From what I understand, Silkies are leary of elevated places due to their lack of flying ability. They also do not roost as most chickens do, and so far, the breeders I have spoken to, and viewed their coops online do not place roosts in their coops or runs for that reason. Ground nesters. Actually think that's pretty darn smart of them!
I don't have roosts for my Silkies but I have the coop raised enough for them to benefit from the shelter underneath the coop. They like the cool dirt for dust baths. I also like the coop off the ground to protect the wood. All our coops are raised because it can get muddy in this area when it rains so much. That means the Silkies have to go up in elevation slightly to get into the coop.