First Coop - A few roost questions

[phryan]

My coop is 9ft x 9.5ft (it was a preexisting shed on the property hence the odd dimensions). The current residents are 8 13 week old red pullets but there another 12 (11 females and 1 male) that will join them in time. I put up 2 temporary 2x4s for roosts at 2.5ft and 5ft, the girls only use the upper one (no surprise). Onto the questions. Is there an optimal height for a roost? Higher than the nest box to keep them from sleeping in them. Is there an issue with too high for them jumping, or getting into where it may get drafty because of the eves (walls are 8ft, eves are covered in hardware cloth, winters are single digits). Two of the girls have figured out how to get onto and sleep on the 2x4 top plates on the gable end walls, any way to discourage this or is it not an issue?