C2: Preservative treatment2.1 GENERAL
As explained in the Design Guidance section, A4 , new European Standards are being introduced which will replace the existing process-type specification with a results-type specification system. Specifiers should check with timber treatment plants and the relevant preservative company's technical departments whether existing British Standards or new European Standards methods of treatment are in use. Both systems depend on preservative penetration and retention in order to be effective. With either the BS or EN system the specifier must decide:
* The desired durability required and assess the likely Hazard Class
* The relevant code of practice
* The type and method of preservative treatment
There are 4 main types of preservatives:
* Tar Oil Preservatives to BS 144 Creosote (but not recommended)
* Inorganic Waterborne to EN 599 e.g. Copper Triazole or ACQ
* Organic Solvents to BS 5707, EN599 e.g. Prevac or Vac-Vac
* Micro-Emulsions to EN 599 e.g. Prevac or Vac-Vac
The three most widely used methods of preservation are (i) micro-emulsions (ii) organic solvent double vacuum for timber not in ground contact and (iii) inorganic water-based pressure treatment for timber in ground contact.
Organic solvent preservation does not have a timber/metal reaction problem and it does not affect timber moisture content (no-redrying). It does not cause dimensional change to timber sections and has little effect on timber colour. It should be specified for close-fitting joinery such as doors and windows. The specification clauses which follow give the British Standard and European approaches to joinery and timber preservation. Specify one system only. Generally do not mix the BS and EN systems together but some of the newer British Standards do refer to the EN Hazard Class system.
2.2 ORGANIC PRESERVATIVE TREATMENT IN ACCORDANCE WITH EXISTING BRITISH STANDARDS FOR OUT OF GROUND CONTACT
Treatment methods and timber to be treated
Double Vacuum organic solvent wood preservative treatment shall comply with B.S. 5707 Part 1, class F/N; i.e. fungicide plus insecticide and timber should be treated in accordance with BS 8417 Table 6.
Double vacuum schedules
Please refer to BS 8417
Treatment charge sheets
Treatment schedule charge sheets shall be provided with all deliveries, copies of which shall be sent to the architect. A treatment docket is not acceptable unless specifically agreed beforehand with the architect. A full treatment charge sheet with complete information is required to be provided to the satisfaction of the architect.
2.3 INORGANIC PRESERVATIVE TREATMENT IN ACCORDANCE WITH EXISTING BRITISH STANDARDS FOR OUT OF GROUND CONTACT BELOW DPC LEVEL AND HAZARDOUS END USES
Treatment with copper organic based wood preservatives should be in accordance with BS 8417. The Critical Value (CV) is specific to each preservative formulation and will vary according to the hazard class.
2.4 PRESERVATIVE TREATMENT IN ACCORDANCE WITH EUROPEAN STANDARDS EN 335, EN 599.
Both the British and European Standards relate the risk of timber decay (whether through fungal or insect attack) to the situation where it is being used. Timber used below DPC level in has a higher risk than timber used internally within a building or for timber use above DPC level. EN 335-1 describes 5 Hazard Classes ranging from Hazard Class 1, where timber is essentially not at any risk of timber decay but may be subject to insect attack, to Hazard Class 5 where timber is permanently exposed to salt water and at its greatest risk of decay.
The European standards require a demonstration of proof that the treatment used has produced the specified results. This can only be achieved through reference to chemical analytical methods. The preservative penetration and retention are intended to be the same as those achieved by current British Standard-based processes. Preservatives such as inorganic concentrates and organic solvent types will continue to be used as they are today. However, water-based micro-emulsion preservatives have been developed (these are also referenced in BS 8417 rather than BS 5268-5) and are available to respond to the requirement for more environmentally compatible preservative treatments. These preservatives can fulfil the role of traditional ones in many circumstances.
Where sapwood is present, or heartwood which does not have adequate natural durability for its intended end use, preservative shall be applied according to its Hazard Class, as defined in EN 335 Definition of Hazard Classes. Other European Standards relevant to the determination of appropriate treatment are EN 350 Natural Durability Classes, EN 351 Penetration and Retention Classes and EN 599 Performance Tests for Preservatives. A certificate of preservation shall be attached to each delivery of each Hazard Class and copies sent to the architect see 2.2.3. The moisture content of the timber to be delivered to site shall not exceed that specified.
Hazard class/service situation
Hazard Class 1:
Floor-boards, floor joists (excluding ground floor), timber above ground floor not
built into external walls.
Hazard Class 2:
Ground floor boards and joists, frame timbers in timber-frame houses, roof timbers,
timbers built into external walls.
Hazard Class 3A:
External cladding, joinery, fascias, bargeboards, soffits and valley gutter timbers
Hazard Class 4:
Sole plates below DPC, fence posts; playground equipment (if in ground contact).
Hazard Class 5:
Marine, salt water contact.
Note: A more detailed table and additional information can be found in the Design Guidance section for designing for durability (A4) .
2.5 RE-TREATMENT OF CUT SURFACES
All cutting, machining, profiling and notching must be completed prior to assembly. Any cutting of treated surfaces must have the architect's approval. Where the architect gives such approval, all exposed surfaces shall be retreated with a liberal brush or spray application of a suitable preservative. Ripsawing, planing and heavy sanding will not be permitted unless the timber is returned for re-treatment.