Flame retardant treatment of wood and wood-based panels for weather protected exterior use (BS EN 1995 Service Class 2)
This specification gives recommendations for the types of substrates and flame retardants that can be used in weather protected exterior situations, e.g. covered walkways, protected overhangs and exterior timbers with a weather resistant maintained coating (see 5.6 for conditions for use of coatings).
The specifier can specify in accordance with BS 476 or European fire test methods provided this complies with Building Regulations Approved Document B where applicable.
For panel products for constructional end uses, the specifier must specify in accordance with BS EN 13986 if CE marking is a requirement.
For solid wood panelling and cladding products for constructional end uses, the specifier must specify in accordance with BS EN 14915 if CE marking is a requirement.
Materials used in such situations generally require a Euroclass C or class 1 surface spread of flame to BS 476: Part 7. In high risk areas they may need to comply with Euroclass B or class ‘O’ as defined in Building Regulations.
Provided protection from direct rainfall is afforded, either by means of an overhang, or as a ceiling on a walkway, or by a weather resistant maintained coating (5.6), then the hazard situation is as for interior situations at high humidities (section 6.3).
As relative humidity increases Type INT1 treatments absorb excess moisture from the atmosphere. This can result in the substrate reaching higher moisture content levels than untreated wood with migration of salts occurring. Excess moisture can also lead to corrosion of metal fastenings, strength losses, adverse effects on finishes and decay.
Only treatment types INT2 and EXT are recommended for these situations.
1.3 Before and after treatment
The material before treatment should be in a suitable condition as recommended in Section 4.
The quality of joinery timber should be in accordance with BS EN 942.
Care should be taken to exclude material infected with incipient decay, particularly in the case of western hemlock.
Plywood for FR1 must meet the following minimum requirements:
BS EN 314-2, Bond class III
BS EN 636, class II
Where European product standards exist, the untreated plywood or board products must meet the requirements of the standard if the material is to be CE marked.
Fabrication after treatment: as far as possible all major machining, particularly sawing and ripping, shall be carried out prior to treatment.
Where post-treatment machining, cross cutting, boring of holes, etc. is unavoidable, the freshly exposed surfaces must be treated as specified by the manufacturer of the treatment involved. This stipulation is particularly important with those treatments, which confer protection from fungal attack, since in these cases, it is essential to maintain a protective
envelope of treated timber if protection is to be afforded under adverse conditions.
Where extensive machining occurs after treatment, e.g. ripping of solid timber sections, then the material shall be returned to the treatment plant for reprocessing.
Impregnation with water-containing flame retardants increases the moisture content of wood. After treatment this needs to be reduced to a level suitable for the end use of the wood (see section 5.3). Drying is usually accelerated by kiln drying.
1.4 Flame retardant treatments and methods application Types INT2 and EXT
Application can be by double vacuum, full cell or empty cell to suit the permeability of the substrate.
1.5 Model specification phrase
WPA FR4 commodity specification: [insert material/species] to be treated to [insert required fire performance class from this list: Euroclass B; or Euroclass C; or Class 1 Surface Spread of Flame to BS 476 Part 7; or Class ‘O’ BS 476 Parts 6 & 7] using a WPA Type [insert INT2; or EXT] formulation* listed in the WPA Flame Retardant Specification Manual Ref: WPA/FRSM E3/OCT 2016.*
* Where specifiers wish to specify use of a particular named flame retardant product, and/or to require that treatment be carried out in accordance with the WPA Benchmark FR Quality Scheme appropriate details can be inserted where an asterisk appears in the above sentences. For treatment in accordance with the WPA Benchmark FR Quality Scheme, the phrase ‘in accordance with the requirements of the WPA Benchmark Flame Retardants Quality Scheme’ should be added to the specification.