My three weekers did the same to get out of the rain. I'm so proud that they were able to figure it out for themselves. I did have to put one of the under a weekers in though. The other under one weekers just followed the big girls in.
I put them outside in the new tractor a week ago. The last 4 days they would go up and down the ramp all day to take naps, but would huddle up on the ground in a corner at night to sleep.
I picked them up and put them in the upstairs roost every night. Not what I call low chicken maintenance.
I guess the big black obelisk from 2001, a Space Odessy landed in their pen and they made an evolutionary leap.
Mine figured it out the day after I put them up there. When I put them up there they all jumped back down, but later that night they were all up there.
I think it helps that it's considerably warmer on the roost because it's closer to their lamp, kinda makes it more inviting.