I built a small fort out of landscape timbers.It has plywood on the roof, and kids go on the roof to play. I want to know what is the best thing to waterproof the plywood, and not get damaged by the kids? Shingles are out because of constant feet on the roof. I have seen sprayed on bed liners for pick-up trucks. Is there such a product out there for roofs?

John, you could probably use the spray-on bed liner on plywood, however I doubt that it would provide much in the means of waterproofing. One thought that I have is to cover it with rubber, you could purchase some of the material that is used for lining landscaping ponds it's around 40-50mil thick and could be applied with an adhesive. This would provide both a waterproof roof and since it's rubber would be non-slip (so the kids don't get hurt).