Can You Lay Composite Decking Over An Existing Wooden Deck?

Is your wooden decking in need of a modern refurb? A frequent query we often get asked is whether or not you can install composite decking directly onto an existing timber decking area.

The short answer is no and in this article we explain the hazards of going down that route and offer tips on how best to combat the effects of moisture build-up within your deck. We will also look at the options you have when installing our composite decking directly onto a pre-existing timber subframe.

This isn’t advised. It is very important to ensure adequate conditions for our composite decking to be installed and we strongly recommend installing composite decking boards on top of a proper joist network. The main reasons for this are: (a) to allow for drainage; and (b) to allow for airflow.

By installing composite decking boards directly onto timber decking means that there is very little allowance for either drainage or airflow, which therefore can ultimately end in your composite deck sitting in water, which will then end in damaged boards due to moisture build-up between the timber boards and the composite deck boards.

Not only this, but the aged timber decking that the composite decking will be sitting on will be prone to rotting and will therefore degrade rapidly over time. We suggest opting for the sensible option of installing our composite deck boards directly onto a joist subframe network.

We don’t advise against this option, but there are certain things that we highly recommend when deciding on this option.

Tip #1 – Ensure that your joist spacings sit at the recommend centres of 250-350mm. Many timber decking projects have slightly wider centres that usually sit at around 500mm so this may require reinforced timber joists. While composite deck boards are extremely hardwearing, particularly in heavy footfall areas, they require a little more support beneath them as they have a slight tolerance compared to ridged timber deck boards which don’t have a huge amount of bend capacity.

Tip #2 – Incorporate a drainage slope to your decking subframe where possible. This is all dependent on how well the decking subframe was built in the first place. When water sits on timber joists, this is when problems start to arise such as rot and warping. We advise incorporating a slight decline to your framework which will combat this allowing water to run off and reduce water pooling beneath your composite deck. The decline rate we advise is approximately 1:100.

Tip #3 – Always use NeoTimber® joist tape when planning a composite decking installation on top of a timber subframe. Whilst we generally recommend opting for composite joists or plastic lumber subframes over timber subframes, we understand that sometimes it is cheaper and easier to lay composite decking onto your pre-existing or newly laid timber subframe. Joist tape will protect your timber joists and provides users with that peace of mind by providing a completely waterproof seal on the surface of the joists. Without joist tape – your timber subframe can result in significant deck frame weathering, resulting in rot.

Tip #4 – Ensure the entire structure is both stable and safe. Think twice about installing long-life composite decking boards onto failing subframes. We have seen this happen in the past whereby customers have installed our decking onto an aged subframe and then ended up ripping their deck boards up to change their subframe later down the line.

Looking for more installation help? Check out our installation guide to help with the laying of your composite decking.