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Architects and specifiers of plywood and all wood based panel products

Architects and Specifiers

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Guide to standards

Specification

Plywood used in the UK is sourced from all over the world and is often manufactured to overseas standards. Plywood manufactured for construction purposes is specified in accordance with EN 636. Products claiming compliance with this standard must also carry the specified markings, which includes a reference to EN 636. This standard was originally in three parts, but was re-issued in 2003 in a single part as follows:

EN 636 : 2003 : Plywood – Specifications

The 2003 edition of EN 636 retains the designations -1, -2 and -3 from the previous three part standard to represent dry, humid or exterior conditions of use.

The environmental conditions for which each of these types of plywood are considered suitable are defined according to the parameters laid down for hazard classes in EN 335-3:1996 “Durability of wood and wood-based products – Definition of hazard classes of biological attack. Part 3. Application to wood-based panels”:

Dry conditions: for interior applications with no risk of wetting, defined in hazard class 1, with a moisture content corresponding to environmental conditions of 20°C and 65% relative humidity.

Humid conditions: for use in protected exterior applications as defined in hazard class 2, with a moisture content corresponding
to environmental conditions of 20°C and 85% relative humidity.

Exterior conditions: for use in unprotected external applications, as defined in hazard class 3, where the moisture content will frequently be above 20%.

EN 636 also introduces bending strength and modulus classes based on bending tests to EN 310. These give a designated strength (F) and modulus (E) parallel and perpendicular to the face grain. An example designation would therefore be F10/20, E30/40.

EN 636 gives minimum values for each of the classes and EN 12369-2 gives corresponding characteristic values for use with each of these classes.

Plywood for use in construction must demonstrate compliance with the Construction Products Directive (CPD). The most straightforward route to achieving this is by demonstrating compliance with EN 13986, which can also involve the application of a CE mark.

The properties required of panels in EN 636 are shown in the table below:

Property Standard Plywood type to EN 636
    Dry Humid Exterior
 
Dimensional Tolerance EN 315
Bonding Quality EN 314-2 Bonding class 1 Bonding class 2 Bonding class 3
Mechanical Properties  Structural

- Characteristic values

- Bending strength
EN 636/EN 310/EN 12369-1 or EN 789/EN 1058
Non-structural
- Bending strength
EN 310
Formaldehyde Emission  Construction EN 13986 E1 or E2 E1 or E2 E1 or E2
Non-construction EN 636/DD ENV 717-1/EN 717-2 E1 or E2 E1 or E2

E1 or E2

Advice on Specification & Treatment of Exterior Plywood

Exterior service, in building terms, covers a wide range of situations from full and severe exposure (sign boards, sea front fascias) to sheltered and protected (soffits under porches and balconies). There are also applications other than in buildings that impose similar stresses on plywood in service.

Two different exterior situations where plywood has a long history of successful use can be recognised:

Exterior but protected - where direct wetting of surfaces and edges is unlikely once construction is completed such usage (sheathing, soffits, sarking, roof decking, etc) is designated as Hazard Class 2 according to BS EN 335-1 1-hazard classes of wood and wood-based products against biological attack. Classification of hazard classes and BS EN 336-3 Application to wood-based panels BS EN 636 2 Plywood - Specifications. Requirements for plywood for use in humid conditions is appropriate to these types of usage.

Full exposure to weather (characterised by the possibility of high moisture content in service, driving rain and full exposure to sunlight). Examples include cladding, fascias, exterior doors. These uses are designated Hazard Class 3 in terms of the risk of biological attack (mould, stain, decay). BS EN 636-3 Plywood - Specifications. Requirements for plywood for use in exterior conditions is appropriate to these types of usage.

The factors of exterior exposure that can be significant to the performance of plywood are:

  • Rain (particularly wind-driven) and where drying-out is slow.
  • Fluctuations in relative humidity.
  • Sunlight, particularly on south-facing aspects and dark coloured surfaces.
  • Fungal organisms.
  • Frost, insect attack and severe chemical pollution may sometimes also have an adverse effect on the service life of plywood.

Rain, either wind-driven or running down the facade of a building, can be rapidly absorbed by unprotected end grain of timber.

Plywood contains much end grain around the edges which must be protected if satisfactory performance is to be achieved. If the edges are not protected, water can enter and give rise to stresses which can eventually cause splitting of the wood and separation of the veneers around the edges even of ‘exterior’ bonded panels. In the shorter term, it often gives rise to unsightly staining behind a finish and can lead to mould growth, blistering of paint and ultimately decay.

It is, therefore, imperative that all edges of all panels are protected if satisfactory performance can be achieved.

Water usually penetrates much more rapidly than is lost. This is particularly the case if Its entry is localised, such as through an area of improperly applied edge sealant. Under these conditions, water can be trapped for long periods behind fairly impermeable finishes, putting considerable demands on the durability or preservation treatment at the plywood and the adhesion of the finish.

Durability of Glue Bonds

Plywoods are produced with glue bonds which range from those suitable for interior uses only, to those which will withstand full exposure for long periods.

BS EN 314-2 Plywood bonding quality. Requirements defines three classes on the basis of test requirements that the
plywood bond must meet:

Class 1: Dry conditions (interior, dry uses)
Class 2: Humid conditions (protected external service, 'damp' internal uses and limited exposure during construction)
Class 3: Exterior conditions (exposure to weather over substantial periods or continuous exposure to relative high humidity)

Full Exterior Bonding

Class 3 bonds are a requirement for plywood to comply with BS EN 636-3. The bonds are largely comparable with what were known as WBP - weather and boil-proof bonds (to BS 6566 Part 8, now withdrawn), except that the requirement that effectively they be made with phenolic-type glues has now been dropped. To qualify, the bond must now meet the stipulated test requirements.

Fire Ratings Explained

The use of Fire Retardant products is a crucial part of safety in public buildings in the event of fire.

The two classes of fire retardant panels are Euroclass B (limited combustibility and low surface spread of flames) and Euroclass C (low surface spread of flames).

Hanson Plywood Ltd offer a number of fire retardant products ex stock. We can also offer treatments on most panel products to these classes.

The Euroclass B & C ratings have superseded the old BS476 Class 0 and Class 1 ratings respectively. Although Euroclass C products are a directly compatible replacement for Class 1 please note that Euroclass B is not a direct substitution for Class 0. Please check carefully with your specification to ensure you order the correct panel.

 

Types of wood-based sheet materials
Product Conditions of Use Applications Standards
OSB (oriented strand board)
OSB/1 Use class 1 (under cover, not exposed
to weather and wetting)
Non-structural interior fitments, including furniture 0.0929m²
OSB/2 Use Class 1 Internal, structural applications, eg exhibition panels, internal walls, shelving, packing, cases 10.76 sq ft
OSB/3 Use Classes 1 and 2 (under cover, not exposed to weather but where high environmental humidity can lead to occasional, but not prolonged, wetting) Timber frame structural sheathing, flat and pitched roofs, wall sheathing, flooring, caravans BS EN 300:2006
OSB/4 Use Classes 1 and 2 Heavy-duty structural applications in humid conditions BS EN 300:2006
Particleboard (chipboard)
Type P1  Use Class 1 Internal non-structural applications, eg general purpose joinery and furniture BS EN 312:2010
Type P2 Use Class 1 Internal, non-structural furniture items, eg melamine-faced, veneered shelving, kitchen and bedroom units BS EN 312:2010
Type P3 Use Class 1 and 2 Non-structural applications similar to those listed under P2 above. However, P3
suitable for use in humid conditions
BS EN 312:2010
Type P4 Use Class 1 Internal structural applications, eg flooring, loft planks, bathrooms, kitchens BS EN 312:2010
Type P5 Use Classes 1 and 2 Structural applications similar to those listed under P4 above, however P5
suitable for humid conditions
BS EN 312:2010
Type P6 Use Class 1 Heavy-duty internal structural applications BS EN 312:2010
Type P7 Use Classes 1 and 2 Heavy-duty structural applications in humid conditions BS EN 312:2010
Plywood*
EN 636 1G Use Class 1 Internal non-structural applications BS EN 636:2003
BS EN 314-2:1993
Class 1 glue bond
EN 636 1S Use Class 1 Internal structural applications
EN 636 2G Use Classes 1 and 2 Non-structural applications in humid conditions BS EN 636:2003
BS EN 314-2: 1993
Class 2 glue bond
EN 636 2S Use Classes 1 and 2 Structural applications in humid conditions, eg timber frame wall sheathing, roofing, flooring
EN 636 3G Use Classes 1–3 (not under cover, not
in contact with ground, exposed to
weather continually or is protected from
weather but subject to wetting)
External, non-structural applications. Additional treatment of the plywood may be required depending on the natural durability of the timber species

BS EN 636:2003
BS EN 314-2:1993
Class 3 glue bond
DD CEN/TS 1099:2007

EN 636 3S Use Classes 1–3 External, structural applications. Additional treatment of the plywood may be required depending on the natural durability of the timber species
Standard MDF (medium density fibreboard)
Type MDF Use Class 1 Internal non-structural applications, eg general joinery, signage, door parts,
fascias
BS EN 622-5:2009
Type MDF .H Use Classes 1 and 2 Non-structural applications in humid conditions, eg kitchen and bathroom joinery BS EN 622-5:2009
Type MDF .LA Use Class 1 Internal, structural applications BS EN 622-5:2009
Type MDF .HLS Use Classes 1 and 2 Structural applications in humid conditions
NB these panels are restricted to instantaneous or short periods of loading
BS EN 622-5:2009
Exterior MDF Use Classes 1-3 External, non-structural applications, eg shopfronts, fascias, doors, cladding See manufacturer’s literature
or www.wpif.org.uk
Flame-retardant MDF Use Class 1 Internal, non-structural application where a Euro Class B and C (formerly Class 0 and Class 1) flame-retardant board is required See manufacturer’s literature
or www.wpif.org.uk
Hardboard
Type HB Use Class 1 Internal, non-structural applications, eg panelling, boxing in pipework BS EN 622-2:1997
Type HB.H Use Classes 1 and 2 Non-structural applications similar to those listed above for Type HB. However,
Type HB.H suitable for use in humid conditions
BS EN 622-2:1997
Type HB.E Use Classes 1–3 Non-structural applications similar to those listed above for Type HB. However,
Type HB.E suitable for use in external applications
BS EN 622-2:1997
Type HB.LA Use Class 1 Internal, structural applications BS EN 622-2:1997
Type HB HLA1 Use Class 2 Structural applications in humid conditions BS EN 622-2:1997
Type HB HLA2 Use Class 2 Heavy-duty structural applications in humid conditions BS EN 622-2:1997
Blackboard/Laminboard
Should comply with requirements of BS EN 636:2003, as these products are included within the definition of plywood Use Class 1 Generally used in applications requiring a product similar to plywood in appearance but at a lower cost. They are normally restricted to interior applications such as joinery, door blanks, furniture and shopfitting Blockboard and laminboard are not commonly marketed as being in accordance with any particular standard. However, blockboard and laminboard are covered by the definition of plywood and should comply with the CPR if they are to be used in construction

* Various grades of light MDF and ultra-light MDF are available. See BS EN 622-5:2009 or consult with manufacturers or visit www.wpif.org.uk

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