within perimeter section 74 it is then able to once again collect debris and hold it until the next removal in the embodiment of fig5 removable screen section 84 is mounted on lower co planar surfaces of the support members as illustrated these co planar surfaces are the upper inner surfaces of the support members 88 90 92 and 94 as in fig4 when desired the support members may be inverted in relation to the installation in the other figures in such an inversion the formerly upper outer surface 96 of support member 88 becomes the lower outer surface and the fastening arrangment strip 106 is fastened to it instead of to an inner surface of the support member in that instance the surface 96 of the support member may either be flush with the bottom surfaces of the beams or may be recessed slightly into the perimeter section 74 so that the removable screen section 84 when installed in its secured position is substantially contained within the perimeter section whether the support members are secured as shown in fig5 or inverted the removable screen section 84 is adjacent to or substantially flush with the bottom surfaces of the beams this slightly recessed arragement has certain advantages for example it moves the adjacent outer edge surfaces of the hook and loop strips into areas protected from the direct blast of heavy.