For years, most conversations about decking focused on what people could see.
Timber or composite? Grey or brown? Grooved or woodgrain?
But the structure beneath the boards is receiving far more attention. Installers are becoming more aware of timber treatment classes, customers are asking better questions and merchants are increasingly being asked to supply something more specific than simply “treated timber”.
One specification appearing more frequently is C24 structural timber pressure treated to UC4.
However, C24 and UC4 are not two ways of describing the same thing. They refer to two separate characteristics of the timber, and understanding the difference matters when selecting materials for decking subframes and other demanding external applications.
C24 describes the timber’s structural strength grade. UC4 describes the treatment class.
Why the timber beneath a deck matters
The decking boards create the finished appearance, but the subframe provides the structural support beneath them.
Depending on the design, a decking subframe may include joists, bearers, posts, blocking and additional framing around board ends, borders, steps and changes in direction.
Once the deck is complete, most of this structure disappears from view. It still has to support the decking, furniture, people and any other loads placed upon it throughout the life of the installation.
That is why the timber used beneath a deck should not be treated as an afterthought.
What is C24 timber?
C24 is a structural strength grade.
Strength grading assesses characteristics that influence how timber performs when used structurally, including its strength, stiffness and density.
Specifying C24 confirms that the timber has been graded to meet a defined level of structural performance. It does not, however, tell you how the timber has been treated or which external environments it is suitable for.
What is UC4 timber?
UC4, also known as Use Class 4, is a timber treatment classification for demanding external applications, including situations where timber may be in direct contact with the ground or fresh water.
The treatment is intended to help protect the timber against biological deterioration in those end-use conditions.
UC4 does not describe the timber’s structural strength. Timber can be pressure treated to UC4 without being structurally graded as C24.
The important distinction
C24 is about strength. UC4 is about treatment.
| C24 | A structural strength grade |
| UC4 | A treatment class for demanding external end uses |
Why specifying “treated timber” may not be enough
Asking for treated timber does not automatically confirm either the timber’s structural grade or the treatment class applied to it.
Similarly, asking only for UC4 timber confirms the required treatment class but does not confirm that the timber has been structurally graded.
The description C24 structural timber pressure treated to UC4 communicates both requirements:
- C24 identifies the structural strength grade.
- UC4 identifies the treatment class.
Clearer specifications help merchants and installers understand exactly what is required and reduce the risk of either element being overlooked.
Where is C24 UC4 timber used?
C24 structural timber pressure treated to UC4 may be specified for demanding external structures where both structural performance and an appropriate treatment class are required.
Potential applications can include certain elements of:
- Decking subframes
- Ground-level external structures
- Landscaping structures
- External supports and frameworks
- Other structural applications exposed to demanding conditions
The correct specification will depend on the design, location and end-use conditions of the individual project.
Timber dimensions, spans, support positions, connections and fixings must also be suitable for the loads the structure will carry. Using C24 UC4 timber does not replace the need for a properly designed and correctly constructed frame.
Why are more customers asking about treatment classes?
Customers have access to more product information than ever before.
They research decking materials, compare warranties, read installation guides and ask installers what will be used beneath the finished surface.
At the same time, installers and merchants are becoming more precise in the way they specify structural and treated timber.
Rather than viewing all treated timber as interchangeable, there is growing recognition that the treatment class should reflect the timber’s intended end use.
This creates an opportunity for merchants to help customers understand that structural grading and timber treatment are separate parts of the specification.
C24 UC4 timber for decking subframes
A decking subframe must provide suitable support for the chosen decking system and be designed for the specific project.
Where Trex composite decking is being installed, the substructure, joist spacing, board layout and additional framing should follow the current Trex UK installation guide.
Additional support may be required around:
- Picture-frame borders
- Breaker boards
- Board ends and butt joints
- Steps and stair treads
- Railing posts
- Changes in board direction
- Areas carrying heavy or concentrated loads
The structure should also be planned with suitable drainage, airflow and moisture management in mind.
Always follow the current installation guide for the decking product being used.
Framing requirements may vary according to the product, board orientation, use of the deck and individual design details.
What should merchants ask?
When a customer requests structural timber for decking or another external project, a few additional questions can help establish what is actually required:
- Is the timber being used structurally?
- What treatment class has been specified?
- Will any part of the timber be in contact with the ground?
- What loads will the structure need to support?
- What dimensions, spans and support centres have been specified?
- Are there installation requirements linked to the finished product?
These questions help distinguish between the timber’s structural requirements and the environmental conditions it will face.
What should installers specify?
When ordering timber for a structural external application, avoid relying on broad descriptions such as “decking timber” or “treated joists”.
Where appropriate for the project, asking for C24 structural timber pressure treated to UC4 makes both the structural grade and treatment class clear.
Installers should still confirm that the timber dimensions, spans, supports and fixings are suitable for the specific design and follow all relevant manufacturer guidance, structural requirements and building regulations.
Arbor Forest Products’ timber treatment capability
Arbor Forest Products supplies a comprehensive range of structural and landscaping timber to merchants throughout the UK.
Our timber treatment facilities allow us to supply products treated for a range of Use Classes, including timber pressure treated to UC4 for demanding external applications.
By combining structural timber supply with extensive treatment capabilities, we can support merchants looking for clearly specified products for decking, landscaping and construction projects.
Remember: C24 and UC4 are not interchangeable
The key point is simple.
C24 tells you about the timber’s structural strength grade. UC4 tells you about the treatment class.
Neither term replaces the other.
Where a project requires structurally graded timber with treatment suitable for demanding external conditions, both parts of the specification should be considered and clearly communicated.
Speak to Arbor about C24 UC4 timber
For more information about C24 structural timber pressure treated to UC4, speak to your Arbor Forest Products account manager or contact our sales team.
- Telephone:
01469 532300 - Email:
sales@arborforestproducts.co.uk
Ask us about C24 UC4 timber
Speak to our team about structural timber for decking subframes and demanding external applications.
