Specifications
| Property |
Detail |
| Nominal size |
47x100mm |
| Finished size |
~45x95mm (after regularising) |
| Available lengths |
2.4m, 3.0m, 3.6m, 4.2m, 4.8m, 5.4m, 6.0m |
| Grade |
Predominantly C24, with some lengths within packs graded to C16 |
| Treatment |
Pressure treated – for use not in ground contact |
| Finish |
Kiln dried, regularised, eased edges |
| Suitable for |
External structural use, not in ground contact |
Nominal vs Finished Size – What Is the Difference?
The 47x100mm size shown on this listing is the nominal size, measured before the timber is regularised – the finished size after planing is approximately 45x95mm. Regularising removes a small amount of material from each face to produce a consistent, smooth cross-section across every piece, which is why finished dimensions are always slightly smaller than nominal. Always use the finished size (45x95mm) when calculating joist hangers, fixings, or any application where exact clearance matters.
What Does C24 Grade Mean?
C24 is a structural strength grade for softwood timber under BS EN 338, indicating the timber has been graded for higher bending strength and stiffness than the lower C16 grade. Many structural applications specify C24 where longer spans or higher loads are required, and span tables set out the strength class needed for a given size, spacing and span. Because grading depends on the individual piece of timber, stock is not guaranteed 100% C24 – the majority of our stock meets this grade, with the remainder typically grading as C16. If your project specifically requires certified C24 throughout, mention this when ordering.
Applications
The 47x100mm structural joist is most commonly specified for floor joists and decking, alongside roofing and partition framing. Common uses include:
- Floor joists – supporting suspended timber floors in ground floor and upper floor construction
- Decking joists – the structural frame beneath deck boards, supported on posts or ledger boards
- Roof joists / rafters – for flat roof and low-pitch roof construction
- Stud wall partitions – internal non-load-bearing and load-bearing partition framing
- Outbuilding and shed framing – floor and wall frames for garden buildings
For decking specifically, joist spacing and span depend on deck board thickness, joist span, and expected loading – always check span tables or consult a structural guide for your specific deck design before finalising joist centres.
Working With Treated Joists
When cutting treated joists to length, always treat the freshly cut end with a compatible end-grain preservative, particularly for any external application. Pressure treatment penetrates the outer section of the timber but does not fully treat the core, so a cut end exposes untreated timber at the most vulnerable point for moisture ingress. For external use such as decking, ensure joists are not in direct contact with the ground – use deck posts, padstones, or joist supports to maintain a clear air gap.
You Will Also Need
Frequently Asked Questions
Can I use 47x100mm joists for floor joists in a house?
Yes, subject to span tables and building regulations for your specific application – joist size, spacing and span must comply with the current span tables for your timber grade, spacing and loading. Always check with a building control officer or structural engineer for load-bearing floor applications.
Are these joists treated for ground contact?
No. These joists are treated for use not in contact with the ground. For any application where timber will sit in or touch soil, a higher ground-contact treatment specification is required.
Why is the actual size smaller than 47x100mm?
47x100mm is the nominal size, measured before the timber is regularised (planed smooth on all faces). The finished size after this process is approximately 45x95mm – this is standard practice across the UK timber industry, not a discrepancy specific to this product. This product is most often referred to as 4 by 2, as it is very close in size to 4”x2”.
What is the difference between C16 and C24 grade?
C24 has higher bending strength and stiffness than C16 under BS EN 338, meaning it can span further or carry greater load for the same cross-section. Many structural applications specify a minimum of C24.
