Hi, Im planning on adding a porch on my back door around 2.5m x 1.5m. Now at the minute the floor area is a concrete base. After drawing out a section of the new floor the current concrete base is the same level has where the sub base would be. So could I just leave it has it is and just put my sand screen on this and continue upwards or would I have to break this up to small pieces to create a granular Sub Base?