Grates are 2 feet x 4 feet by 2-5/8" thick and are made of high-density polyethylene (HDPE). The openings in the grate are approximately 1 x 1-1/4 inches.
We have used this grate with 8" x 16" concrete block spaced as illustrated. In this application we have added as much as 400 lb of stone and still be able to walk on it. This is presented as a guide only. Your own application may or may not require additional support.