Writing a precise timber specification is one of the most consequential decisions in the procurement phase of any construction project. A well-drafted specification eliminates ambiguity, ensures competitive tendering, reduces site disputes, and delivers the structural performance the design demands. A poorly written one invites substitution, quality failures, and programme delays.
This guide provides a systematic approach to writing structural timber specifications for construction tenders, following NBS-style conventions and referencing current European standards.
Why Timber Specifications Matter
Timber is a natural material with inherent variability. Unlike steel or concrete — where a grade designation guarantees near-identical performance — timber properties vary by species, growth region, moisture history, and grading method. The specification is the single document that controls this variability and ensures the delivered product matches the designer's assumptions.
A specification that simply states "C24 timber" leaves critical questions unanswered:
- What moisture content at delivery?
- What dimensional tolerances?
- What surface finish?
- What certification is required?
- What preservative treatment, if any?
- How should it be stored on site?
Each unanswered question becomes a potential dispute during construction.
Structure of a Timber Specification
A professional timber specification follows a logical hierarchy, moving from general requirements to specific product clauses. The recommended structure is:
1. Scope and General Requirements
This section defines what the specification covers and establishes the overarching quality framework.
SCOPE
This specification covers all structural softwood timber for use in:
- Primary structural frame (floor joists, roof rafters, wall studs)
- Secondary structural elements (noggings, blocking, battens)
- External timber cladding substrate framing
GENERAL REQUIREMENTS
All timber shall comply with the Construction Products Regulation (EU) 305/2011
or UK equivalent (UKCA marking) and carry valid CE/UKCA marking.
2. Standards and References
List all applicable standards. This creates a clear contractual framework:
| Standard | Title | Application |
|---|---|---|
| EN 338 | Structural timber — Strength classes | Grade specification |
| EN 14081-1 | Timber structures — Strength graded structural timber | Grading requirements |
| EN 336 | Structural timber — Sizes, permitted deviations | Dimensional tolerances |
| EN 13183-1/2 | Moisture content of sawn timber | MC measurement |
| EN 335 | Durability of wood — Use classes | Treatment specification |
| EN 351-1 | Durability of wood — Preservative-treated solid wood | Treatment penetration |
| EN 14080 | Timber structures — Glued laminated timber | Glulam specification |
| EN 15497 | Structural finger jointed solid timber | Finger-jointed members |
3. Material Specification Clauses
This is the core of the document. Each clause should be unambiguous and measurable.
Strength Class and Grading
The strength class clause is the foundation of any structural timber specification:
STRENGTH CLASS
All structural timber shall be strength class C24 to EN 338, unless
otherwise noted on structural drawings.
GRADING METHOD
Timber shall be machine stress graded (MSG) in accordance with EN 14081-1
by a grading body holding valid accreditation from a national accreditation
body (e.g. UKAS, DAkkS, RISE).
MARKING
Each piece shall be indelibly marked with:
- Strength class (C24)
- Grading body identification mark
- CE/UKCA marking reference number
- Standard reference (EN 14081-1)
- Species or species group
For projects using C24 structural construction timber, the specification should explicitly state the acceptable species group. While EN 338 makes strength class independent of species, some projects have aesthetic or durability reasons to prefer specific species.
Moisture Content
Moisture content (MC) is arguably the most critical parameter after strength class. Timber delivered too wet will shrink, twist, and potentially develop mould:
MOISTURE CONTENT
Structural timber for internal use shall have a moisture content not
exceeding 18% at time of delivery, measured in accordance with EN 13183-2
(resistance method).
Target moisture content: 16% ±2%
Maximum permitted: 20%
Measurement: Minimum 10% of delivered pieces, selected randomly
Timber for enclosed wall panels or insulated floor cassettes shall have
a moisture content not exceeding 16% at delivery.
Joinery-grade timber shall have a moisture content of 10–14% at delivery,
appropriate to the equilibrium moisture content of the installed environment.
Dimensional Requirements
Specify both the target dimensions and the tolerance class:
DIMENSIONS
Timber sections shall be as indicated on structural drawings.
Regularised (planed) timber shall comply with EN 336 Tolerance Class 1:
- Dimensions ≤100 mm: ±1.0 mm
- Dimensions >100 mm: ±1.5 mm
Length tolerance: +25 mm / −0 mm (to allow site cutting)
SURFACE FINISH
Structural timber for concealed locations: sawn finish acceptable
Structural timber for exposed locations: planed all round (PAR) to
achieve surface roughness ≤0.5 mm
Certification and Chain of Custody
Responsible sourcing is now a baseline requirement for most projects:
CERTIFICATION
All timber shall be sourced from legally harvested forests and carry
valid chain-of-custody certification under one of the following schemes:
- Forest Stewardship Council (FSC)
- Programme for the Endorsement of Forest Certification (PEFC)
- Or equivalent scheme recognised by the project sustainability assessor
DOCUMENTATION
The contractor shall provide, prior to delivery:
- Chain of custody certificate number
- Declaration of Performance (DoP) for each product
- Species identification and country of origin
- Preservative treatment certificate (where applicable)
Preservative Treatment
Treatment specifications must reference the use class system:
PRESERVATIVE TREATMENT
Timber in Use Class 1 (internal, dry): No treatment required
Timber in Use Class 2 (internal, risk of wetting): Factory-applied
surface treatment with fungicidal preservative
Timber in Use Class 3.1 (external, above ground, coated): Pressure
treatment with copper-based preservative to EN 351-1, minimum
sapwood penetration 6 mm
Timber in Use Class 3.2 (external, above ground, uncoated): Pressure
treatment with copper-based preservative to EN 351-1, minimum
sapwood penetration 12 mm, heartwood penetration 6 mm
Timber in Use Class 4 (ground contact): Pressure treatment with
copper-based preservative to EN 351-1, full sapwood penetration,
minimum heartwood penetration 6 mm
TREATMENT CERTIFICATION
Treatment shall be carried out by a facility holding valid quality
assurance certification (e.g. WPA Benchmark scheme or equivalent).
Treatment certificate to be provided for each batch.
Writing Effective Specification Clauses
The Anatomy of a Good Clause
Every specification clause should follow this structure:
- What — the requirement (measurable, unambiguous)
- How — the standard or method of verification
- When — the point at which compliance is assessed
- Consequence — what happens if the requirement is not met
Example of a well-written clause:
Structural timber shall achieve strength class C24 to EN 338 [WHAT],
verified by machine stress grading to EN 14081-1 [HOW], assessed at
point of manufacture with grade stamp visible on delivery [WHEN].
Timber not bearing a legible grade stamp shall be rejected and removed
from site within 24 hours at the contractor's expense [CONSEQUENCE].
Common Specification Errors to Avoid
Error 1: Specifying species instead of strength class
❌ "All structural timber shall be Scandinavian Whitewood" ✓ "All structural timber shall be strength class C24 to EN 338. Preferred species: Picea abies (Norway Spruce) or equivalent achieving C24 grade."
Specifying only species does not guarantee structural performance and may limit the supply chain unnecessarily.
Error 2: Omitting moisture content requirements
❌ "Timber shall be kiln dried" ✓ "Timber shall have a moisture content not exceeding 18% at delivery, measured to EN 13183-2. Kiln drying certificate to be provided."
"Kiln dried" is not a measurable specification — timber can be kiln dried and then re-absorb moisture during storage or transport.
Error 3: Vague quality language
❌ "Timber shall be of the best quality" ✓ "Timber shall comply with EN 14081-1 visual override requirements: no wane exceeding 1/3 of any face, no splits exceeding 2× section width in length, no insect damage."
"Best quality" is subjective and unenforceable. Measurable criteria create clear pass/fail assessment.
Error 4: Failing to specify tolerances
❌ "47 × 200 mm C24 timber" ✓ "47 × 200 mm C24 timber to EN 336 Tolerance Class 1 (±1.0 mm on 47 mm dimension, ±1.5 mm on 200 mm dimension)"
Without tolerance specification, any piece within the standard's loosest tolerance class is technically compliant.
Error 5: Incomplete treatment specification
❌ "Timber shall be treated" ✓ "Timber shall be pressure-treated with copper-based preservative to achieve Use Class 3.2 performance per EN 335, with minimum sapwood penetration of 12 mm per EN 351-1. Treatment to be carried out post-machining."
Specification for Engineered Timber Products
Glued Laminated Timber (Glulam)
BSH glued laminated beams require additional specification clauses beyond solid timber:
GLUED LAMINATED TIMBER (GLULAM)
Strength class: GL24h to EN 14080
Lamination thickness: Maximum 45 mm
Adhesive type: Type I (MUF or PRF) for Use Class 3 exposure
Type II (MUF) acceptable for Use Class 1 and 2
Delamination resistance: Tested to EN 14080 Annex C
Finger joint strength: Minimum 24 N/mm² (matching beam class)
Surface quality: Industrial finish (for concealed members)
or Visible quality (for exposed members — no open defects
on visible faces, filled knots acceptable)
DIMENSIONAL TOLERANCES (GLULAM)
Cross-section dimensions: ±2 mm
Length: ±5 mm or ±0.1% of length, whichever is greater
Straightness: Maximum deviation L/500
Pre-camber (where specified): ±10% of specified camber
Cross-Laminated Timber (CLT)
CLT specifications require particular attention to panel properties:
CROSS-LAMINATED TIMBER (CLT)
Manufacturer: Shall hold valid ETA (European Technical Assessment)
for the CLT system proposed
Lamination grade: Minimum C24 for longitudinal layers,
minimum C16 for transverse layers
Number of layers: As indicated on structural drawings (minimum 3)
Panel thickness: As indicated on structural drawings ±1 mm
Adhesive: Formaldehyde-free polyurethane (PUR) or equivalent
Edge bonding: Required / Not required (as specified)
Surface quality: Non-visible / Industrial visible / Residential visible
Fire Performance Specification
Fire performance clauses are increasingly important as timber is used in larger buildings:
FIRE PERFORMANCE
Structural timber members required to achieve fire resistance shall be
designed to EN 1995-1-2 using the reduced cross-section method.
Charring rate: 0.65 mm/min for softwood (EN 1995-1-2 Table 3.1)
Additional char depth (d₀): 7 mm
Where fire-retardant treatment is specified:
- Treatment type: Pressure-impregnated fire retardant
- Classification achieved: Euroclass B-s1,d0 to EN 13501-1
- Durability: Treatment shall maintain classification for the
design life of the building (minimum 60 years for permanent structures)
- Certification: Test reports from notified body, less than 5 years old
Procurement and Delivery Clauses
Lead Time and Programming
LEAD TIME
The contractor shall allow the following minimum lead times from
order confirmation to delivery:
- Standard C24 sections (stock sizes): 2–4 weeks
- Non-standard sections or lengths: 4–8 weeks
- Glulam beams (standard sizes): 4–6 weeks
- Glulam beams (custom sizes/curves): 8–12 weeks
- CLT panels: 10–16 weeks
- Fire-retardant treated timber: Add 2–4 weeks to above
DELIVERY SCHEDULING
Timber deliveries shall be coordinated with the construction programme
to minimise on-site storage duration. Maximum on-site storage before
installation: 4 weeks (subject to adequate protection).
Storage and Protection
SITE STORAGE
Timber shall be stored on level bearers at minimum 150 mm above ground,
with bearers at maximum 1200 mm centres to prevent sagging.
Protection: All timber shall be covered with breathable waterproof
sheeting (not polythene) within 24 hours of delivery. Sheeting shall
be secured against wind uplift and arranged to allow air circulation.
Engineered timber products (glulam, CLT) shall be stored in accordance
with the manufacturer's written instructions. Factory wrapping shall
remain intact until immediately prior to installation.
Quality Assurance and Inspection
Inspection Regime
INSPECTION
The following inspection regime shall apply:
PRE-DELIVERY
- Review of DoP and certification documents
- Confirmation of lead times and delivery schedule
- Factory inspection (for engineered timber products over £50,000 value)
ON DELIVERY
- Visual inspection of 100% of delivered pieces
- Moisture content testing of minimum 10% of pieces (random selection)
- Dimensional check of minimum 5% of pieces
- Grade stamp verification on 100% of structural members
- Reject any pieces with: missing grade stamp, visible decay,
excessive distortion (bow >10 mm/2 m, twist >5 mm/2 m),
or moisture content exceeding specified maximum
DURING INSTALLATION
- Verify correct grade is used in correct location
- Check moisture content before enclosure (must be ≤16% before
covering with insulation or boarding)
- Photograph all structural connections before covering
Rejection Criteria
Define clear, measurable rejection criteria to avoid disputes:
| Defect | Rejection Threshold |
|---|---|
| Missing or illegible grade stamp | Reject 100% |
| Moisture content >20% | Reject piece |
| Bow (flatwise) | >10 mm per 2 m length |
| Bow (edgewise) | >8 mm per 2 m length |
| Twist | >5 mm per 2 m length |
| Spring | >8 mm per 2 m length |
| Splits (end) | >2× section width |
| Wane | >1/3 of any face |
| Blue stain (structural) | Accept (no structural effect) |
| Blue stain (visible grade) | Reject piece |
| Insect damage (active) | Reject piece |
| Insect damage (historic, treated) | Accept if structurally sound |
Template: Complete Timber Specification Clause
Below is a complete example clause suitable for inclusion in an NBS-style specification:
SECTION L20: TIMBER FRAMING
PRODUCTS
110 STRUCTURAL SOFTWOOD TIMBER
- Strength class: C24 to EN 338
- Grading: Machine stress graded to EN 14081-1
- Species: Picea abies (Norway Spruce) or Pinus sylvestris (Scots Pine)
- Moisture content at delivery: Maximum 18%, target 16% ±2%
- Dimensional tolerance: EN 336 Tolerance Class 1
- Surface finish: Regularised (planed) all structural members
- Certification: FSC or PEFC chain of custody
- Marking: CE/UKCA marked, grade stamp on each piece
- Treatment: None (Use Class 1 — internal, permanently dry)
120 STRUCTURAL SOFTWOOD TIMBER (EXTERNAL)
- As clause 110, except:
- Moisture content at delivery: Maximum 20%, target 18% ±2%
- Treatment: Pressure-treated to Use Class 3.2 per EN 335
- Preservative: Copper-based, minimum sapwood penetration 12 mm
- Treatment timing: Post-machining, pre-delivery
- Treatment certificate: Required for each batch
130 GLUED LAMINATED TIMBER (GLULAM)
- Strength class: GL24h to EN 14080
- Lamination species: Picea abies
- Adhesive: Type I (MUF or PRF)
- Surface quality: Visible quality (exposed members)
- Finish: Sanded to 120 grit, ready for coating
- Moisture content: 12% ±2%
- Dimensional tolerance: ±2 mm cross-section, ±5 mm length
- Certification: FSC or PEFC chain of custody
- Factory finish: One coat clear preservative base coat
EXECUTION
210 GENERALLY
- Install timber at moisture content within 4% of expected
equilibrium moisture content for the installed environment
- Do not enclose timber with moisture content exceeding 16%
- Provide minimum 10 mm clearance between timber and masonry/concrete
- All cuts and notches to be treated with brush-applied preservative
matching the factory treatment specification
220 STORAGE
- Store on level bearers at minimum 150 mm above ground
- Cover with breathable waterproof membrane within 24 hours
- Maximum on-site storage: 4 weeks before installation
- Separate species and grades to prevent mixing
230 INSPECTION AND TESTING
- Contractor to maintain timber delivery log recording:
date, supplier, quantity, grade, MC readings, certificate references
- Architect/engineer reserves right to independent testing at any time
- Non-compliant material to be removed from site within 48 hours
Digital Specification Tools
Modern specification writing increasingly uses digital platforms:
NBS Chorus / NBS Source
NBS provides pre-written clauses for timber specification under work sections L20 (Timber framing), Z12 (Timber products), and Z22 (Preservative treatments). These clauses are regularly updated to reflect current standards and can be customised for each project.
BIM Integration
For BIM-enabled projects, timber specifications should be linked to the model through:
- IFC material properties matching specification clauses
- COBie data drops including certification references
- Embedded product data sheets from manufacturers
Specification Checklist
Before issuing a timber specification for tender, verify:
- Strength class stated with EN 338 reference
- Grading method specified (MSG preferred)
- Moisture content limits defined with measurement method
- Dimensional tolerances stated with EN 336 class
- Surface finish requirements clear
- Certification scheme(s) identified
- Treatment specified by use class with penetration requirements
- Fire performance requirements included (if applicable)
- Delivery and storage requirements stated
- Inspection and rejection criteria defined
- Documentation requirements listed
- Lead times acknowledged in programme
Coordination with Other Trades
Timber specifications do not exist in isolation. Coordinate with:
Structural Engineer: Confirm strength class, connection details, fire resistance period, and any special requirements (pre-camber, specific species for durability).
Building Services Engineer: Confirm notching and drilling limitations, service zone requirements, and acoustic performance needs.
Fire Engineer: Confirm fire resistance period, whether fire-retardant treatment is required, and surface spread of flame classification.
Sustainability Assessor: Confirm which certification schemes earn credits under the project's assessment method (BREEAM, LEED, HQM).
Contractor/Subcontractor: Confirm availability of specified products, lead times, and any buildability concerns with the proposed specification.
Updating Specifications During Construction
Specifications are living documents. Common reasons for specification changes during construction include:
- Supply chain disruption requiring alternative species or source
- Design changes requiring different strength class
- Value engineering proposals from the contractor
- Discovery of site conditions requiring enhanced treatment
All changes must be formally documented through:
- Contractor's request for substitution (with technical justification)
- Designer's assessment of equivalence
- Formal instruction (Architect's Instruction or equivalent)
- Updated specification clause issued to all parties
Never accept verbal agreements to change timber specifications — the consequences of using incorrect material may not become apparent for years.
Related Resources
- Structural Timber Guide — comprehensive reference for timber construction
- C24 Structural Construction Timber — product specifications and availability
- BSH Glued Laminated Beams — engineered timber beam solutions
- Timber Moisture Content Guide — understanding MC in construction