have:
15 chickens, although I'd like to be able to squeeze in maybe 3 more in the future.
smallest will be faverolle, largest will be brahma (standards)
coop dimensions are 4 feet wide, 16 feet long, 6 feet high

Questions:
1) I want to have 2 parallel roosts. I am thinking 8 feet long. Is that enough? Would 10 feet long be better?

2) Are same height roosts better? Or stepped - one higher than the other?

3) The roosts will be parallel to a wall. How far should the roost be from the wall?

4) Is the roosts are the same height, how much space should be between the roosts? And if the roosts are stepped, same question?

5) How far should my poop board extend on either side of the roosts?

6) I have read the actual roosting surface should be 2 inches wide, 3 inches wide, and 4 inches wide. What do you think is best and why?

I think that's it for now. Thanks in advance!

 

[THINGUM]

#1 =1ft per chicken (15 chx=15feet)
#2= Same
#3=minimum 12"
#4=16"
#5=12" (and 8 to 10" BELOW ROOST)
#6= all will work, chickens don't wrap their feet around for balance but perch on tree limbs naturally --go figure

 

[kwork]

I have another question to add: How much headroom above the roost is ideal? Is 18" sufficient?

Also, why so much space below roost for poop board (boot tray)? I would think 5-6" would be plenty (but what do I know??)

 

[THINGUM]

18" is sufficient of they use a ladder to get there, but if they fly up then 24 is better.
I have read here somewhere that poop boards between 8 and 10 inches is a starting point. Consider that they may "splash" if closer and the fact that their "aim" is somewhat... erratic
All of this is Home Design For a Chicken, my friends remind me that "ya ain't buildin' a gran piana ya know!"