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.
The decking seen above was specifically designed to support the weight of a large hot tub. You can see my Lay-Z-Spa Helsinki review and understand why this tub with its 1,123-litre capacity needed such a robust timber deck. That’s over a tonne in weight!
What is a Decking Frame?
A decking frame is the load-bearing structure that sits beneath your deck boards. It typically consists of:
- Decking Frame (Rim Joists) – The outer frame that forms the perimeter of the deck. Rim joists define the overall shape of the structure and provide fixing points for the internal joists.
- Joists – The main horizontal structural timbers that span across the frame and directly support the decking boards. They are typically spaced at regular centres to suit the decking material being used.
- Noggins – Short cross pieces installed between joists to prevent twisting, improve rigidity, and help distribute loads more evenly across the frame.
- Support Posts – Vertical timber posts used where the deck is raised above ground level. They transfer the weight of the decking frame down to the foundations below and provide additional structural support.
- Footings / Bases – The foundations that support the decking frame. These may be concrete pads, paving slabs, ground screws, adjustable pedestal supports or concrete footings, depending on the site conditions and deck design.
- Joist Hangers / Brackets – Galvanised steel connectors used to securely fix joists to rim joists or beams. They strengthen the frame and help prevent movement over time.
- Ground / Sub-base – The prepared ground beneath the decking. This should be level, well-drained and, where required, compacted with hardcore to provide a stable foundation and minimise future settlement.
- Damp Proof Membrane (Optional) – A weed-suppressing and moisture-resistant membrane laid beneath the frame to help reduce vegetation growth and minimise moisture rising towards the timber. Although not essential for every installation, it can improve long-term durability, particularly on soil or poorly drained ground.
I also like to apply a strip of waterproof joist tape along the top of each joist before installing the decking boards. This creates a protective barrier that prevents water from sitting on the timber around screw holes, helping to reduce moisture ingress and extend the lifespan of the decking frame.
The decking frame distributes weight evenly and prevents sagging, movement, and premature failure of your deck.
Best Timber for Deck Frame
What should you actually use to build a decking frame that lasts? Pressure-treated softwood is the standard choice in the UK, and for good reason.
Pressure treatment forces a chemical preservative deep into the timber under vacuum, rather than just coating the surface, giving it far better resistance to rot, moisture and insect attack than untreated wood, which won’t last in an outdoor frame. You’ll find it at most timber and builders’ merchants labelled as treated sawn timber or carcassing timber, usually treated pine, spruce or fir.
Hardwoods like oak or iroko can technically be used, but they’re considerably more expensive and rarely necessary for a domestic deck. Unless budget genuinely isn’t a factor, stick with treated softwood.
Choosing timber for your decking project comes down to a trifecta of importance: budget, strength, and quality.
The next question is which size and strength grade you need, and that comes down to strength class (C16 or C24) and the span you’re working with, covered in detail below.
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.
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 |
Decking Joist Span Table (150 x 50mm / 6×2)
Maximum clear span for the most commonly used decking joist size, based on 1.5kN/m² domestic loading. Joists spanning more than 2.5m should have noggins fitted at mid-span.
| Joist Centres | Max Span (C16) | Max Span (C24) |
|---|---|---|
| 400mm | 2.97m | 3.32m |
| 500mm | 2.76m | 3.07m |
| 600mm | 2.59m | 2.89m |
Decking Beam Span Table (Double 150 x 50mm)
For a double beam built from two 150 x 50mm sections bolted together, the standard build-up for supporting joists.
| Beam Centres | Max Span (C16) | Max Span (C24) |
|---|---|---|
| 1.2m | 2.59m | 2.89m |
| 1.8m | 2.16m | 2.51m |
| 2.4m | 1.87m | 2.27m |
Figures based on Timber Decking: The Professionals’ Manual (TRADA), assuming standard 1.5kN/m² domestic loading. For heavier loads such as hot tubs, or spans beyond these tables, consult a structural engineer.
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. Depending on your construction, you could use a guide like this to calculate the clear span of the timber you are using and the number of support posts required.
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.
Fixing It All Together: Joist Hangers, Noggins and Ledger Boards
Choosing the right timber is only half the job. I’ve seen more decks fail from poor fixing than from the wrong grade of wood, so here’s how I actually put a frame together on site.
Joist Hangers
Where a joist meets a beam or a ledger board, I sometimes use a galvanised or stainless steel joist hanger rather than just nailing or screwing through the face. One end of every joist can be face fixed with a hanger to stop it twisting under load. This is a genuine building regulation point, not just good practice. Try to avoid hangers that wrap over the top of the joist. They sit proud of the frame and get in the way when you’re fixing the deck boards down.
Noggins (blocking)
Any joist spanning more than 2.5m needs a row of noggins fitted at the midpoint. I fit mine using the same size timber as the joists themselves, staggered slightly rather than in a dead straight line, it makes them far easier to nail or screw in place and it stiffens the whole frame against twisting. On the hot tub deck I built, I ran noggins at closer centres than the minimum given the extra point load, worth doing on any deck carrying serious weight.
You might be wondering what these odd-looking posts are sticking out of the middle of my deck. They are for a hammock! You can read about my hammock decking build here.
Ledger Boards
If you’re attaching a deck to the house, the ledger board (sometimes called a wall plate) needs to be at least the same size as your joists, bolted to sound masonry with a 10mm gap left behind it using washers or packers. That gap isn’t optional. It lets water drain away rather than sit against the wall. Just as important, never cut into or bridge the damp proof course when you’re fixing it. That’s the most common mistake I see on DIY decks attached to houses.
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.
