Structural timber in the UK is strength-graded to give engineers and builders a guaranteed minimum performance level. The two grades you will encounter on virtually every UK domestic project are C16 and C24 — the numbers refer to the characteristic bending strength in N/mm². Picking the wrong one doesn’t always cause immediate problems, but it can result in excessive deflection, springy floors, or — in the worst case — failure of a critical member.
What the Grade Numbers Mean
The ‘C’ stands for conifer (softwood), and the number is the characteristic bending strength in N/mm² — the 5th-percentile value below which only 5% of pieces are expected to fall. This is defined in BS EN 338: Structural timber — Strength classes.
The grade is assigned either by visual grading (an inspector examining each piece for knots, slope of grain, checks and wane) under BS 4978 (UK visual grades GS and SS map to C16 and C24 respectively) or by machine grading (a machine measures stiffness and outputs a grade automatically). Timber must be marked with the CE mark, strength class, and the grading body’s identification.
Key Mechanical Properties
| Property | C16 | C24 |
|---|---|---|
| Characteristic bending strength (fm,k) | 16 N/mm² | 24 N/mm² |
| Characteristic tensile strength (ft,0,k) | 10 N/mm² | 14 N/mm² |
| Characteristic compression strength (fc,0,k) | 17 N/mm² | 21 N/mm² |
| Mean modulus of elasticity (E0,mean) | 8,000 N/mm² | 11,000 N/mm² |
| Mean density | 310 kg/m³ | 420 kg/m³ |
| Characteristic density (ρk) | 310 kg/m³ | 350 kg/m³ |
The mean modulus of elasticity is particularly important for floor joists: stiffer timber (C24 at 11,000 N/mm²) deflects less under the same load, which is why C24 is preferred or required for longer-span domestic floors to meet the serviceable deflection limit of span/400 under variable load.
Common Sections
UK structural timber is supplied in a limited range of cross-sections, typically regularised (planed all round) to consistent dimensions. The dominant thickness in domestic framing is 47 mm:
| Section (mm) | Common use |
|---|---|
| 47 × 75 | Non-structural noggins, light duty |
| 47 × 100 | Stud partitions, short ceiling joists |
| 47 × 125 | Floor joists (short spans), ceiling ties |
| 47 × 150 | Floor joists, ceiling joists, rafters |
| 47 × 175 | Floor joists (medium spans), ridge boards |
| 47 × 200 | Floor joists, flat roof joists |
| 47 × 225 | Floor joists (long spans), flat roof, transfer |
For built-up beams (two or three pieces nailed together), sections such as 47 × 200 or 47 × 225 can be combined into 94 × 200 or 3/47 × 200 members where a steel beam would be over-specified.
Spans: Floor Joists
Allowable span depends on the grade, section, spacing, and loading. For domestic floors, the standard imposed load is 1.5 kN/m² (habitable rooms) with a dead load of approximately 0.5–0.7 kN/m². Joists are commonly spaced at 400 mm or 600 mm centres.
Indicative maximum clear spans from the TRADA Span Tables for Solid Timber Members in Floors, Ceilings and Roofs (TRADA document valid under Eurocode 5):
| Section | Grade | 400 mm centres | 600 mm centres |
|---|---|---|---|
| 47 × 150 | C16 | ~2.7 m | ~2.4 m |
| 47 × 150 | C24 | ~3.1 m | ~2.8 m |
| 47 × 175 | C16 | ~3.2 m | ~2.9 m |
| 47 × 175 | C24 | ~3.7 m | ~3.4 m |
| 47 × 200 | C16 | ~3.8 m | ~3.5 m |
| 47 × 200 | C24 | ~4.4 m | ~4.0 m |
| 47 × 225 | C16 | ~4.4 m | ~4.0 m |
| 47 × 225 | C24 | ~5.0 m | ~4.6 m |
These are approximate indicative values only. The actual permissible span must be read from the appropriate TRADA span table or calculated under BS EN 1995-1-1 (Eurocode 5), accounting for the specific loading, notch positions, and end conditions.
Cost Comparison
C24 carries a modest premium over C16 because it is graded from better-quality timber — slower-grown, tighter-grained material with fewer large knots. The premium varies with the market, but as a guide in mid-2026:
| Grade | Typical cost ex-merchant (47 × 150, 4.8 m lengths) | Cost/m³ |
|---|---|---|
| C16 | £7–£9 per length | £250–£330 |
| C24 | £9–£12 per length | £310–£420 |
Premium is typically 15–30% for C24 over equivalent C16. On a typical semi-detached extension floor, the additional cost of upgrading from C16 to C24 throughout is usually £80–£200 in material — often worthwhile to allow smaller sections, reduce dead load, or meet a longer span.
Where Each Grade is Used
C16 is standard for:
- Stud-frame partitions (load-bearing and non-load-bearing)
- Short-span ceiling joists (up to ~2.5 m)
- Noggins, blocking and trimmer frames
- Roof rafters on shorter pitches where the span is limited
- Temporary works and formwork
C24 is required or preferred for:
- Floor joists spanning over ~3.0 m (to control deflection)
- Trussed rafter chords and webs (most truss manufacturers specify C24 minimum for their designs)
- Flat-roof decking joists where live and dead loads combine
- Engineered timber systems (LVL, glulam) which are graded to equivalent or higher classes
- Situations where section depth is restricted and maximum span must be extracted from the timber
Moisture Content and Treatment
All structural timber should be at or close to its equilibrium moisture content (EMC) for the service condition before it is enclosed. For internal dry conditions, EMC is approximately 10–14%; for external or exposed conditions, 15–19%. The standard Service Class 1 (dry indoor) and Service Class 2 (damp or intermittent wetting) are defined in Eurocode 5. Timber supplied at ‘kiln dry’ (KD) typically has a moisture content below 20% but should be allowed to acclimatise.
Structural timber used in ground contact or in situations of Service Class 3 (permanently wet or exposed to weather) must be pressure-impregnated with preservative to BS 8417 and the relevant Use Class (UC3 for above-ground exterior, UC4 for in-ground).
Standards Reference
- BS EN 338: Strength classes for structural timber
- BS 4978: Visual strength grading for softwood
- BS EN 1995-1-1 (Eurocode 5): Design of timber structures
- BS 8417: Preservation of wood — code of practice
- TRADA Span Tables: Practical span tables for UK domestic construction (aligned to EC5)
- Approved Document A (Structure): References EC5 and span tables for deemed-to-satisfy compliance
