We have 2 closets in our basement that had no shelving. I thought about installing flat pieces of plywood as shelves but the span of 48 inches was too much for that. I cut the plywood into 2 and a half inch strips and turned them on edge. I made runners on each end of the strips to hold them on edge to form shelves. This had great structural integrity and can easily hold 500 lbs or 230 kg.
