If I move this brush up and down, it stayes the same
But if I dupplicate this brush I get some invisible triangles that makes walls disappear
what could be the problem? Thank you very much for your help.
It must be something with other additive brush because after I deleted it my problem was gone. But I still dont know why only when I mad a new copy but with originial brush it worked normal.
So anyway this is what I am doing, block of flats from real life 
This is my second day in unreal engine so I am preaty noob ( I never even used any other programs before, but now I will try creating nice indie game)
![alt text][1]
