Structural underlayment support system and panel for use with paving and flooring elements

An underlayment support panel for supporting a plurality of paving elements, the underlayment support panel having a core, a top side, and a bottom side, the panel being made from one of an expanded polypropylene bead material or an expanded polyethylene bead material and configured as a water impervious layer of the paving system, the core defining a recovery characteristic such that a deformation from a concentrated compressive load applied for a short duration onto the top side returns the top side to a generally planar condition, the core further defining a first thickness, at least one of the top side and the bottom side including a plurality of projections, the projections having a height extending from the core and defining a second thickness, wherein the ratio of the second thickness to the first thickness is in a range of 5 percent to 25 percent.