A decking frame (also called a decking base or decking subframe) is the structural skeleton that supports your deck boards and carries the weight of the entire deck. Choosing the correct decking frame timber and the right timber size for your decking frame is essential for strength, longevity, and safety.
In this guide, I’ll cover the best timber for decking frames, recommended sizes for joists and beams, spacing guidelines, and practical tips for building a long-lasting decking base.
Table of Contents
- What is a Decking Frame?
- Best Timber for Deck Frame
- Using Pressure-Treated Timber for Your Decking Frame
- Best size timber for decking joists
- What Size Timber for a Decking Frame?
- Building a Solid Decking Base
- C16 vs C24 Timber for Decking Frames
- Final Thoughts on Decking Frames and DIY Decking
- Frequently Asked Questions
What is a Decking Frame?
A decking frame is the load-bearing structure that sits beneath your deck boards. It typically consists of:
- Joists (horizontal timbers supporting deck boards)
- Bearers or beams (larger timbers supporting joists)
- Posts or ground supports
- Fixings and connectors
The decking frame distributes weight evenly and prevents sagging, movement, and premature failure of your deck.
Best Timber for Deck Frame
The most commonly used timber for decking frames in the UK is pressure-treated softwood such as:
- Treated pine
- Treated spruce
- Treated fir
These timbers offer an excellent balance of strength, durability, and cost. You’ll commonly find that most timber and builders’ merchants offer these types labelled as treated sawn timber, or carcassing timber.
Hardwoods such as oak or iroko can be used for decking frames, but they are significantly more expensive and rarely necessary for domestic projects.
Choosing timber for your decking project comes down to a trifecta of importance: budget, strength, and quality.
Most people are happy to use softwood-treated timber but some people get creative with their deck framing, seeing as there as so many species to choose from – cedar, redwood, douglas fir and spruce, to name a few. Whilst pressure-treated softwood tends to be the favoured material to build decking, if your budget allows, you can digress from the softwood division and opt for hardwood such as oak for your decking areas.
As a rule of thumb, though, pressure-treated wood is the way to go. We advise against using untreated softwood timber varieties in decking frames; untreated softwoods won’t get you far in terms of longevity and the overall preservation of your deck.
Using Pressure-Treated Timber for Your Decking Frame
Pressure-treated timber is a popular choice for timber frames due to its affordability and resistance to insect attack, moisture and rot. Unlike untreated wood and most softwood timbers (that lack natural resistance to the elements), it is infused with chemicals that endow it with impeccable rot and moisture resistance.
On the topic of pressure-treated timber, it should be noted that while most UK websites might throw around Americanisms like ‘pressure-treated lumber’, here in the UK we simply refer to it as ‘timber’ or ‘wood’. The irony is that some species are indeed imported from America.
Best size timber for decking joists
Once you have decided on the type of timber for decking joists, it is time to select the size.
Commonly used joist sizes are 4-by-2, 6-by-2 and 8-by-2. The size that you use will depend on the width of your deck area and how often you can support the joists down to the floor. Depending on your construction, you should use a guide like this to calculate the clear span of the timber you are using and the number of support posts required.
What Size Timber for a Decking Frame?
The size of timber you need for a decking frame depends on:
- Joist span (distance between supports)
- Joist spacing
- Deck height
- Expected load
For most domestic decks, the following sizes are commonly used:
Typical Timber Sizes for Decking Frames (UK)
| Timber Size (mm)Common Use | |
|---|---|
| 100 x 50 (4×2) | Small decks, short spans |
| 150 x 50 (6×2) | Standard joists |
| 200 x 50 (8×2) | Long spans / bearers |
As a general rule:
- 4×2 timber is suitable for short spans and low-level decking
- 6×2 timber is the most popular size for decking joists
- 8×2 timber is often used for beams or where extra strength is required
What size Decking Board?
The following indicates the distance between the joist centres to provide suitable support for your decking.
Decking joist Spacing Guide
| Deck board thickness | Recommended joist spacing |
|---|---|
| 20mm boards | 300–400mm centres |
| 25–27mm boards | 400–500mm centres |
| 32mm boards | Up to 600mm centres |
Thicker decking boards allow wider joist spacing. If in doubt, reduce spacing for extra rigidity.
Installing additional decking boards to your joists may allow you to select a smaller size of timber, saving on space and cost.
I would advise that you always consult a professional before undertaking any construction project to ensure that your deck is built safely and correctly.
Protecting Your Timber Frame
Water is your deck’s biggest enemy so I always advise adding a layer of waterproof tape over the top of the joist to prolong the life of the decking. Here’s an example of a deck I built recently.
Building a Solid Decking Base
A strong decking frame starts with a well-prepared decking base. If the base is unstable or poorly drained, even the best-quality decking frame timber can twist, sink, or rot prematurely.
Your decking base must be:
- Level – to ensure the decking frame sits flat and the boards finish evenly
- Well-drained – to prevent water pooling beneath the frame
- Properly supported – to carry the weight of the decking and foot traffic
Common decking base options
There are several reliable ways to create a stable base for your decking frame:
- Concrete pads – Ideal for supporting posts and bearers on larger or raised decks
- Paving slabs – A popular choice for ground-level decking frames
- Adjustable pedestal supports – Useful for fine height adjustment on hardstanding surfaces
- Ground screws – A fast, low-disruption alternative to digging concrete footings
Whichever method you choose, the ground beneath should be excavated and compacted before installation. Laying a compacted sub-base of MOT Type 1, finished with a layer of gravel or pea shingle, greatly improves drainage and helps prevent standing water beneath the decking base. Ground preparation and digging can sometimes leave surrounding lawns a little worse for wear, but any damaged turf can easily be repaired with lawn seed and garden supplies from Rural Supplies.
Taking the time to prepare a solid decking base will significantly increase the lifespan, stability, and overall performance of your decking frame.
C16 vs C24 Timber for Decking Frames
When choosing structural timber for a decking frame, you’ll often see two strength grades: C16 and C24.
C16 timber is the most commonly stocked structural grade in the UK. It offers good strength for most domestic decking projects, especially low-level decks with regular support spacing. It may contain more knots and visual imperfections, but it remains perfectly suitable when installed correctly.
C24 timber is a higher structural grade with greater strength and stiffness. It typically has fewer knots, straighter grain, and better load-bearing performance. C24 is ideal for:
- Raised decks
- Longer joist spans
- Areas carrying heavier loads
In practical terms, C16 is adequate for many small and medium decks, while C24 provides extra peace of mind where additional strength is beneficial. If budget allows, upgrading to C24 for your decking frame can improve rigidity and long-term performance.
Final Thoughts on Decking Frames and DIY Decking
Building a reliable decking frame is the foundation of any successful decking project. By choosing the right decking frame timber, selecting the correct timber size for your decking frame, and constructing a solid, well-drained decking base, you greatly increase the strength, stability, and lifespan of your deck.
For most domestic projects, pressure-treated softwood is the best choice, while 150 x 50mm (6×2) timber remains the most commonly used size for decking joists. Higher-grade C24 timber offers additional strength and fewer imperfections than C16 and is ideal for raised decks or longer spans, although C16 can still be suitable for smaller, low-level builds when properly supported.
I’d recommend buying your timber from reputable sources like Travis Perkins. It’s responsibly sourced, and they’ll drop it to your door free of charge when spending a certain amount.
I’ve built over six decks over the years, and one thing I’ve learned is that each build gets better as you go. You start to understand what spacing works best, where extra support makes a difference, and how important good drainage really is. Taking a little extra time at the framing stage always pays off in the long run.
If you’re planning to build your own deck, our full DIY decking guide walks you through each stage of construction, from preparing the base to fixing the boards.
Frequently Asked Questions
What is a decking frame?
A decking frame (also called the decking base or subframe) is the structural timber framework that supports your deck boards. It includes joists, bearers and supports that carry the load and keep the deck rigid and level.
What timber is best for a decking frame?
For most UK domestic projects, pressure-treated softwood is the best choice for a decking frame because it offers good strength, affordability and resistance to moisture, rot and insect attack. Avoid untreated timber for outdoor deck framing.
What size timber should I use for a decking frame?
The right size depends on span, support spacing and load. As a general guide, 100 x 50mm (4×2) suits short spans, 150 x 50mm (6×2) is the most common joist size for many decks, and 200 x 50mm (8×2) is often used where longer spans or extra strength are needed.
What size timber should I use for decking joists?
Common decking joist sizes include 100 x 50mm (4×2), 150 x 50mm (6×2) and 200 x 50mm (8×2). The correct choice depends on the distance between supports (clear span) and the spacing of your joists.
How far apart should decking joists be?
Joist spacing is usually set by your deck board thickness. Thinner boards typically need closer centres (for example 300–400mm), while thicker boards can sometimes span wider centres (up to 600mm). If in doubt, reduce spacing for a stiffer deck.
How do I build a solid decking base?
A decking base should be level, well-drained and properly supported. Common base options include concrete pads, paving slabs, adjustable pedestal supports and ground screws. A compacted sub-base of MOT Type 1 finished with gravel or pea shingle improves drainage and helps prevent standing water beneath the frame.
Is C24 timber better than C16 for a decking frame?
Yes. C24 is generally stronger and has fewer defects than C16, making it a good option for longer spans, heavier loads or raised decking. For many smaller, low-level decks, C16 may be sufficient when correctly supported.
Can I use untreated timber for a decking frame?
It’s not recommended. Untreated timber can absorb moisture and rot quickly outdoors. Use pressure-treated timber suitable for external use, and ensure the frame has airflow and drainage beneath it.
