Aluminium or composite joists – which provide a more stable deck?

What function do joists serve beneath composite decking boards?

Joists form a kind of skeleton for the terrace. The composite decking board is fixed to them using fixing clips and other components specified for the system in question. Their main purpose is to provide suitable support for the boards and transfer loads. A well-built structure also helps maintain the correct gaps, ventilation and surface geometry. It is therefore a mistake to treat joists as ordinary battens that can be replaced with any similar profile. The structure should be built in accordance with the decking board manufacturer's instructions and the project requirements. 

Composite joists – what are their characteristics?

Composite joists are often offered as part of a complete WPC system. Like composite decking boards, they are made from materials containing wood and polymer components, although the exact composition depends on the manufacturer. A key advantage is their resistance to typical conditions beneath a terrace and their compatibility with system fixing components. WPC joists work particularly well in low-level structures where they have regular, stable support. A typical example might be a terrace built on a prepared concrete slab, where the joist has suitably spaced support points in accordance with the system instructions. 

Advantages of composite joists. 

One of the arguments in favour of composite joists is their natural compatibility with many WPC decking board systems. With a complete system, the manufacturer may supply the decking board, joist, starter clips, intermediate clips and finishing components. This makes it easier to select components and reduces the risk of accidentally combining products that are not compatible. Composite joists are also resistant to moisture in the way typical of WPC materials and do not require impregnation like traditional timber components. They are therefore well suited to many standard projects at detached houses. 

Are composite joists stable?

Yes – provided they are used correctly. However, it is important to distinguish the stability of the finished system from the rigidity of the profile itself. A composite joist should not automatically be treated as a structural beam capable of spanning large distances. Many WPC profiles require relatively frequent, stable support. If the support points are spaced too far apart, the structure may flex more than the manufacturer intended. Therefore, during installation, you should check not only the spacing of the joists beneath the decking boards, but also the maximum spacing between the joists' own support points. These are two different measurements.

Aluminium joists – why are they so popular?

Aluminium joists are increasingly used in professional decking structures. Their greatest advantage is the favourable ratio of weight to stiffness, along with aluminium’s resistance to atmospheric corrosion thanks to its natural oxide layer. A suitably selected aluminium profile can create a very rigid and stable structure. This is particularly important for larger decks, systems installed on adjustable supports and more demanding projects. Aluminium also offers high dimensional consistency. The profiles are straight and predictable, making it easier to create a level surface for composite decking boards. 

Do aluminium joists provide a more stable deck?

If we compare profiles intended for a specific project and correctly selected for the structure, aluminium joists generally offer greater rigidity than standard WPC joists. This is one of the most important differences between these materials. However, this does not mean that every aluminium profile will automatically be better than every composite joist. The profile cross-section, wall thickness, support method, loads and the overall structural design all matter. A thin profile used outside its intended application may also fail to meet expectations. That is why you should always assess the specific system, not just the material the joist is made from. 

Aluminium joists on adjustable supports.

This is where aluminium demonstrates one of its greatest advantages. Adjustable decking supports allow the structure to be raised above the ground and the deck height to be set precisely. This solution is used, among other places, on concrete slabs, roofs and balconies, and for projects that require the surface level to be raised. Combined with suitably designed aluminium profiles, it can create a very stable structure. However, the height and spacing of the supports, the method of attaching the profiles and any bracing should all be tailored to the design. The higher the structure, the more important it becomes to brace it correctly. 

And what about moisture?

Moisture regularly builds up beneath decking. Rain can pass through the gaps between the boards, so the structure must be suitable for outdoor conditions. Both high-quality composite joists and aluminium profiles used correctly perform well in these conditions. Regardless of the material, however, ensure that water can drain away freely. The structure should not be designed in a way that allows water to pool beneath the deck. With aluminium, you should also select the fasteners correctly and take environmental conditions into account, particularly where the structure may come into contact with other metals or an aggressive environment. 

Which joists are easier to install?

Much depends on the deck structure. On a low, level concrete slab, composite joists can be a simple and practical solution. The system is straightforward, and installation can be carried out using compatible accessories. Aluminium profiles need to be cut appropriately and require suitable connecting components. On the other hand, their straightness and rigidity help when levelling larger structures. For a professional team, neither solution should pose a problem. More important than ease of installation alone is building the structure in accordance with the design.

Joist spacing for composite decking boards.

Whether we choose aluminium or WPC, we must follow the maximum joist spacing for composite decking boards. Many systems use spacing of around 30–40 cm between centres for a standard board layout, but this is not a universal rule. Each model may have different requirements. If the manufacturer specifies a maximum spacing of, for example, 35 cm, using a more rigid aluminium joist does not mean you can automatically increase it to 50 cm. The decking board needs an appropriate number of support points, and its instructions specify the maximum permitted spacing of the structure beneath the surface.

Joist spacing and support spacing – don’t confuse these measurements.

This is one of the most important aspects of the whole subject. Joist spacing is the distance between consecutive joists beneath the boards. Joist support spacing, by contrast, indicates how often the joist itself must be supported by concrete, pads, supports or another structure. It is therefore possible for joists to be spaced 35 cm apart, while each one needs support at different intervals along its length. In a suitably designed system, a more rigid aluminium profile may allow different spans than a composite profile. However, the specific values must be based on the properties of the profile used and the design. 

Deck stability depends on more than the joist material.

You can buy a very good aluminium profile and still build an unstable deck. How is that possible? If the supports are poorly positioned, the ground is unstable, suitable bracing is missing or the components are connected incorrectly, the quality of the joist alone will not solve the problem. The stability of a composite deck is affected by all of the following: 

• joist type and cross-section,
• joist spacing,
• spacing of support points,
• ground quality,
• how the structure is connected,
• deck height,
• loads,
• how the boards are installed,
• expansion gaps and ventilation. 

A deck should therefore be treated as a complete structure.