Specification guidance
When specifying wood based panels or indeed any product, a number of things need to be considered in that process. Not all that is considered will be relevant but all must be considered. In certain situations a precise specification is needed so that an appropriate material can be found to meet that specification. A good specification will ensure that an appropriate product is purchased for the end use that is specified.
All performance tests are the same for all wood based panels except those tests that are carried out for factory production control purposes. As a designer or architect the test values of concern are those performance tests for a particular end use such as characteristic strengths and stiffnesses or tests for flooring, roofing or walls or the water vapour resistance values for walls etc. rather than the factory production tests. The manufacturer or agent should be able to present the product information required for the end use specified and it should not be assumed that all panel products are the same in terms of performance or suitability for a particular end use. For example, to specify ‘plywood’ or ‘OSB’ for a structural application would not be enough information, you would need to specify much more precisely as there will be requirements such as strength, requirements for different service classes etc.
In EN 13986, BS 8103-3 and EN 12369 parts 1 to 3 there are tabulated design values for various aspects that are related to the panel products meeting certain minimum requirements, usually a technical class and/or density.
PB, OSB, Fibreboards, SWPs and LVL are fairly easy to specify as they are based on simple technical classes (e.g. OSB/3 for structural use in service class 2) and density (e.g. >600 kg/m3), and have a good number of performance tests that easily relate to those technical classes in EN 12369-1, BS 8103-3 or EN 13986.
On the other hand plywood is not as simple. It does have structural data in EN 12369-2 but the route to getting it is not as simple as meeting a technical class. Plywood has 3 technical classes EN 636-1, EN 636-2 and EN 636-3 for its suitability in dry, humid and exterior conditions and in order to declare structural values from EN 12369-2, the manufacturer has to state the bending strength and stiffness classes. These classes are declared for both directions of the board, e.g. F10/20 E30/40, the mean density is also required. Some plywoods (specific types of Finnish, Swedish, Canadian and North American) but not all can conform with tabulated values in BS 8103-3. EN 13986 and the tabulated values within are based on density. When specifying for exterior use, consideration of the wood species and the presence of sapwood should be taken into consideration in terms of durability and service life, ENV 1099 can be used for guidance.
Cement bonded particleboard is not featured in many tabulated property tables except in EN 13986 where it is based on density. If structural values are required then this will need to be tested.
Specification check list
• Service class (dry, humid or exterior as defined in the Eurocodes)
• Use class (previously hazard class as per EN 335-1 where use class 1 & 2 are essentially dry and humid and use classes 3, 4 & 5 are basically various severities of exterior use)
• Desired service life in years.
• Maintenance schedule.
• Location of panel
• Appearance of finished item
• Panel thickness
• Veneer thickness(plywood only)
• Number of ply (plywood only)
• Face Species natural durability
• Core species natural durability
• Preservative treatment (plywood only)
• Fire treatment
• Edge profile
• Edge sealing
• Joint treatment: Adhesive/dry
• Moisture content at time of installation and that when in service (relates to expansion and shrinkage)
• Type of finish (some finishes might be required to obtain the desired service life or durability):
– Finish
– Colour
– Gloss level
– Build up
• Preparation of panel for finishing
• The wood’s legality and sustainability status:
• Chain of Custody evidence
• Scheme: FSC
• Structural credibility for your required purpose (3rd party product certification or test results / claims based on CE marking or BSI Kitemark and equivalents)
• Substrate (i.e. what the board is to be fixed to e.g. C16 softwood studs/solid background)
• Fixings size
• Fixings spacing
• Fixing material
• Fixing preparation (if any)
• Unsupported edges
• Fastening type: screw, nail. bolt and head profile, drive type: slot/cross/etc.
• Fastening material and finish
• Fastening spacing:
• Fastening edge distance
• Setting out of boards: (e.g. staggered joints or expansion gaps)
• Board size
• Any other requirements specific to an end use such as:
– Thermal conductivity
– Acoustic performance
– Acoustic isolation
– Water vapour transmittance/resistance
– Reaction to fire classification
– Airtightness
– Windtightness
– Off-gassing
