I want to store some plastic bins in an attic crawl space and have been trying to figure out how to calculate the pounds per square foot the space can handle. I have measured the distance between joists (24" oc), the distance between support walls (12'), and the joist size (2"x4"). I don't know the type of wood, but the house was built in a suburb of Pittsburgh, PA about 50 years ago. I laid 1/2" plywood over the joists. I have 23 storage bins. Twenty-one of them are 31"Lx15"W with a mean weight of 52 lbs (range 23-80 lbs). Two bins are 40"Lx20"W and weigh 115 and 120 lbs. I have 6 cardboard boxes (mean 31 lbs) that I also would like to store there. The total weight of all storage boxes is 1567 lbs. I want to stack the small bins in pairs (weight range 96-115 lbs) on the plywood between joists about 2 feet off the attic center on either side. There is a support wall under the attic center. The large bins and cardboard boxes will be stored without stacking. I have looked at span charts and other references online, but cannot determine the actual psf this space can handle. Can anyone help?