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What Base Do You Need for Composite Decking?

Best Decking Joists for your Subframe

Safety is, of course, the main priority when completing any project that will be regularly used by you and your family. It is necessary when constructing decking to build a stable subframe that supports your decking boards for decades to come. Your frame is the most important part of your decking project, and therefore it is imperative to use the best joists you are able to in order to achieve the best project possible. The process of building a solid substructure is crucial to the decking’s longevity. In fact, the frame’s strength is more important than the strength of the decking boards.

Which Material To Use For Your Substructure

One of the first aspects of your project, and the most important for safety and longevity, is building a substructure on your foundation. You can utilise various different materials for your joist frame, such as wood, composite or aluminium. We recommend composite for this process, as its installation is as easy as wood but will last longer, and it is cheaper over a 10 year period. If you don’t plan to be in the property over 10 years then a timber subframe will provide better value for money. The length of time you plan to stay in the property and keep the decking is key upon choosing the right material for the subframe. We have compiled a few questions that are frequently asked by customers regarding joist framing:

Can I use wooden joists?

Yes, you can choose from wooden, composite or aluminium joists. A common misconception is that wood joists cannot be used when installing composite decking, which is not the case. We recommend that timber joists be used with a first-generation composite board, as they have a similar life expectancy. Whereas a second-generation board’s life expectancy exceeds this significantly, which would mean having to replace your joists before the boards need to be replaced.

Why would I use composite joists?

Composite joists have a longer lifespan than wooden joists and therefore have a much better value for money over time. Due to there moisture resistance you don’t have to raise them up from the ground, this means you don’t require posts, if you are constructing on concrete, paving or any other hard surface. This saves time on installation and any additional cost of material.

Are composite joists expensive?

The upfront cost of composite joists may be more expensive initially than wooden joists. However, they are cheaper than aluminium, and are a good long-term investment. Due to their extensive lifespan and minimal maintenance required, this will save you money in the long run on replacement or repairs that will be necessary for wooden joists.

Why do composite joists outlast wooden joists?

The reason that composite material lasts longer than wood is due to the composition of the material. Composite is created through the process of mixing wood fibres and recycled plastics, which are heated and cooled, then moulded into shape. This means that the joists have the properties of both wood and plastic, and therefore the benefits of both materials. In turn, composite will resist moisture due to its plastic aspect, which prevents rotting or warping from taking place, as it will in wood.


Are there different types of wooden joists?

Yes, there are untreated or treated wooden joists available. We would always recommend treating wooden joists, as this protects the material from moisture and insects, therefore improving its longevity. There are various different types and strengths of wooden joists. Softwoods are not as strong as hardwood timber but can still be utilised as long as they pass the grading process. Softwoods include pine, fir and cedar. On the other hand, some examples of hardwoods are oak and teak.

Please note, joist thickness affects durability when creating a structurally sound subframe. The thicker the joist, the stronger the joist. This means a more structurally sound frame.


Joist Spacing and height

When fitting your subframe, you will need to put spacing between your joists to allow for thermal expansion. It is imperative, as if you do not, this will not only distort the decking appearance with cracking and splitting but will damage the board’s stability. For domestic properties, you will need to have a joist centre of 300mm between your joists, to support your decking boards and prevent any sagging or flexing. When constructing for a commercial project, the spacing needs to be smaller to add extra strength and stability, with joist centres of 200mm. Additionally, you will need to consider height when choosing your joist material, as the higher the decking, the stronger the material required. Here we have collected some regularly asked joist questions:

 What will happen if I do not leave expansion gaps?

If you do not leave expansion gaps in your subframe, during thermal expansion, the joists will butt against each other, causing the subframe to become unstable and out of line, resulting in the decking boards doing the same. This is not only aesthetically unappealing but compromises the structural integrity of your decking, which is highly unsafe for you and your family.

What happens if you do not leave the correct joist centres?

If the subframe does not support the decking boards accurately, the boards will start to flex in the areas they are unsupported. This leads to an unsightly decking appearance and damage to your boards, which is both costly and time-consuming to fix.

Does the height of the decking affect the material needed?

Timber joists can still be used for raised decking. However, we would strongly consider a more durable joist for the safety aspect of your project. Therefore, if you are constructing a raised decking, composite or aluminium joists are the better option.


In conclusion, composite joists are the best option for your decking project. They are the ideal choice to ensure longevity, value for money and durability for support. Don’t be put off by the higher initial upfront cost, as composite is an investment and will pay for itself over time through the lack of maintenance or repair required. Remember that if you are investing in composites, you will need to leave gaps for thermal expansion and use the correct joist centres in order to achieve this lifespan.


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