Complete Guide to 4×2 Timber: Sizes, Grades & Uses
The 30-second answer: 4×2 timber (100mm x 50mm nominal size) is the UKâs primary structural component for internal stud walls, roof rafters, and floor joists. For internal framing, 4×2 CLS is the industry standard due to its rounded edges and eased dimensions, while Sawn Treated 4×2 is typically the best choice for external or damp-prone structural applications.
Key Takeaways:
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CLS (Canadian Lumber Standard) is planed and rounded for easy handling in internal partitioning.
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Sawn Treated timber is pressure-impregnated with preservative for protection against rot and insects.
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C16 and C24 are both structural grades; C24 offers higher density and fewer knots for longer spans.
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Actual vs Nominal: A “4×2” measurement refers to the size before processing; the actual finished dimensions will be smaller depending on the finish (CLS vs Sawn).
What is 4×2 Timber?
4×2 timber is a versatile structural softwood, traditionally measured in inches but sold in metric equivalents (100mm x 50mm). It is the backbone of UK residential construction, primarily utilised for framing, load-bearing partitions, and roofing. When selecting timber, you must distinguish between the “nominal” size (the size it is cut from the log) and the “finished” size (the size after kilning and machining).
Understanding the difference between C16 and C24 is essential, as these strength classes determine the load-bearing capacity of your structure. Both grades are kilned to reduce moisture content, ensuring stability during the timber frame construction process.
Options & Variations: CLS vs Sawn Treated
Choosing the right 4×2 depends on the environment of the installation and the required finish. In the UK market, the two primary variations are CLS and Sawn Treated.
Note: While we do not sell untreated sawn timber due to its limited durability in structural applications, we provide both high-quality CLS for internal use and pressure-treated sawn timber for structural longevity.
| Feature | 4×2 CLS Timber | 4×2 Sawn Treated Timber |
| Finish | Planed all round with rounded edges. | Rough sawn surface with square edges. |
| Actual Size | Typically 38mm x 89mm. | Typically 47mm x 100mm (Regularised). |
| Treatment | Untreated (Internal use only). | Pressure-treated (UC2) for rot resistance. |
| Primary Use | Internal stud walls and framing. | Roof rafters, joists, and external structures. |
Competitive Edge: The “Regularised” Advantage
Most 4×2 sawn timber in the UK is now “regularised,” meaning it has been passed through a machine to ensure a consistent width and thickness. This is vital for floor joists; without regularisation, slight variations in timber height would result in an uneven floor surface that would require significant sanding or shimming before laying floorboards.
How to Choose: C16 vs C24 Grade
When you buy 4×2 timber, it will be stamped with a structural gradeâeither C16 or C24. Both are high-quality, sustainable softwoods, but they serve different structural needs.
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C16 Grade: The most common structural grade for internal walls and rafters. It provides sufficient strength for most domestic applications and is the standard for 3×2 CLS and 4×2 internal framing.
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C24 Grade: This is a premium grade with fewer knots and higher density. It is the industry standard for high-load areas or where longer spans are required without additional vertical support.
Decision Framework: If you are building standard internal stud partitions, C16 4×2 CLS is the recognised standard.
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If you are installing floor joists or heavy-duty roof rafters, C24 Sawn Treated is typically the best choice for its superior structural integrity and ability to meet stricter building regulations.
4×2 Timber Span Tables & Applications
Timber sizing is governed by British Standards (BS EN 338) to ensure safety. Below is a general guide for common 4×2 applications.
1. Internal Stud Walls
For non-load-bearing walls, 4×2 CLS is spaced at 400mm or 600mm centres. This provides a rigid frame for 12.5mm plasterboard. If the wall is load-bearing, C24 grade is often specified to support the weight of the floor or roof above.
2. Floor Joists (Approximate Spans)
While 4×2 is common for mezzanine floors or small shed bases, larger domestic floors usually require 6×2 timber or 8×2 timber.
| Timber Size | Spacing (Centres) | C16 Max Span | C24 Max Span |
| 47 x 100mm | 400mm | 1.72m | 2.03m |
| 47 x 100mm | 600mm | 1.38m | 1.66m |
3. Roof Rafters
In traditional “cut and pitch” roofs, 4×2 Sawn Treated timber is frequently used for rafters, supported by purlins to reduce the clear span. For these structural connections, ensure you use the correct timber connectors to maintain joint strength.
Practical Execution & Pitfalls
When working with 4×2 CLS or 3×2 CLS, following these steps ensures a professional finish:
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Acclimatisation: Store timber in the room where it will be installed for 48 hours to prevent warping after installation.
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Levelling: Use a laser level or a 1.2m spirit level to ensure the “sole plate” (bottom timber) is perfectly flat before vertical studs are added.
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Fixing: Use 90mm or 100mm framing nails or screws.
Common Pitfall: Using untreated 4×2 CLS for ground-floor joists without a damp-proof membrane (DPM). Even for internal work, any timber in contact with masonry should be treated or protected by a DPM to prevent “rising damp” from rotting the wood.
Soft Conversion FAQs
What is the difference between 4×2 and CLS?
4×2 is the general size description. CLS timber is a specific type of 4×2 that has been planed and rounded for internal framing. Standard 4×2 is often rough-sawn and larger in actual dimensions.
Can I use 4×2 for decking joists?
Generally, no. For decking subframes, the industry standard is at least 6×2 treated timber to ensure the structure can handle the weight of the boards and people safely. Using 4×2 would require significantly more support posts to prevent “bounce.”
Is C24 always better than C16?
Not necessarily. While C24 is stronger, C16 is perfectly adequate for the majority of internal stud walls. You only need C24 when the structural calculations or spans specifically require the higher strength class.
Project-First Recommendation
For most internal renovation and stud-wall projects, the best starting point is 4×2 CLS. It provides the most consistent finish for plasterboarding. View available lengths and specifications here.
If you are dealing with structural roofing or floor joist work where moisture may be present, Sawn Treated 4×2 (C16/C24) is typically the best choice. Check current stock levels here.
For a broader overview of timber grades and species, consult our All You Need to Know About Timber guide or read our specific guide on 6×2 timber for structural work.
