At 5 months, they may not be mature enough to want to go to roost.
I've had some groups start roosting at two months and some not till 6 months or later.
I'm wondering what breeds of chickens you have that they would need heat lamps?
Perhaps they don't like the heat in there.
Also, I can't image enough ventilation coming from slots.
If the heat emitters are in there for heat/comfort, what happens if your power goes out. They need to become...
I wouldn't worry if they are predator proof.
I have had plenty of hens and roosters sleep in trees over time in much harsher climates through rain, snow, wind and temps down to zero. They had the option of a dry well ventilated building and just suddenly opted for the better ventilated trees.
I...