Composite decking is a durable and resilient product to add to your garden; however, your decking’s strength actually comes from the subframe below. Your composite decking requires a foundation, to provide it with stability and the strength to withstand footfall.

If you are deciding whether or not to add composite decking to your garden, you will also need to think about your foundation first. Within this article, we will explore why you need a foundation for your composite decking and the important aspects to consider regarding your composite decking’s foundation. 

from ground to up

Why Do I Need a Foundation for My Composite Decking?

All composite decking projects require a foundation or subframe to provide strength and stability to the decking boards. Your foundation is what gives your decking its longevity and when built correctly, it will extend the lifespan of your decking even further.

The smaller the joist centres, the stronger the decking will be, as the gaps aren’t as wide, meaning additional support and less chance of bowing or bending. At Composite Warehouse, we advise joist centres at 300mm for residential projects and 200mm for commercial projects. These guidelines are to be kept within our warranty. 

Your decking subframe can be made from wood or composite. However, it is highly recommended to opt for composite joists when choosing the material for your decking subframe, especially when fitting composite decking boards, due to their lifespan.

Composite joists are resistant to rotting and warping, meaning they will last for decades, reducing the need to replace or repair your subframe a few years down the line, as is necessary with wooden joists. 

Hidden Fastener System

Another reason your composite decking requires a foundation is due to its installation process. Composite decking boards are designed with a groove channel running along the side of the boards, which are made to fit your decking clips into.

Composite decking boards are not created to be drilled directly through but into your subframe below. Your starter clips and hidden fasteners will be screwed into your joist system and then your decking boards can be slid into them utilising these grooved channels. 

This system ensures a seamless finished look to your project without any visible screws poking out. Not only is this more aesthetically appealing but it will also retain the structural integrity of the boards. Additionally, these clips allow for thermal expansion, which reduces the chance of any damage such as cracking when the boards expand and contract in temperature changes. 

Surface Foundations You Can Build Composite On

Whether you have a soft surface such as soil or grass in your garden or a hard foundation such as concrete, you can build composite decking in your exterior space. The surface foundation you have in your garden will determine the installation process needed to fit your composite decking. 

If you have a surface foundation such as concrete to build on top of, then the installation process is simpler and won’t require raising the decking using posts, as necessary when building on soil. Despite this, it is recommended to slightly raise your subframe using pedestals for drainage and airflow purposes.

This, of course, is only possible when the foundation is flat, even and stable to begin with. If your concrete base is damaged or uneven, then you will need to fix that before building any subframes on top. Even if your concrete base is in good condition, you still need to build a subframe to support your composite decking on top. 


To conclude, yes, you need a foundation for your composite decking. Composite decking is fitted using a hidden fastener system, which requires joists to be screwed down. Your decking foundation is what will provide your decking with strength and stability throughout its lifespan, creating a safe deck for you and your family.

The installation process will differ due to the surface you are building your decking foundation on, which must be taken into consideration. Your composite decking subframe can be wooden or composite, but it is advised to use composite joists for maximum durability.


Leave a Reply

Your email address will not be published. Required fields are marked *

Your email address will not be published. Required fields are marked *