The roofs, they do nothing! AAAAAAAAAAAAAAAH! IT BUUUUUURNS!
You need at least a double protection for it to count IIRC. So a vehicle roof next to another vehicle roof (or door, or something with a roof). Elsewise it’s assumed that a fair bit of rain blows in through the side to get underneath the single roof.
For a cell to be inside, it must have a roof and all adjacent cells must have unpathable terrain (wall, door, window, quarterpanel, board, windshield, vehicle door, etc) or roofs. This is true for structures and vehicles both. The notable difference between structures and vehicles is that structures have roofs over open doors and windows, while vehicles typically do not (although iirc you can build a roof over your door or windshield). Inside vs outside decides whether you get wet in the rain, burned by acid rain, sunlight for solar power, light for reading, light-sensitivity penalties, glare, and so forth.
Always check if the tile you stand on is inside or outside.
To make any tile tagged “in” you must either build roofs around this tile or build impassable obstacles.
D - doors
T - tile with roof
When doors are closed - the tile is tagged “In”. When opened - “Out”.