butadiene additives such as coloring agents strength enhancing agents or friction modifying agents may be added to the rubber referring to fig7 a c and 8 the rubber grommet 24 is disposed about the perimeter of at least one retaining wall 40 of the tray 12 the rubber grommet 24 advantageously provides support and impact cushioning in the lateral and longitudinal directions with respect to the tray 12 this is particularly useful to prevent damaging of fragile flooring materials 22 when they are joined together the grommet 24 resembles grout and grout joints because when the floor structure is assembled the rubber grommet 24 engages a region of the flooring material 22 on an adjacent tile preferably the rubber grommet 24 is sufficiently resilient to maintain its shape when a bare region of flooring material 22 is placed against the rubber grommet 24 as detailed later herein as best depicted in fig4 and 7 c the rubber grommet 24 and the rubber matrix 20 are a unitary piece the region of the rubber grommet 24 surrounding the perimeter of the retaining walls 40 along the tray lower surface 28 forms an outer edge of the rubber matrix 20 the monolithic matrix 20 and grommet 24 unexpectedly provides enhanced cushioning absorption of footfall impact and noise and eliminates shifting of the new floor because energy transfer is limited to between the rubber matrix 20 and rubber grommet 24 instead of to the floor structure unit 10 and flooring material 22 furthermore the monolithic structure eliminates debris from reaching the sub floor 28 if any debris invades the flooring unit 10 the debris is trapped between the region of the rubber grommet 24 surrounding the perimeter of the retaining walls 40 that forms the.