Hi, I want to make the floors in my levels with custom plannar reflection inside the material I’m using. I know that the idea is rendering the whole scene upside down and simulating reflections that way. thanks 
You shouldn’t have to do anything custom since as of 4.12 UE4 supports planar reflections now.
https://docs.unrealengine.com/latest/INT/Engine/Rendering/LightingAndShadows/PlanarReflections/