Chickens can be quite sensible when it starts to rain and most chickens are relatively hardy but you will still need to give them some form of shelter in addition to the hen house so they can shelter from the wind and rain, particularly if they are kept out in the open where there is little natural cover.
Experienced birds will run for cover if they think it’s going to be more than a light shower and will often huddle under a hen house or bush to keep themselves from getting too wet. There are however exceptions to the rules.
Chicks that are old enough to be outside and young birds in their first year do not usually have the sense to shelter, they will stand around getting wet. Other birds that haven’t learned to shelter from the rain are Ex-battery hens as they have spent so long inside sheltered buildings and to be fair to them, they don’t really understand what rain is as they have never experienced it before. Some breeds / hybrid chickens can handle the wet better than others and are more hardy in the poor weather conditions but breeds like the Poland (the one with all the head-gear) or the Silkie can suffer and need to be either kept in a covered run or you will need to keep an eye on them.
Birds that get soaked through can be at risk of hyperthermia. Their body temperature soon drops, especially if there is added wind chill factor involved.
It is a good idea to provide some form of shelter from the rain and the prevailing wind if possible for your chickens. Many birds will either not use the hen house, or, due to their position in the pecking order will not be able to go inside when the higher order birds are just inside the doorway.
Shelter can be quite basic, corrugated sheeting over a run, or a house lifted 40 cm off the ground to provide room underneath for birds to shelter, or even an old fence panel leaning against a wall or shed. Whatever you choose, ensure it is secure and won’t blow away and try to provide some protection from the prevailing winds.
found this article at https://poultrykeeper.com/housing-chickens/sheltering-chickens-rain/