Roost "fails" when your HP is full, so I was wondering...
You remain a flying type.
If you Roost at full health, Roost will fail and as such the effect from Roost won't take place and you'll remain a flying type.
I run flying leads so I can get Tailwind up and I've tried to predict being outsped many times, so I've had this happen to me, I also tried it ingame just to double check.
When you use Roost at full health, you don't lose your flying type ability, no matter your health.
I hope that I was helpful!