Composite joists are usually manufactured from wood, plastic and a bonding agent. In terms of performance, they sit somewhere between timber and aluminium. However, composite joists can fluctuate massively in quality and, in some cases, can be worse than timber. Check carefully and avoid poor quality composite joists at all costs as they can sag and fail in a remarkably short space of time.
Premium quality composite joists can benefit from a reasonable lifespan, and require replacing long after a timber substructure but before an aluminium frame.
Composite joists are weaker than aluminium joists, making them less versatile. They are also prone to expansions and contractions that can cause twists, splits and an uneven finish. Composite joists normally depend on brackets for installation. It makes them a little trickier to install.
In conclusion, aluminium decking substructure joists provide the optimal level of support for decking boards and are exceptionally easy to install. With a continuous fixing channel which guides the screws in and offers high pull-out strength, the aluminium substructure brings strength and versatility that cannot be achieved by composite or timber decking joists.