What Joists Should You Use for Composite Decking?

When making design decisions concerning composite decking, your choice of joist material is all-important. Joists create the decking subframe on which you lay the boards or paving. Your choice of material will determine how durable your decking is, how it is installed and how it performs.

For instance, materials perform in different ways when wet or dry. Some can swell and contract as conditions change. However, others demonstrate relatively little variability. In this article, we explore what joists you should use for composite decking and analyse the factors influencing your decision.

Joists for composite decking

Typically, composite decking joists are manufactured from one of three materials. Below, we examine each in greater depth.


Aluminium joists are by far the best option for construction professionals who prioritise joist strength, longevity, reliability, and performance, but desire a lightweight, versatile material. They boast a fantastic lifespan and excellent durability, minimising the maintenance required over the product’s lifetime and effectively minimising the need for replacement.

aluminium joists
Mahogany Decking

RYNO’s aluminium decking joist is specifically designed to facilitate quick, straightforward installation and allow for various configurations and applications. It is non-combustible and complies with government fire regulations concerning external terrace and balcony constructions on residential buildings. Unlike timber, aluminium does not expand or contract to a significant degree.


Timber is the traditionalist’s choice of joist material. Historically, it was used to create the majority of decking subframes. Although relatively easy to work with and affordable, timber has several weaknesses, which is why other materials are supplanting it and becoming increasingly popular in modern construction projects.

For instance, timber degrades far more rapidly than either composite or aluminium joists, resulting in a much shorter expected lifespan than alternative materials. Timber decking joists for example can suffer from water damage, and begin to sag and warp, leading to distortion and damage to the decking surface above. This situation can leave you with a dangerous substructure and a costly remediation and repair project.

Timber joists require careful installation and regular maintenance. Just as crucially, timber is combustible and can pose a safety risk, and will not comply with the UK government’s fire regulations for residential buildings.


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.

RYNO composite decking solutions

For RYNO, versatility is a core component of the TerraSmart® Composite decking solutions. To learn more about our composite decking and aluminium joists, head to our Smart Decking Systems page or get in touch with the RYNO team to discuss your needs and how we can help you with your next decking project.

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