A UK Guide to Decking
Subframes & Joists

Safety

Safety should be the main priority when constructing any project that will see regular use by you and your family. It is essential to build a stable and durable subframe that will support your decking boards for years to come.

The process of building a solid substructure is crucial to the decking’s longevity. In fact, our expert view is that the frame’s strength is more important than the strength of the decking boards themselves. A beautiful deck on a weak frame is a failure waiting to happen.

Timber Joists

In our experience, timber joists are the most common material used for decking subframes, mainly due to their price. If you use good wood to build your decking joists, the decking will be structurally sound and not prone to collapse over time. A correctly installed timber subframe will last up to 10-15 years, making it a suitable choice if that aligns with your long-term plans for the property.

• Pressure-Treated Pine:

  This is the most common and affordable choice, making it ideal for those on a smaller budget. The timber is infused with chemicals to make it resistant to rot and insects. While not as aesthetically pleasing as hardwood, it is versatile and easy to work with.

• UC4 (Use Class 4) Timber:

  “UC4” refers to the level of treatment the wood has received. This timber is specifically treated for Ground Contact, meaning it can handle being wet for longer periods without rotting. If your deck frame is sitting directly on the ground or in a damp area, you should specifically request UC4 timber to extend the lifespan of your frame.

Expert Tip: Pressure-treated pine joists are readily available at any local timber or builders’ merchant. The main decking joist available here in the U.K. is… pressure treated softwood. We strongly recommend using C24 graded timber for any decking sub-structure to ensure the best long-term strength and durability.

Customer Note: Understanding C16 vs. C24 vs. UC4 When shopping for joists, you will see terms like C16 and C24. It is vital to understand the difference:

• C16 (Standard): This is often cheaper timber used for internal stud walls. It can have more knots and may warp slightly.

• C24 (Structural): This is a higher “Strength Grade”. It is cut from slower-grown trees, meaning it is denser, stronger, and straighter than C16.

• Our Recommendation: Always ask for “C24 Graded, Pressure Treated Timber”. The small extra cost gives you a straighter frame and significantly better structural strength for decades.

Aluminium Joists

Aluminium joists are another “fit and forget” solution. They are lightweight, extremely strong, and offer Class A fire resistance, making them a great choice for rooftop terraces or commercial projects. Just like composite, they won’t rot, rust, or splinter, making them perfect for ground-level decks where raising the frame is not possible.

Top Tip: Most aluminium joists have an extra “Thermosetting” coating. This layer helps insulate the metal against thermal expansion and further protects it against corrosion and wear.

How Far Apart Should Your Joists Be?

While different brands may have slightly different guidelines, our expert advice based on years of experience is simple:

• For residential composite decking, the distance between the centre of one joist and the centre of the next must not exceed 300mm (30cm).
• For commercial projects with heavier footfall, or if you are laying boards diagonally (at a 45-degree angle), you must reduce this spacing to 250mm (25cm).