When installing composite decking, many people assume the substructure (joists) can also be made from composite material (WPC joists). While composite joists are sometimes used in low-load applications, they are generally not recommended as the primary structural support for outdoor decking.

This is not because composite joists are unusable, but because they have several limitations compared with aluminium or treated timber joists. The main concerns involve structural strength, long-term stability, and installation performance.

Below are the key reasons.

1. Structural Strength Is Lower

Composite joists are made from the same material as composite decking boards: a mixture of wood fibres and plastic.

While this material works well for decking boards, it is not ideal for load-bearing structural framing.

Compared with aluminium or structural timber, WPC joists have:

• Lower stiffness (modulus of elasticity)

• Higher deflection under load

• Reduced load-bearing capacity

Because of this, composite joists require much closer spacing, often around:

Whereas aluminium or timber joists can commonly support:

This increases installation complexity and cost.

2. Greater Thermal Expansion

Composite materials expand and contract more than metal.

For joists installed outdoors, this can lead to:

• Frame movement in hot weather

• Potential deck surface noise

• Misalignment of clips and boards over time

Aluminium joists, by comparison, have much better dimensional stability.

3. Long-Term Structural Creep

Another issue with WPC structural components is creep.

Creep is a material's slow deformation under constant load over time.

For decking frames supporting:

• Furniture

• Planters

• Groups of people

Composite joists may gradually sag if not properly supported.

This is why most professional installers prefer structural materials with higher rigidity, such as aluminium or structural timber.

4. Screw Holding Performance

Decking screws and clips rely on strong material to maintain their grip.

Composite joists generally have lower screw-holding strength compared with timber or aluminium systems.

Over time, screws may:

• Loosen

• Shift slightly under repeated load

This can lead to:

• Deck movement

• Surface noise

5. Durability in Structural Applications

While WPC boards are highly durable as decking, using the same material for structural framing can subject it to additional stress.

Joists must handle:

• Continuous load

• Structural stress

• Fastener tension

For this reason, many manufacturers recommend using more rigid framing systems to ensure long-term stability.

When Are Composite Joists Acceptable?

Composite joists may still be used in certain situations, such as:

• Ground-level decking

• Small patio areas

• Decorative platforms

• When installed over solid concrete bases

In these cases, the load is distributed across the base rather than fully supported by the joists.

Better Alternatives for Deck Framing

For most outdoor decking projects, installers prefer:

Aluminium Joists

Advantages:

• Very high structural strength

• Minimal expansion

• Long lifespan

• Straight and stable

Treated Timber Joists

Advantages:

• Strong load capacity

• Excellent screw holding

• Widely used in residential decks

Final Thoughts

Composite joists are not necessarily “bad,” but they are generally less suitable for structural decking frames due to their lower stiffness and long-term stability compared with aluminium or treated timber.

For most outdoor decking projects, a stronger substructure ensures the deck remains:

• Stable

• Quiet

• Durable for many years

Choosing the right framing material is just as important as selecting the decking boards themselves.