Hello, if the ceiling/floor overlap will flicker, please check whether the height from the top is 0. The minimum value of the height from the top in the custom ceiling and wall is 1 and cannot be 0. Because if it is 0, it overlaps with the original ceiling/original floor. 

Enter the ceiling custom, double left click the ceiling to get into the 2D design canvas, change the height from the top to 1.