A double joist is a type of framing that involves using two joists (horizontal support beams) instead of one to create a stronger and more stable substructure for composite decking. The double joist technique involves spacing the two joists closer together than standard spacing and placing them side by side. The composite decking boards are then secured to both joists, providing additional support and reducing the risk of sagging or warping over time.

The framework of a composite decking installation is important because it serves as the foundation for the entire structure. The substructure or framing provides the necessary support for the decking boards and ensures that they remain level and stable over time. In addition, the framing can help to prevent moisture damage, rot, and decay, which can compromise the structural integrity of the entire installation. Proper framing techniques, including the use of double joists where appropriate, can help to ensure a safe, durable, and long-lasting composite decking installation.