Assembling a 197,000-square-foot rooftop to accommodate a living tapestry of native plant species is challenging enough. Add to that the technical problems posed by the roof's extreme dips and slopes. How to keep the plants and soil from sliding off? Rana Creek, who worked with Piano to design the roof, developed and patented a solution called the BioTray®. They used 50,000 porous, biodegradable trays made from tree sap and coconut husks as containers for the vegetation. These trays line the rooftop like tile, yet enable the roots to grow and interlock, binding the trays together like patchwork.