Thanks for your fantastic question. Edging is a very important aspect of Natural Paving installation system. The minimum edging we recommend is 100 x 25 rough sawn H4 treated pine for Clutha-garden grade, and 100 x 75 rough sawn H4 treated pine posts for Waiau-Car grade. It is a more substantial lump of wood because cars should be able to run over it without damage. Both these sizes are commonly available and economical. The edging is placed at a level 20mm above the top of the plastic grid. It is there to keep the stones at the edge from rolling off. There should be 15 to 20mm depth of stones covering the grid, not more and not less.