Am going to put a concrete floor down in my old out house. Their is brick floor already in there, which has been their about 100 years and has become uneven over that time, but is quite stable now. i am going to put my floor over this. The area is 3 metres by 2 metres. Can i just use balast to level this out or do i need hardcore? As the lowest point is 3 to 4 inches lower than the highest in some areas? Dont want to use a wacker if dont have to.