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plastic decking joists A long-lasting deck needs a subframe that's built to last too. Our Plastic Decking Joist provides a durable, rot-proof foundation for composite decking and other outdoor decking installations.
Made from solid recycled plastic, these decking subframe joists won't absorb moisture or rot like timber. This is particularly important beneath decking, where trapped moisture and limited airflow can cause traditional wooden joists to deteriorate over time.
Unlike timber, recycled plastic can be installed in direct contact with the ground without requiring protective treatment. This makes our plastic decking joists particularly useful for low-level and ground-level decking installations where moisture resistance is essential.
Key benefits include:
Using recycled plastic joists beneath long-lasting composite decking helps create a more durable complete system, without relying on a timber frame that may need replacing before the decking boards above it.
For detailed fitting information, please see our decking installation instructions in the Downloads section.
Price includes VAT
There is no minimum order
Our Plastic Decking Joist is a solid recycled plastic structural profile designed for building durable, moisture-resistant decking subframes.
Product: Plastic Decking Joist / Decking Subframe
Colour: Black
Size: 47 × 97 × 3100 mm
Construction: Solid
Material: 100% recycled plastic
Warranty: 25 years*
Suitable for: Composite decking, garden decking and outdoor decking subframes
*The 25-year warranty covers material defects and material failure. It does not cover incorrect installation or product misuse.
Our solid recycled plastic decking joists can be cut and drilled using conventional woodworking tools.
Hand saw
Chop saw with a suitable wood or metal cutting blade
HSS drill bits
Standard timber screws and bolts
Single-thread screws
When fixing the joists, holes should be pre-drilled approximately 3–4 mm larger than the screw diameter to allow for the natural expansion and movement of recycled plastic.
The black colour of the joists comes naturally from the recycled raw materials used during manufacturing. As a result, some variation in shade and appearance may occur between products.
If your project requires a more consistent, homogeneous black finish, please contact LL Plastic before ordering.
The quantities below provide an approximate guide to the plastic decking joists and supporting legs required for common decking sizes.
Joist size: 47 × 97 × 3100 mm
| Decking Area | Approx. Joists | Approx. Legs |
|---|---|---|
| 3 × 3 m | 20 | 16 |
| 3 × 4 m | 24 | 20 |
| 4 × 4 m | 30 | 25 |
| 4 × 5 m | 41 | 30 |
| 5 × 5 m | 70 | 36 |
| 6 × 6 m | 76 | 49 |
These quantities are provided as a general guide. Actual requirements may vary depending on the decking design, joist spacing, ground conditions and installation method.
For lower decking structures below 600 mm in height, we recommend using 80 × 80 mm recycled plastic posts as supporting legs.
For decking structures above 600 mm, we recommend 100 × 100 mm supporting legs, subject to the requirements of the individual installation.
Using recycled plastic for both the decking joists and supporting legs creates a subframe that won't rot through prolonged exposure to moisture or contact with the ground.
Before installing your plastic decking subframe, please read our detailed installation guidance for recommended construction methods, spacing and fixings.
View the Decking Subframe Installation Guide:
https://www.llplastic.co.uk/webshop-downloads---1
