Fossil-fuel and rubber-derived waste stream conversion to composite lumber substitutes or barrier members; the composites having material properties and uses of greater value than the solid waste stream components separately or together. Preferred combinations including waste materials derived from waste carpet, waste tires, and waste bituminous roofing shingles, all enormous problems for landfill disposal. In a range of formulation ratios, when combined with a binder, new and marketable products are made from solid waste. Improved resistance to rot, to water, and to weathering is exhibited in synergy with improved compressive and flexural strength, enabling production of a wide variety of useful and environmentally-friendly structural products, for example. Product weight and strength can be engineered to suit and may be structural members for architectural, engineering or agricultural use. Advantageously, the new products themselves can be re-used—by an end-of-life process for making more new products, achieving the capacity to make and remake multigenerational products from solid wastes and to reduce loading of landfills.