I'm shacked up at a warehouse on the main street in Muldraugh - used a sledge to destroy the railing - then built flooring - and it's raining inside despite having a roof. I had print screened out of pause menu w/e - but in the first shot you can still clearly see the bounds of the rain -- hadn't noticed anything like that with other weather patterns though. Also - just to clarify, my guy does get wet in that area.
Edit: Also - just remembered - earlier in the playthrough, I had placed a T.V. in the floored area - and no matter where it was placed in that area, it wouldn't receive power - but if placed on the pre-made floor, it would.
Edit: Tried removing the final railing and some floors to see if that had an effect -- also tried making flooring on the roof - in case it wasn't registering the roof -- well the final screenshot has a roof made over it - floor tiles placed on the existing roof - and you can see in the SS that any tile that had been removed is counted as inside the building - where ones that have floor tiles - even rebuilt after removal of railing - are counted as outside