Treated Softwood Decking

There are two ways to specify treated wood – 1. by referring to the British Standard BS8417 or 2. by using the Wood Protection Association (WPA) commodity code system :

Option 1: Using reference to the British Standard

The British Standard for wood preservation is BS8417. This code of practice defines treatment by preservative penetration and retention requirements by species, component use class, desired service (15, 30 or 60 years) and type of preservative..

The essential information that needs to be communicated to the timber supplier is:

  • the species of wood;
  • the component description, g. fence post, cladding) and use class
  • the desired service  life – 15, 30 or 60 years plus,
  • the preservative type, in instances where a specific preservative is preferred or required by the service life/end-use conditions

Sample specification clause using BS8417:

“[insert species if desired] fence posts to be treated with preservative [insert preservative type if desired] according to the recommendations in BS 8417 for a use class [insert use class number e.g. 4] application from which a desired service life of [insert number-15, 30 or 60] years is required.”

How to Specifying treated timber Andrew Goto
Treated Waney Edge Cladding

Option 2: Using the  Wood Protection Association (WPA )Commodity Code System

Interpreting the penetration and retention results set out in BS8417 is not easy without detailed knowledge about wood characteristics and preservative types.To avoid the pitfalls of incorrect specification, the WPA developed a simple system of codes based on the commodity that required preservative protection.

These “C” (Commodity) Specifications have been used for many years and are called up by other specification bodies such as NBS and NHBC. The only information that needs to be communicated to the timber supplier is:

  • component description, g. 18×94 timber cladding for use class 3 application
  • commodity specification, g. C6 for external timber cladding above ground

There are currently twelve WPA Commodity Codes. These are set out below. The precise details of each specification are set out in the WPA Manual: Wood Preservation Specification & Practice

C1      Timber in cooling towers

C2      Timber permanently or intermittently in contact with sea or fresh water

C3      Fencing timber

C4      Agricultural/horticultural timbers

C5      Non-load-bearing external softwood joinery and external fittings (excluding cladding) not in ground contact

C6      External timber cladding

C7      Timber for use in buildings in termite infested areas

C8      Constructional timbers (excluding walls of timber-framed houses)

C9      Timber framed housing

C10    Hardwood external joinery

C11    Wood-based board and engineered wood products

C12    Decking timber

More information on:

Biological Treatment preservatives and processes


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