Structural Timber Grades: C16 vs C24 Explained

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, which awards the visual grades GS and SS, or by machine grading (a machine measures stiffness and outputs a grade automatically). Visual grades are converted to a strength class through the species tables in BS EN 1912.¹

The conversion depends on the species and where it grew. Imported European whitewood (spruce) graded GS and SS becomes C16 and C24.¹ British-grown spruce graded GS is only C14.² That is why most visually graded C24 on sale in the UK is imported. Graded timber is stamped with the strength class, the grading body’s identification and a UKCA or CE mark.

Key Mechanical Properties

These are the characteristic values in the 2016 edition of BS EN 338.³

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 for longer-span domestic floors, where deflection rather than strength usually sets the size.

Common Sections

UK structural timber is supplied in a limited range of cross-sections, usually regularised, meaning machined on the edges to a consistent depth, rather than planed all round. The dominant thickness in domestic framing is 47 mm:

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 span tables use an imposed load of 1.5 kN/m², with separate columns for dead loads (the floor’s own boards, ceiling and finishes) of up to 0.25, 0.5 and 1.25 kN/m².⁴ Joists are commonly spaced at 400 mm, 450 mm or 600 mm centres.

Timber Development UK (formerly TRADA) publishes free span tables for C16 and C24 domestic floor joists, calculated to Eurocode 5. For an imposed load of 1.5 kN/m² and a dead load of up to 0.5 kN/m², they give these maximum clear spans for 45 mm regularised joists:⁴

At the same depth and spacing, C24 spans about 11% further than C16.⁵

These tables are a guide. 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 is graded from better-quality timber, with fewer and smaller knots, but at merchant retail in 2026 it costs almost exactly the same as C16. At Travis Perkins in September 2026, a 3.6 m length of regularised, treated 47 × 150 in C16 was £22.94 excluding VAT.⁶ The same length in C24 was £23.09 excluding VAT.⁷

Where you buy matters more than the grade: sawn, treated C24 carcassing from an independent merchant has been around £698 per m³. Because the grade makes so little difference to the price, specifying C24 to allow a shallower joist or a longer span usually costs little or nothing extra.

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:

• Longer floor joist spans, where deflection usually governs and C24 reaches about 11% further than C16 at the same depth
• Trussed rafters, although most UK truss makers use TR26, a grade specific to trussed rafters, rather than C24⁸
• Flat-roof decking joists where live and dead loads combine
• 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 the equilibrium moisture content (EMC) it will settle at in service before it is enclosed. Eurocode 5 groups this into service classes: in Service Class 1 most softwoods stay at or below about 12% moisture, in Service Class 2 (covered, but humid or unheated) at or below about 20%, and Service Class 3 is anything wetter, such as timber exposed to the weather.⁹ Timber sold as kiln dried or ‘dry’ graded has an average moisture content of 20% or less, so it can still shrink as it dries to indoor levels and should be allowed to acclimatise.⁹

Structural timber in Service Class 3 or in ground contact should be naturally durable or pressure-treated with preservative to BS 8417 for the relevant Use Class: UC3 for exterior timber above ground, UC4 for timber in contact with the ground or fresh water.⁹

Standards Reference

• BS EN 338: Strength classes for structural timber¹
• BS 4978: Visual strength grading for softwood¹
• 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 compliance¹⁰

The rules in this note are those that apply in England. Scotland, Wales and Northern Ireland have their own building standards and permitted development rights.