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Sustainability · For Architects
Timber preservative treatment specification depends entirely on Use Class — getting it wrong leads to either premature rot (under-specified) or unnecessary cost and environmental impact (over-specified). This template covers correct specification per EN 335 and EN 351-1.
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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
UC1 (internal dry) → UC4 (in ground/water contact). Define from EN 335 service environment.
Vacuum-pressure impregnation per EN 351-1 (the only acceptable method for UC3-4).
P1–P9 per EN 351-1. Defines depth of preservative penetration.
kg/m³ active ingredient. Specified per Use Class and species treatability.
Copper-based (typical), boron, or organic. Specify if specific allergens / restrictions apply.
WTPC scheme membership or equivalent. Treatment certificate per consignment.
On-site cuts must be re-treated with end-grain preservative on UC3-4 applications.
Example Clause (NBS Format)
TIMBER PRESERVATIVE TREATMENT Use Class: UC4 (in ground contact / fresh water) per EN 335 Treatment process: Vacuum-pressure impregnation per EN 351-1 Penetration class: P8 minimum Retention: Per WTPC tables for species and Use Class Active ingredient: Copper-based water-borne preservative (e.g., Tanalith E or equivalent) Certification: - Treatment plant: WTPC (Wood Protection Association) member - Treatment certificate per consignment, traceable batch number on each piece - Marking: Treated timber stamp visible on each piece On-site cuts: - All cut ends, drilled holes, and re-machined surfaces in UC3 or UC4 service shall be re-treated with end-grain preservative per manufacturer specification - Provide field application kit with each delivery for site use
Copy and paste this clause into your tender. Adjust dimensions, grades, and project-specific requirements.
Common Errors
Specifying "treated timber" without Use Class
Correction: Always state UC1, UC2, UC3, or UC4 per EN 335. Without it, suppliers default to lowest treatment.
Surface application instead of pressure treatment
Correction: Brush / spray treatment only suitable for UC1. UC3-4 require vacuum-pressure impregnation.
No re-treatment specification for site cuts
Correction: Pressure-treated timber loses protection at every cut. Specify field re-treatment for UC3-4.
Reference Standards
Frequently Asked
UC4 — fence posts are in ground contact. UC3 (above-ground external) is insufficient and the post will rot at the ground line within 5–10 years.
UC3 if joists are clear of ground (200mm+ clearance) and ventilated. UC4 if joists are within 200mm of ground or in ground contact.
Modern copper-based preservatives are safer than older CCA treatments but still require gloves, dust masks during cutting, and avoiding contact with potable water sources. Read the WTPC handling guidance for the specific treatment.
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