If you're buying structural timber, you'll often see C16 and C24 alongside familiar sizes such as 3x2, 4x2 and 6x2.
The main difference is simple:
C24 is a higher strength grade than C16. That doesn't mean you automatically need C24.
C16 is suitable for many everyday building jobs. C24 is more likely to be used where greater structural performance is needed, particularly for longer spans or more demanding construction.
For straightforward DIY jobs, you may not have drawings telling you which grade to use — and that's normal. What you're building, the size of timber, how far it needs to span and whether it needs to be treated can all matter more than simply choosing the highest grade.
For larger structural projects such as extensions, floors, roofs and loft conversions, the required timber sizes and grades are much more likely to be given on drawings, calculations or a timber schedule.
C16 vs C24 at a glance
| C16 timber | C24 timber | |
|---|---|---|
| Structural timber? | Yes | Yes |
| Which is stronger? | Lower strength grade | Higher strength grade |
| Suitable for general construction? | Yes | Yes |
| Common for everyday framing and DIY work? | Yes | Yes, although often unnecessary |
| Longer or more demanding spans? | Can be suitable | More likely to be chosen |
| Can it be used for joists or rafters? | Yes, where suitable | Yes |
| Does the grade mean treated? | No | No |
| Is C24 automatically better? | No | No |
| Usually the lower-cost option? | Often | Usually not |
The important thing is to choose the right grade for the job, rather than assuming C24 must always be better because the number is higher.
What is C16 timber?
C16 is a strength grade for structural softwood timber.
It is commonly used for general construction and timber framing where its strength is suitable for the job.
Typical uses include timber framing, stud walls, decking subframes, sheds and garden structures, general carcassing and some floor or roof construction where the timber size and design are suitable.
Reynolds stocks C16 structural timber in common carcassing sizes including 3x2, 4x2 and 6x2.
The important thing to remember is that C16 tells you the strength grade — it doesn't tell you what size timber you need.
A 3x2 and a 6x2 can both be C16 while being suitable for very different jobs.
What is C24 timber?
C24 is also strength-graded structural softwood, but it meets a higher structural strength class than C16.
That makes it useful where a project needs greater strength or stiffness.
You're more likely to see C24 used for longer floor joists, roof rafters, larger structural sections, more demanding frames and projects where C24 has specifically been called for.
Reynolds carries a broader range of C24 structural timber, including larger sections for more demanding building work.
C24 isn't a completely different type of timber to C16.
Think of them as two structural strength grades, with C24 being the higher grade.
What do C16 and C24 actually mean?
The C refers to coniferous softwood.
The numbers are part of the technical system used to classify structural timber by strength.
Unless you're designing the structure yourself, you don't need to understand or calculate the engineering figures behind them.
For most customers, the useful version is simply:
C16 = structural timber suitable for many general applications.
C24 = higher-strength structural timber, useful where greater performance is needed.
Is C24 better than C16?
Not necessarily.
C24 is stronger. That doesn't make C16 inferior timber.
If C16 is suitable for your project, changing everything to C24 may provide little practical benefit while costing more.
C24 makes sense when its additional structural performance is useful or when the project specifically requires it.
So rather than asking:
“Which grade is best?”
A better question is:
“Which grade does this particular job need?”
Which should I use for my project?
This is usually what customers actually want to know.
| Project | What should you consider? |
|---|---|
| Domestic decking subframe | C16 is commonly used where timber sizes, spans and supports are suitable. C24 can be used for more demanding designs. |
| Normal garden fencing | C16 vs C24 usually isn't the main decision. Purpose-made posts, rails and boards are normally more relevant. |
| Shed or simple garden frame | C16 can be suitable for general framing. |
| Internal stud wall | C16 is commonly suitable for general framing where a higher strength grade isn't required. |
| Floor joists | C16 or C24 can be used depending on timber size, span, spacing and load. |
| Roof rafters | Either grade may be suitable depending on the roof design. C24 becomes useful for more demanding spans. |
| Pergola or garden structure | C16 can suit straightforward supported structures. Larger spans need more careful timber sizing. |
| Extension or larger structural build | Use the timber grade and dimensions specified for the project. |
| Longer or more demanding spans | C24 is more likely to be required. |
These are general examples rather than structural calculations.
The grade is only part of the decision. Timber size, span, spacing, support and treatment can all matter.
Do I need C24 for decking?
Not automatically.
C16 is commonly used for domestic decking subframes where the timber sizes, joist spans and support arrangement are suitable.
We're talking about the timber structure underneath the decking boards here — the joists and supporting frame — rather than the decking boards themselves.
For a straightforward garden deck, jumping to C24 simply because it's stronger isn't necessarily the right approach.
What may matter more is whether you've chosen the correct joist size, how far the joists are spanning, how closely they're spaced, how they're supported and whether the timber is appropriately treated.
C24 can become useful for more demanding decking structures or longer spans, but it isn't automatically required for a normal domestic deck.
What about fencing?
For an ordinary garden fence, C16 vs C24 often isn't the main buying decision.
You're more likely to need purpose-made treated fence posts, rails, feather edge boards or fence panels.
Structural carcassing timber may be useful for certain frames and garden structures, but someone putting up a normal feather-edge fence shouldn't assume every part needs to be C16 or C24.
Choosing the correct fencing products is normally more important.
C16 or C24 for floor joists?
Both C16 and C24 can be used for floor joists where the design allows it.
The important thing is that grade alone doesn't tell you whether a particular joist is suitable.
Floor joist selection also depends on timber width and depth, the distance it needs to span, spacing between joists, loads and how the joists are supported.
C24's higher structural performance can make it useful for longer or more demanding spans.
For an extension, loft conversion or new structural floor, you're much more likely to have drawings or structural information specifying the timber required.
If a particular C24 size has been specified, don't simply replace it with the same size C16 timber without checking.
C16 or C24 for roof rafters?
C16 can be suitable for roof construction where the design allows it.
C24 is more likely to be useful where longer spans or greater structural performance are required.
Again, grade isn't the only factor.
Rafter size, spacing, span, roof construction and loading all affect the timber required.
A small shed roof is very different from the roof of a house extension.
For substantial roof construction, work from the timber sizes and grades specified for that project.
C16 or C24 for stud walls?
C16 is commonly suitable for general timber framing and studwork.
For a straightforward non-load-bearing internal stud wall, there is usually little reason to automatically choose C24 simply because it's stronger.
The correct timber size, spacing and construction are normally more relevant.
If the wall is load-bearing or forms part of a structural design, follow the specification for the project.
When is C24 actually worth choosing?
C24 becomes particularly useful when the project requires its additional structural performance.
That may be because a longer span is required, the structure is carrying greater loads, greater stiffness is needed, larger structural work is involved or the drawings specifically call for C24.
For everyday DIY work, though, don't assume:
“Stronger = safer = better.”
Using the correct timber size and building the structure properly are just as important.
Does C24 have fewer knots?
Generally, C24 has tighter limits on characteristics that can reduce structural strength, including significant knots.
That means some C24 timber may look cleaner or more consistent than C16.
However:
C24 is a structural strength grade, not an appearance grade.
C24 timber can still contain knots.
If the timber will remain visible and appearance matters, consider the finish as well as the required structural grade.
Is C24 more durable than C16?
Not simply because it is C24.
This is an important distinction.
C16 and C24 tell you about structural strength.
They don't automatically tell you how well a piece of timber will resist decay or whether it's suitable for an exposed outdoor application.
That's where treated/tanalised vs untreated timber becomes another part of the buying decision.
Does C24 mean treated or tanalised?
No.
This is probably one of the most useful things to understand before buying structural timber.
C16 and C24 = strength grade.
Treated/tanalised or untreated = treatment.
They describe different things.
Reynolds stocks C24 timber in both treated/tanalised and untreated options, depending on the product and size.
You can therefore have two pieces of timber that are both C24, with one treated and the other untreated.
Choosing C24 alone doesn't tell you whether the timber is right for the environment where you plan to use it.
What is tanalised timber?
Tanalised timber is timber that has been preservative treated under pressure.
You'll also commonly hear it described simply as treated timber.
Treatment is relevant where additional protection against decay and insect attack is required for the intended application.
But treatment doesn't change C16 into C24.
The strength grade and the treatment are separate parts of choosing the timber.
A useful way to think about it is:
What strength grade do I need?
Then:
Does the timber need to be treated?
Treated or untreated C24: which do I need?
That depends on where and how the timber will be used.
For protected construction where preservative treatment isn't required, untreated C24 may be suitable.
Where the project requires preservative-treated structural timber, choose the appropriate treated/tanalised option.
Don't make the decision based purely on whether the project is inside or outside, particularly for important structural work.
If a treatment requirement is given on the project specification, follow it.
For more straightforward DIY garden work, choose timber intended and appropriately treated for the application.
Can I use C16 instead of C24?
There's a difference between choosing timber for a straightforward DIY project and substituting timber on a structural design.
If you're building a normal garden project and no particular grade has been specified, choose timber appropriate for what you're building.
But if drawings or calculations specifically state C24, don't simply replace it with the same size C16 timber.
The design may rely on the higher structural properties of C24.
Can I use C24 instead of C16?
C24 is the higher strength grade, but that doesn't mean you can ignore everything else about the timber specification.
You still need the correct width, depth, length, treatment, spacing and construction.
C24 doesn't fix the wrong timber size.
If a job requires a particular size of timber, don't assume a smaller section becomes suitable simply because it is C24.
Do I need plans to choose between C16 and C24?
Not for every project.
Someone building a straightforward garden deck, shed, stud wall or basic garden frame may not have formal structural drawings at all.
That's normal.
For these projects, start with what you're building and make sure the timber size, span, support and treatment are suitable.
The bigger and more structural the project becomes, however, the less you should be guessing.
For extensions, new structural floors, substantial roof work, loft conversions and other important load-bearing structures, you're much more likely to have drawings, structural calculations or a timber schedule specifying the required sizes and grades.
If you have that information, use it.
What size C16 or C24 timber do I need?
This is a separate question from strength grade.
You'll commonly see carcassing timber described using familiar sizes such as 3x2, 4x2, 5x2, 6x2, 8x2 and 9x2.
Always check the actual finished metric dimensions on the product as well.
The correct size depends on what you're building.
For a garden deck, this might depend on joist spans and supports.
For general framing, the intended structure matters.
For a structural floor or roof, the required section may be stated on the drawings.
Size and strength grade work together.
A higher strength grade is not a substitute for selecting the correct timber size.
So, should I buy C16 or C24?
For general DIY and building work, C16 is suitable for many everyday applications. Don't automatically pay for C24 if the job doesn't require it.
For longer spans or more demanding structural work, C24 is more likely to be useful because it is the higher strength grade.
If you've been given drawings or a timber schedule, use the grade and size shown.
If you're building a straightforward garden project, you may not have formal plans. Start with what you're building and make sure the timber size, span, support and treatment are suitable.
If you're carrying out substantial structural work and aren't sure what timber is required, don't guess. Get the requirements checked before ordering.
Buying C16 and C24 timber from Reynolds
Reynolds stocks C16 structural timber for general carcassing and building work, alongside a broader range of C24 structural timber for projects requiring the higher strength grade.
Depending on the product and size, Reynolds also supplies C24 in treated/tanalised and untreated options.
Before ordering, check:
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What are you building?
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What timber size do you need?
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Do you need C16 or C24?
-
Does it need to be treated/tanalised?
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What lengths and quantity do you need?
If you're working from drawings, a timber schedule or a materials list and aren't sure which Reynolds product matches it, speak to the Reynolds team before ordering.
C16 vs C24 FAQs
Which is stronger, C16 or C24?
C24 is the higher structural strength grade.
C16 is still structural timber and is suitable for many general construction applications.
Is C24 always better than C16?
No.
C24 is stronger, but if C16 is suitable for the job there may be no practical benefit in choosing C24.
What does the C mean in C16 and C24?
The C refers to coniferous softwood.
The number forms part of the structural strength classification.
Is C16 good enough for decking?
C16 is commonly used for domestic decking subframes where the timber size, joist spans and support arrangement are suitable.
For outdoor projects, also make sure the timber is appropriately treated for its intended use.
Do I need C24 for decking?
Not automatically.
C24 can be useful for more demanding structures, but a straightforward domestic deck may use appropriately sized and treated C16 timber.
Can C16 be used for floor joists?
Yes, where the timber size, span, spacing and loads are suitable for C16.
For significant structural building work, use the grade and section required by the design.
Can C16 be used for roof rafters?
Yes, where the roof design allows it.
C24 may be useful or specified for more demanding spans.
Does C24 mean treated timber?
No.
C24 is the structural strength grade.
Treatment is a separate specification, and Reynolds stocks C24 in both treated/tanalised and untreated options depending on the product.
Is C24 better outside?
Not simply because it is C24.
Outdoor suitability and durability need to be considered separately from strength grade.
Does C24 have fewer knots?
C24 generally has tighter limits on strength-reducing characteristics such as significant knots, but it isn't a knot-free or decorative appearance grade.
Can I replace C24 with C16?
Not without checking where C24 has been specified structurally.
Changing strength grade can affect whether the timber is suitable for the design.
Do I need plans before buying C16 or C24?
Not for every DIY project.
For larger structural work such as extensions, floors, roofs and loft conversions, drawings or calculations are much more likely to specify the required grade and timber dimensions.
C16 vs C24: the main thing to remember
C24 is stronger than C16, but stronger doesn't automatically mean better for your project.
C16 is suitable for many general building and DIY applications. C24 becomes particularly useful where greater structural performance, longer spans or a specific C24 requirement is involved.
And don't choose timber based on grade alone.
Think about the grade, timber size, span, treatment and what you're actually building.
That's a much better way to choose the right timber than simply buying C24 because it has the bigger number.