Glulam treated to C3. Use Class 3 (UC3) — often referred to as “C3” in shorthand — is designed for external applications above ground, where the timber is exposed to frequent wetting but not in contact with soil or water. This treatment level is defined under BS EN 335 and implemented via BS 8417 and the WPA Code of Practice
Treatment Requirements
To meet UC3 standards, timber must be:
• Pressure treated with preservative
• Achieve minimum penetration and retention levels as per BS 8417
• Suitable species (e.g. pine, spruce) must be permeable or pre‑incised
• Often specified with a 15–30 year desired service life.
Common Mistakes to Avoid
• Using UC2 indoors timber outdoors — it will fail quickly
• Assuming all treated timber is UC3 — treatment class must be specified *
• Using UC3 timber in ground contact — that requires UC4.
*Sample Specification
“All softwood components to be pressure treated to Use Class 3 (BS EN 335) in accordance with BS 8417, using an approved preservative (e.g. Tanalith E), with a minimum desired service life of 15 years, applied by a certified treatment plant.”
Click here for full Data on Wood Preservation.
Class of Preservative Protection UC
1. Above ground, covered. Permanently dry. Insect risk.
2. Above ground, covered. Occasional risk of wetting.
3a. Above ground, not covered. Exposed to short spells of wetting (several days). Normally coated if exposed directly to bad weather.
3b. Above ground, not covered. Exposed to frequent wetting (several weeks)
4. In contact with ground or fresh water. Permanently exposed to wetting.
The following Desired Service Life categories apply to Celcure products:
Components in Use Class 1 & 2 – 60 years.
Components in Use Class 3 – 15 and 30 years.
Components in Use Class 4 – 15 and 30 years