Composite and plastic decking: product in review | Architecture & Design

The use of composite and plastic decking products as an alternative to timber may still be relatively new in Australia but there's already a vast range of products available and it's growing.

Wood-plastic composite (WPC) deck products are the dominant type, although there are rice husk-plastic and other fillers, as well as entirely plastic versions.

The main reason for its popularity is its low maintenance but other benefits include its standardised size and straightness, as well as its termite, water and UV resistance.

Composite and plastic products are produced by a machined extrusion process which ensures complete standardisation of products in terms of colour and shape as well as systematic control of the chemical additives.

Changes to regulations around hardwood timber supply, leading to cost increases, have made composite products more competitive in many cases now, and it's predicted their share of Australian decking market will increase.

Being an imitation to wood, it is no equal substitute for real timber. However a growing variety of colours, styles and textures are now offered and timber imitation products are becoming increasingly comparable.

Commonly cited issues with composite decking products are scratching and splitting, colour fade and stains, and a high expansion rate. This is due to poorer material compositions like high ratio of softwood sawdust and soft polyethylene. Manufacturers say this was a problem with the earliest generation products and cheap imports, but improvements have been reported and warranties are now offered by suppliers to suit these advances.

Colour fade is still a problem, attributed to UV protection on that exposed cellulose content. Some suppliers have addressed this by using 100 per cent PVC products or plastic coatings over the top of boards which also protects from scratches, stains, mould and slipping.

Generation one products were made from recycled pine saw dust and polyethylene (milk bottles) and this soft plastic and soft wood combination translated into a product that was easily scratched by furniture and the like.

An opposing view supports the environmental characteristics of using a high level of recycled materials, like polyethylene (milk bottles) over PVC, but they are arguably less suitable for most applications.

The two main types of plastic used in in Australian decking products are high density polyethylene (HDPE) and polyvinyl chloride (PVC).

HDPE is a softer, more flexible substance, and is the most common plastic used in Australian WPC decking. It can be sourced from recycled materials like milk bottles or from pure polymer.

Recycled materials that use lower grade recycled HDPE, which is not chemically designed for exterior use, may result in quicker deterioration of the product. The alternative HDPE, from polymer, is made from virgin material, i.e. petroleum or oil, so although stronger it suffers under sustainability measures.

HDPE is also less fire resistant and suppliers have had to combat this by including fire retardant additives in their manufacturing processes.

PVC is harder, fire resistant and UV resistant, making it the most commonly used exterior plastic – think gutters, down pipes etc - however, its hard surface lends itself to installation difficulties and an underfoot feeling that is dissimilar to real timber.

The wood fibre in WPC also plays an important role in its performance and is used as a filler to reduce the production costs. Any product with more than 55 per cent timber fibre could jeopardise its longevity. Similarly, the type of wood product determines the strength of WPC decking and as with normal decking, hardwood is generally considered to perform better than soft wood fibre in WPC boards.

Boards are manufactured via an extrusion process, whereby all the ingredients, including colorants, coupling agents, UV stabilisers, blowing agents, foaming agents, and lubricants are placed in a large hopper, mixed up, cooked and squeezed out to the desired board profile.

They come in either hollow or solid boards. Hollow boards have an increased risk of absorbing atmospheric moisture and require a more accurate installation, but are cheaper.

Boards generally come with either an embossed finish or ready for brushing with an additional coating. Brushing removes lubricates in the manufacturing process and can include much needed slip resistant benefits. A downfall of brushing is that it leaves wood fibres exposed to staining which is more difficult to remove than on normal timber decking.

Different boards require different installation methods and an important thing to look out for in the installation instructions are the materials’ expansion co-efficient.

Some boards will expand and contract as much as 1mm with every four degree Celsius fluctuation in temperature. In this case, special screws and fixing systems are used and spacing between boards and abutting walls are larger.

WPC is also weaker than wood so installing a composite deck on a foundation originally used for a wood deck will require more joists. Other than this, composite boards are installed much the same as traditional decking although most suppliers will recommend pilot holes before drilling or using a nail gun.

Below are a few products from leading suppliers we've sized up on comparable measures.

You can also source information on the latest lines available in Australia on our directory Infolink, click here.

Composition: What the decking board is made from is very important. The ratio of wood fibre versus plastic in WPCs is very important as is the type of timber and plastic used. More on this importance here:

Colours: Most suppliers offer more than one colour and some more than five. Nearly all suppliers offer a brushed and smooth version of their products. More on that here:

Water absorption rate: Because of the nature of wood fibre, it is important that a decking board’s water absorption rate is known. Different applications will obviously necessitate different needs in terms of water absorption. The rates supplied are generally full submersion over 24 hours.

Thermal Expansion Coefficient: It is important to understand how much the boards will move, especially under Australian summer conditions. We tried to use ‘linear expansion ratings’ where possible, which measures the change in length of the board at temperature fluctuations and therefore affects everything from board spacing to joist positioning.

Warranty: All warranties are based on residential applications. Note that most manufacturers require a warranty card that is returned within 30 days.