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When planning a new deck, most people naturally focus on the part they'll actually see: the decking boards. Colour, finish, layout and style all matter, but there's another part of your deck that's just as important to its long-term performance.
The decking joists and subframe underneath support the entire installation.
Traditional timber joists can be vulnerable to prolonged moisture, particularly beneath decking where airflow may be restricted. This can leave you with perfectly usable decking boards sitting on top of a subframe that has begun to deteriorate.
That's why plastic decking joists provide a practical alternative. Manufactured from recycled plastic, they create a durable, moisture-resistant foundation for composite and other decking systems without the rot concerns associated with timber.
At LL Plastic, our Plastic Decking Joists are designed specifically for long-term outdoor use and are available in several sizes to suit different decking projects.
Decking joists are the structural profiles that sit underneath your decking boards, creating the framework or decking subframe that supports the finished surface.
While timber has traditionally been used for this job, recycled plastic decking joists provide an alternative for projects where resistance to moisture and long-term durability are particularly important.
LL Plastic's joists are manufactured from solid recycled plastic and can be used to construct decking subframes and other suitable outdoor structures.
They can also be cut and drilled using conventional tools, making recycled plastic a practical material for constructing outdoor frameworks.
Moisture is one of the biggest challenges facing any outdoor timber structure.
A decking subframe can be particularly exposed because it sits underneath the decking boards and is often relatively close to the ground. Rainwater and moisture can find their way into the structure, while restricted airflow can make it more difficult for damp areas to dry.
Over time, repeated exposure to moisture can contribute to deterioration and rot in timber joists.
This creates an important consideration when installing modern composite decking.
You might choose decking boards because they're designed to provide years of low-maintenance outdoor use, but if those boards are installed on a less durable subframe, the supporting structure can potentially become the weak point in the system.
A long-lasting deck should therefore be planned from the ground up.
The fundamental difference between plastic and timber decking joists is how the materials respond to moisture.
Timber can absorb water and may require preservatives or other protective treatments for prolonged outdoor use.
Solid recycled plastic does not behave in the same way. It won't rot through exposure to moisture and doesn't require the regular painting, staining or preservative treatments associated with timber.
Other advantages of recycled plastic decking joists include:
For homeowners, landscapers and contractors, this can mean considerably less concern about maintaining the structure hidden underneath the decking.
Yes. In fact, plastic joists and composite decking can make a particularly useful combination.
Composite decking is commonly chosen because customers want a durable outdoor surface without the regular painting, staining and maintenance associated with traditional timber decking.
Using timber underneath that low-maintenance surface can therefore leave part of the structure susceptible to the very maintenance problems you're trying to avoid.
Pairing composite decking with recycled plastic joists allows you to consider durability across the complete decking system, rather than just the visible surface.
If you're still deciding on your finished decking surface, explore our Composite Decking & Accessories alongside our plastic joists to plan the complete installation.
One of the biggest advantages of recycled plastic joists is their resistance to moisture.
Unlike untreated timber, solid recycled plastic doesn't rot through prolonged exposure to damp conditions. This makes it particularly useful when constructing low-level or ground-level decking, where keeping a conventional timber framework completely isolated from moisture can be difficult.
The appropriate design will still depend on the size, load, ground conditions and construction of your individual deck, so the subframe should always be planned correctly before installation.
Yes.
One misconception about recycled plastic construction products is that they require specialist machinery to work with.
LL Plastic's Plastic Decking Joist (Black) can be cut using conventional tools, including suitable hand saws and chop saws, and drilled using HSS drill bits. Standard timber screws and bolts can also be used where appropriate.
Because recycled plastic naturally expands and contracts with changes in temperature, fixing holes should allow for movement. For LL Plastic's joists, we recommend pre-drilling fixing holes approximately 3 to 4 mm larger than the screw diameter.
Always check the relevant installation guidance for the specific profile you're using before starting your project.
Different decking projects require different profiles, which is why the appropriate joist should be selected according to the design and requirements of the installation.
LL Plastic supplies recycled plastic decking joists in different dimensions to suit a range of decking subframes and outdoor projects.
Rather than choosing a joist based on size alone, factors such as spans, loads, support spacing and the overall design of the structure should also be considered.
You can compare the available options in our Plastic Decking Joists range.
For more technical projects, LL Plastic also provides product information and installation guidance through our Downloads & Installation Guides.
Plastic decking joists can provide the supporting structure for different suitable decking systems.
For a contemporary garden finish, they can be combined with composite decking boards, providing an attractive finished surface above a moisture-resistant recycled plastic subframe.
Alternatively, customers looking for an outdoor surface manufactured from recycled plastic can explore our Solid Recycled Plastic Decking Boards.
Composite and recycled plastic decking have different characteristics, so the best option will depend on the appearance, application and performance you're looking for.
Recycled plastic can also be used for supporting components within appropriate decking structures, reducing reliance on timber underneath the finished deck.
LL Plastic supplies a wide range of recycled plastic posts and profiles that can be used across landscaping and outdoor construction projects.
The correct supporting structure will depend on factors including the height of the deck, loads, spans and individual installation requirements. Always ensure the complete structure is appropriately designed for its intended use.
The useful lifespan of any decking structure depends on its design, installation, loads and environment, so it's important not to treat any construction material as indestructible.
However, recycled plastic eliminates one of the major causes of deterioration associated with conventional timber subframes: rot caused by moisture.
LL Plastic's Plastic Decking Joist (Black) is currently supplied with a 25-year warranty covering material defects and material failure, subject to the applicable warranty terms and excluding incorrect installation or misuse.
This makes recycled plastic particularly attractive when the aim is to construct a deck for long-term outdoor use.
One of the main reasons for choosing recycled plastic for an outdoor subframe is the minimal ongoing maintenance required.
Unlike timber, the joists don't need to be periodically painted, stained or treated with wood preservative to protect them against rot.
Once correctly installed, they're designed to provide a durable supporting structure underneath the finished decking.
Of course, the overall deck should still be inspected and maintained appropriately. Drainage should remain clear, fixings should be checked where necessary and the decking surface should be maintained according to the manufacturer's recommendations.
Using recycled plastic in long-life construction products gives existing plastic material another useful application rather than relying entirely on virgin materials.
This approach is closely aligned with the principles of a circular economy, where resources are kept in productive use for longer.
UK sustainability organisation WRAP describes a circular economy as one in which products and materials are kept in circulation through processes including maintenance, reuse and recycling.
For anyone interested in the wider principles behind keeping materials in circulation, you can read more from WRAP about the circular economy.
Choosing recycled plastic doesn't mean that every project automatically becomes sustainable, of course. Material use, product lifespan, transport, installation and eventual end-of-life all matter.
But using an existing waste material to manufacture a durable product designed for long-term outdoor use can form part of a more resource-conscious approach to construction and landscaping.
Choosing attractive, durable decking boards is only half the job.
The framework underneath determines how those boards are supported, and replacing a failed subframe can mean removing the decking above it simply to gain access.
That's why it's worth thinking about the entire structure at the planning stage.
A recycled plastic decking subframe offers a rot-proof, low-maintenance alternative to conventional timber joists and can be combined with composite or recycled plastic decking boards to create a system designed for long-term outdoor use.
Explore our Plastic Decking Joists to compare the available profiles, view our Plastic Decking Joist (Black) for individual product information or read our Composite Decking Installation Guide for more help planning your decking project.
