Wood-based panels and cedar shakes and shingles in weather exposed use (BS1995 Service Class 3)
This specification gives recommendations for the types of substrates and flame retardants that can be used in weather exposed situations, e.g. cedar shakes and shingles, unprotected playground equipment and other exposed timbers.
The specifier can specify in accordance with European or BS 476 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 EN14915 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 0 as defined in Building Regulations.
For cedar shakes and shingles an AA rating to BS 476: Part 3 is required.
For unprotected timbers exposed to direct rainfall the risk is one of chemicals being resolubilised and washed out of the substrate.
Only Type EXT and INT2 (with a suitable maintained coating) offer long-term resistance to leaching in 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 selection must meet the following minimum requirements:
BS EN 314-2, Bond class III
BS EN 636, class III
However, BS EN636, class II might be acceptable in instances where a type INT2 process also contains preservative properties in addition to the flame retardant ones. 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 of application
Type LR treatment or type INT2 (with a suitable maintained coating) may be used in a weather-exposed situation where long-term durability is required. Application is by vacuum/pressure full cell processes.
1.5 Model specification phrase
WPA FR5 commodity specification: [insert material/species] to be treated to [insert required fire performance class from this list: Euroclass C; or Euroclass B; or Class 1 Surface Spread of Flame to BS 476 Part 7; or Class ‘O’ BS 476 Parts 6 & 7; or an AA rating to BS 476: Part 3], using a WPA Type EXT or Type INT2 (with a suitable maintained coating) formulation* listed in the WPA Flame Retardant Specification Manual*.
* 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 Bench FR Quality Scheme, the phrase ‘in accordance with the requirements of the WPA Benchmark FR Flame Retardants Quality Scheme’ should be added to the specification