When you’re building a garden room, whether a humble shed, a summer house, or a fully insulated timber-framed garden room, the choice of structural timber matters. The grades C16 and C24 refer to the strength class of the timber, and each has different implications for strength, span, stability, and ultimately the safety and performance of your build.
Table of Contents
Key Takeaways from this Article
- C16 vs. C24: C24 is stronger, stiffer, and cleaner than C16, making it best for load-bearing or exposed timber; C16 works for vertical loads, or lighter, internal structures.
- Use Span Tables: Proper sizing and spacing of joists, beams, and rafters prevent deflection or failure; span tables are essential.
- Garden Room Recommendation: For floors, roofs, and long spans, choose C24; C16 is fine for light internal studs, but all timber must be graded and treated if exposed.
Understanding the Grading System
- The “C” in grades like C16 and C24 stands for “conifer”, indicating softwood (e.g. pine, spruce) sourced from coniferous trees.
- The number (16 or 24) relates to the minimum bending strength (in N/mm²) that timber must achieve to be classed in that grade.
- Grade-marking is important: structural timber should be clearly stamped with its strength class. Timber without a visible strength-grade stamp should not be used for structural applications.
Thus, the two grades reflect different levels of strength and suitability: while both are “structural timber,” C24 carries stricter grading requirements, fewer knots, straighter grain, and higher density, resulting in greater strength, stiffness, and reliability.
The difference between C16 and C24 timber comes down to strength, stiffness, and overall quality. Both are structural softwoods graded under the European C-strength class system, but C24 is the higher grade.
Key Differences
| Property | C16 | C24 |
|---|---|---|
| Strength | Standard structural grade | Higher structural strength |
| Stiffness | More flexible | Stiffer (less deflection) |
| Quality | More knots and defects allowed | Fewer knots, more uniform, higher density |
| Span capability | Shorter spans for the same size | Longer spans for the same size |
| Weight | Usually slightly lighter | Slightly heavier (denser timber) |
| Cost | Cheaper and more widely available | More expensive |
What are the Technical Differences?
According to industry data, timber in class C24 typically has:
- Higher bending strength (minimum 24 N/mm² vs. 16 N/mm² for C16)
- Greater stiffness, higher modulus of elasticity, and higher characteristic density, meaning it resists bending or deflection more effectively under load.
- Tighter growth grain (often from slower-grown trees, e.g. Scandinavian or Baltic softwood), resulting in fewer and smaller knots, straighter pieces, and improved dimensional stability.
In practice, this means C24 is more robust and reliable, particularly for structural uses where spans are long, loads are significant, or stability matters (e.g. floors, roof rafters, decking frames, wall studs under load).
Why Span Tables Matter
When designing structures like a garden room, you cannot rely on strength class alone; you need to ensure the sizes and spacing of timbers are appropriate for the span and loading. That’s where span tables come in.
- The publication of span tables for both C16 and C24 timber by Timber Development UK (TDUK) offers guidance on the load-bearing capacity of softwood members in floors, ceilings, and roofs of domestic dwellings.
- Another authoritative resource is Eurocode 5 Span Tables (4th edition), widely regarded as the standard for structural timber specification in the UK and Europe. These tables show permissible sizes, spans, and spacings for members graded C16 or C24 under certain load conditions.
By consulting span tables, you can determine, for example, what joist size (depth × width), spacing (centres), and support spacing are needed to safely carry the load of a garden-room floor, roof, or ceiling. This helps avoid overloading, excessive deflection, or structural failure.
What it Means for Building a Garden Room
When building a garden room, you should choose timber based on what you’re asking the structure to do. Here’s a rough guide:
| Use or requirement | Recommended timber grade / notes |
|---|---|
| Internal wall studs (non-loadbearing or light loads) | C16 – sufficient and cost-effective |
| Floor joists, roof rafters, beams, or anywhere that needs to span a distance or support loads (floor, roof, heavy doors/windows) | C24 – safer and more stable “premium” structural timber |
| Any exposed structural timber (e.g. roof beams, floor joists in ceilings, visible frame) where appearance and fewer defects matter | Prefer C24 for cleaner look, straighter grain, fewer knots |
| Where timber is used outside, or ground contact is possible | Ensure appropriate preservative treatment (e.g. UC-grade treatment) — strength class does not equate to durability by itself. timberonline.co.uk+2Pasquill+2 |
In short, for a garden room, especially if it has a floor, loft/ceiling, or larger open spaces, C24 is usually the safer bet. C16 can be used for lighter interior work, or where spans are small and loads are light.
Why I Recommend C24 for Floors and Roofs, and C16 for Walls
I recommend using C24 timber for floors and roofs because its greater strength and stiffness mean it can span longer distances with fewer supports, making the build simpler and more reliable. Its straighter, cleaner appearance also makes it easier to work with, especially if structural elements will be visible, such as floor joists or roof rafters.
For internal walls or non-load-bearing partitions, C16 timber is sufficient. Since the load on these walls is vertical rather than horizontal, the timber isn’t spanning gaps but carrying weight along its strongest dimension. This makes C16 a cost-effective choice for studs and partitions where the higher strength of C24 isn’t necessary.
Using span tables for both grades ensures your timber sizes and spacing are based on engineering guidance, helping your garden building remain safe, stable, and compliant with building standards.
Conclusion – What I’d Recommend if You’re Building a Garden Room
If you’re planning to build a garden room, whether a small summerhouse, a large insulated space, or anything in between, I’d generally recommend using C24 kiln-dried softwood, graded and stamped to the C24 mark, especially for load-bearing components like floor joists, roof rafters, beams, and long spans.
For lighter, internal, or non-load-bearing elements (e.g. internal stud walls, noggins, non-structural partitions), C16 can be acceptable, but only if you use appropriate sizes and spacing guided by span tables, and only if you accept its limitations (more defects, less stiffness, shorter spans).
Finally, whatever grade you choose, make sure the timber is graded and stamped, and if it’s to be used outdoors or in contact with moisture, make sure it’s preservative-treated and sealed.
References & Further Reading
- The grading system for structural timber (C-grades defined by European/UK standards), see “What is C16 and C24 Timber?” from Buildworld. Buildworld
- Span-table guidance for C16 and C24 timber, tables published by Timber Development UK (TDUK) for joists, floors, roofs. Timber Development UK
- Further span table guides and references – Timber Beam Calculator UK
