Solar roof structures can now be built entirely using Structural Solar. These are ultralight, high efficiency solar panels, which are integrated directly in a special roof structure. The structural part located under the photovoltaic panels is made of an Expanded Polystyrene Structure (EPS), weighing 2.5 lbs Solar roof structures can now be built entirely using Structural Solar. These are ultralight, high efficiency solar panels, which are integrated directly in a special roof structure. The structural part located under the photovoltaic panels is made of an Expanded Polystyrene Structure (EPS), weighing 2.5 lbs. per cubic foot. EPS contains a large amount of air, giving it superior thermal insulation properties, so it keeps outside heat from getting in and does not let cool or warm indoor air escape.