Top Down Fog of War

I have been away from UE development, but looking to get back in. I have always wanted to develop a Top Down game that has a fog of war to it.

Not sure I am overlooking something, hoping someone can point me in the right direction.

Basically, I want to have my actor walk around a set of buildings, and when they walk into a specific building or room the roof or top is removed but not completely. Basically the fog would remove a portion of the roof as they move around.

Removing the whole roof is OK to do, but I would need the FOG to still be active to limit what can be see inside.

Any help on implementation or links I have missed to tutorials would be appreciated.