Thanks to the ongoing popularity of backyard decks and growing awareness of the need for cost-effective, sustainable alternatives to solid timber, composite decking has become increasingly popular. The unprecedented number and variety of composite decking products gives designers more freedom than ever when specifying decks, but also complicates the selection process – it is often difficult to tell the options apart and know where to start. Below, we take the stress out of composite decking specification, setting out the key considerations that must be taken into account.

1. Hollow versus soild

Composite boards are supplied in two varieties: hollow and solid. The current generation of composite boards have refined, circular cavities that are distinct from the rectangular cavities of older generations of board. As is the case with most materials, the manufacturing cost of composite decking is proportional to the weight of the new material. Hollow boards are lighter – and therefore cheaper – than solid composite boards, but may have reduced structural integrity, strength, durability, and effective life. Water sometimes enters the holes in the boards cannot get out, causing progressive degeneration. This is particularly true of older boards with rectangular hollows, which generally do not offer any long term saving in comparison with solid boards.

2. Second versus third generation boards

Second generation composite boards are those that have been on the Australian market for around 10 years. They have the same colour throughout their cross section and have no coating, making them prone to fading and staining. Conversely, third generation products are manufactured via co-extrusion, which produces and bonds an outer layer to a second generation core. The outer layer may be a different colour to the core, and adds a protective layer that is resistant to fading and stains.

3. Width and length of board

It is important to note that strength is primarily determined by density and thickness and width has very little impact. Generally, the wider the boards, the fewer that need to be cut and laid, and hence the lower the installation cost. Length must also be carefully considered: a common length of composite boards in Australia is 5.4m, which corresponds with hardwood lengths and fits into a container easily, thus reducing costs. However, long lengths may be very heavy and difficult to manoeuvre: shorter lengths or pre-cut project-specific lengths are generally the most economic option and save on installation labour.

4. Thickness and maximum joist span

Generally, the thicker the board, the more rigid it will be. Rigid boards are more capable of supporting their weight and loads than thin ones, allowing use of fewer joists at larger spacings, thus maximising savings. The relationship between thickness and rigidity also means that thinner boards will require more supporting joists placed at closer intervals. In many cases, this undermines the cost savings posed by the lower material volumes of thin boards. Always check the maximum joist spacing recommended for a composite board, which may be less than the standard 450mm (18”) for a timber residential deck.

5. Profile and installation method

Labour is always a substantial component of the total installation cost of a deck, and varies depending on the installation method. Most composite boards have a “hidden fixing” method where a channel in the edge of the board clips directly to the joist. However, different boards may have different recommended installation methods, and it is important to check which applies to your selected product before proceeding.

Crucially, composite timber must be laid with gaps between fixed objects and at board butt joints to accommodate expansion and contraction. Most manufacturers will void warranty if gapping does not comply with installation instructions.

6. Finish

In the past, composite was chosen for its realistic “timber look” that closely recreated the colours and textures of natural timber. Now, in response to changing market needs, composite products are available in a wide variety of colours, textures, and finishes. Most third generation composite boards are brushed with metal brushes to wear down the plastic and create a more richly textured, natural looking surface. As photographs cannot accurately depict the finish and texture of composite board, it is important to view samples in person before making a decision. 

7. Colour

Composite decking is available in a wide variety of colours, but is primarily specified in yellowy, reddish, and dark shades of brown. These options are typically rounded out with less woody shades such as grey and beige. Again, it is important to view samples in person before making a decision, as colours may vary significantly between manufacturers and two products with the same colour name may differ greatly in reality. Composite boards can be made in a variety of colours including bespoke options to match a colour scheme.

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