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Structural Timber · For Architects
A complete C24 structural timber specification protects your project from wrong moisture content, ungraded timber, and uncertified material — three of the most common causes of timber defects on construction sites. This template provides a tender-ready clause aligned with NBS conventions and EN 14081-1 requirements.
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NBS-format clause, common errors checklist, and standards reference. Editable PDF format ready for your tender documents.
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Specification Checklist
Specify C24 explicitly — never "structural grade" or "stress graded". Reference EN 338.
Require machine stress graded (MSG) per EN 14081-1. Visual grading produces inconsistent quality.
Spruce/Pine/Fir (SPF) is standard. Specify single species only when architecturally required.
12% (±2%) at delivery for internal Service Class 1 use. 16% (±2%) for SC2.
CE marked per EN 14081. FSC or PEFC chain of custody for sustainability compliance.
Reference EN 336. Specify regularised or planed if precision required.
Sawn (default) or Planed All Round (PAR) — affects cost and lead time.
Specify Use Class 3 or 4 preservative treatment if exposed to weather or ground contact.
Example Clause (NBS Format)
STRUCTURAL TIMBER (C24) Specification: - Strength class: C24 to EN 338 - Grading: Machine stress graded to EN 14081-1 - Species: Spruce/Pine/Fir (Picea abies / Pinus sylvestris) - Moisture content: 12% ±2% at delivery (Service Class 1) - Marking: CE marked with grade stamp visible on each piece - Certification: FSC or PEFC chain of custody required - Tolerances: To EN 336, regularised - Sections: As shown on structural drawings - Lengths: Standard mill lengths, no end-jointed material unless specified Acceptance: - Reject any piece exceeding 14% moisture content on delivery - Reject any piece without legible CE grade stamp - Verify chain of custody documentation on first delivery
Copy and paste this clause into your tender. Adjust dimensions, grades, and project-specific requirements.
Common Errors
Specifying "structural grade timber" without strength class
Correction: Always state C16, C24, or C30 explicitly — these are defined strength classes per EN 338.
Omitting moisture content
Correction: Always specify target MC (typically 12% ±2% for SC1). Without this, suppliers can deliver wet timber.
Accepting visual grading only
Correction: Specify machine stress graded — visual grading is permitted but produces less consistent results.
No CE marking requirement
Correction: CE marking is mandatory under the Construction Products Regulation. Always require it.
No certification specified
Correction: Specify FSC or PEFC CoC — required for BREEAM credits and EUDR compliance.
Reference Standards
Frequently Asked
C24 has 50% higher bending strength (24 vs 16 N/mm²) and significantly higher stiffness. C24 is the standard structural specification for floor joists, roof rafters, and load-bearing studs in European construction.
Not necessarily. The most common specification is "Spruce/Pine/Fir (SPF)" which allows the supplier to deliver any species meeting C24 grade. Specify a single species only when architecturally required (e.g., visible larch).
For external structural use (covered, ventilated — Service Class 2), specify 16% ±2%. For fully exposed external (Service Class 3), specify 18–20%. Most internal heated buildings require 12% ±2% (Service Class 1).
Yes. CE marking is mandatory for structural timber sold within the EU under the Construction Products Regulation (EU 305/2011). UKCA marking applies in Great Britain post-Brexit.
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