I'm going to be installing a paver patio over an existing poured slap patio. it is surrounded on two sides by brick wall, and a third side by pressure treat wood ( the side of a house build on concrete tubes ). The fourth side is the down hill side, and I would like to allow it to drain. Since there is something constraining on all sides, do I really have to cement the perimeter pavers down, or can I just install again existing surfaces.