Panel Products - Service Classes & Hazard Classes

Service classes and Hazard classes of panel products

Certain types of wood-based panels can be used in demanding exterior situations. However to provide the desired performance and service life they must be correctly specified, detailed, installed and maintained.

Exterior use covers a wide range of situations from full exposure, where the panels may be in contact with salt or fresh water to more protected situations where there is a risk of occasional wetting, such as soffits under porches and lorry decks.

There are two important concepts; ‘service class’ and ‘hazard class’, both of which, amongst other things, must be considered when specifying a wood-based panel for exterior use.

Service classes are used in structural design to define the environmental conditions in service which affect the mechanical performance of the panel under certain humidity and/or wetting conditions. Three service classes are defined in the structural design codes, BS 5268-2 and BS EN 1995-1-1, as shown in Table 1.

In some of the European (EN) panel product standards, these three service classes are referred to as “dry”, “humid” and “exterior” conditions respectively.

Table 1 Service classes and examples (from BS EN 1995-1.1).

Service class Temperature Approx RH* EMC** Examples from Eurocode National Annex
1 20°C 65% 12% Warm roofs, intermediate floors, timber-frame walls – internal and party walls
2 20°C 85% 20% Cold roofs, ground floors, timber-frame walls – external walls, external uses where member is protected from direct wetting
3 Conditions leading to higher moisture contents than service class 2 >20% External uses – fully exposed

* RH Relative humidity: level not to be exceeded for more than a few weeks each year

** EMC Maximum equilibrium moisture content for most softwoods. Note: in similar conditions the EMC of panel products typically will be lower

Hazard classes (or Use classes as they are likely to be termed in future) deal with the risk of biological attack in the form of fungi or insects. They are defined in BS EN 355 and summarised in Table 2.

Table 2 Hazard classes as defined in BS EN 335-1 and –3.

Hazard classes as defined in BS EN 335-1 Hazard classes as defined in BS EN 335-3
Situation in which the wood or wood-based product is: Moisture content : Risk of mould and fungal attack: Risk of insect (and/or other invertebrate) attack:
1 Under cover, fully protected from weather and not exposed to wetting No higher than that resulting from exposure to a temperature of 20° C and RH of 65% Regarded as being dry, and thus the risk of attack by surface moulds or by staining or wood-destroying fungi is insignificant.

For plywoods attack is possible, its frequency and importance depends upon the geographical region. Attack by beetles can also depend upon veneer species and thickness.

2 Under cover and fully protected from the weather but where high environmental humidity can lead to occasional but not persistent wetting Can occasionally attain or exceed that which would result from exposure to a temperature of 20ºC and RH of 90% Moisture content can occasionally increase to a level which can allow growth of wood-destroying fungi. Disfigurement of decorative panels can also occur as a result of surface moulds and staining fungi.
3 Not covered and not in contact with the ground. Either continually exposed to the weather or protected from the weather but subject to frequent wetting Frequently above 20%

Often liable to attack.

Disfigurement of decorative panels can also occur as a result of surface moulds and staining fungi.

4 In contact with the ground or fresh water and thus permanently exposed to wetting Permanently above 20%

Liable to attack.

Use of plywoods only appropriate if the inherent and/or conferred board properties are adequate. This often involves treatment with a chemical preservative.

5 Permanently exposed to salt water Permanently above 20% and wholly or partially submerged in salt water

Liable to attack.

Use of plywoods only appropriate if the inherent and/or conferred board properties are adequate. This often involves treatment with a chemical preservative.

Attack by invertebrate marine organisms is the principal problem. For the above water portions, risk of insect attack is similar to hazard class 1. Termites can be an additional problem in certain geographical regions.

Service classes and hazard classes are used for determining the suitability and treatment of products for construction purposes, where failure of the product would not be acceptable in terms of maintenance or safety. It is worth noting that panels may be used outside their service class where failure or degradation of the product would not be dangerous, or where a limited service life is acceptable.

Factors affecting performance

The factors of exterior exposure that can have a significant effect on the performance of wood-based panels include:

Correct design, detailing, workmanship and maintenance can help to minimise the effects of some of these factors. Wood-based panel products, because of their nature will perform differently to large sections of solid timber of the same species, and differently to each other, especially in an external environment.

Wood-based panels are susceptible to moisture ingress through both faces, and in particular, via the board edges, resulting in an increase in moisture content and consequent swelling of the panel.

The edges of fibreboards, particleboards and OSB are coarser textured than the faces and are susceptible to moisture ingress. In plywood, exposed end grain and/or wood fibres at the edges facilitates moisture ingress. The presence of core gaps/veneer overlaps can create localised areas of ridging on plywood faces as a result of changes in moisture content.

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

Moisture ingress can cause unsightly staining behind a translucent/clear finish. If wood-based panels remain wet in service for prolonged periods and without adequate ventilation or the drying effect of wind, fungal decay may lead to severe degradation of the panel.

Water will penetrate into wood-based panels much more rapidly than it is lost by drying to air under normal conditions. This is particularly the case if its entry is localised, such as through an area of improperly applied edge sealant or large exposed core gap in plywoods. Under these conditions, water can be trapped for long periods behind fairly impermeable finishes, putting considerable demands on the adhesion between the wood substrate and the surface coating or finish. Continuous fluctuations in relative humidity can also be problematic, and cause dimensional changes to wood-based panels, particularly fibreboards, particleboards and OSB which are more susceptible to swelling across the thickness of the board. Repeated wetting and drying of unfinished plywood panels can result in lathe checks opening up on the face veneer surface. Similarly, the surface strands/flakes of wood used to produce OSB boards can degrade and peel off, if left permanently exposed to the elements and without an appropriate coating.

UV light and sunlight will also cause a rapid change of colour of unprotected wood-based panels. Prolonged exposure will lead to general lightening/greying of the wood surface and a protective finish is necessary unless a rough, bleached, weathered appearance is acceptable.

Suitability of wood-based panels for use in service classes 2 and 3

The suitability of a wood-based panel for use in a particular service class is largely dependent on the glue bond quality and its ability to withstand certain in-service conditions for prolonged periods of time. Each wood-based panel product Standard defines the requirements that shall be met in order to claim compliance with a service class 2 or 3 glue bond. These are shown in Table 3

Table 3 Wood based panels suitable for use in service classes 2 and 3.

Panel type Suitable product ‘grades’ or ‘classes’
  Service class 2: Service class 3:
Plywood (BS EN 636) EN 636-2 EN 636-3
Cement bonded particleboard
(BS EN 634)
Class 1 Class 1
Class 2 Class 2 -
Oriented strand board (OSB)
(BS EN 300)
OSB/3, OSB/4 -
Medium density fibreboard - MDF
(BS EN 622-5)
MDF.H, MDF.HLS* Exterior MDF
(available but not defined in a
BS or EN product standard)
Hardboard (BS EN 622-2) HB.H, HB.HLA1, HB.HLA2 HB.E **
Medium boards (BS EN 622-3) MBL.H, MBH.HLS1*, MBH.HLS2*   MBL.E **
Softboards (BS EN 622-4) SB.H, SB.HLS  SB.E **
Particleboard (BS EN 312 P3, P5, P7   -

* restricted to instantaneous or short term load duration

- No suitable products defined

** Should only be used in exterior situations if a treatment of proven exterior durability (coating or otherwise) has been applied to relevant surfaces and edges

Durability of wood species for wood-based panels

The natural durability principles and lists of commonly used wood species are given in BS EN 350-1 & -2 and guidelines for the durability requirements for each hazard class are given in BS EN 460.

In basic terms, each wood species used in a wood-based panel must be considered for its natural durability and then treatment or coatings can be applied, if required, depending on the hazard class appropriate to the end use. Table 4 shows that the majority of particle or fibre-based panel products are not suitable for use in hazard class 3 and above situations, with the exceptions of exterior grades and cement bonded particleboard. Consideration of species durability is therefore most relevant to plywood.

An exterior bonded plywood is probably the most commonly used panel for exterior construction applications. Due to the almost infinite number of wood species combinations possible, extra care needs to be taken when specifying plywood for situations where biological attack may occur.

The fact that a plywood employs Class 2 or 3 bonds to BS EN 314-2 (which would correspond to BS EN 636-2 and 636-3 respectively), in its manufacture, does not automatically imply that it is suitable for long term exterior exposure in its ‘as-received’, natural state. If there is a risk of prolonged wetting and a good service life is required then the wood component of plywood must be sufficiently durable, ie resistant to fungal or insect attack, in addition to being adequately bonded. Most situations of exterior use require that a durable veneer species, preservative treatment or an appropriate factory applied coating/finish is needed for satisfactory long-term service.

Plywood can be pressure impregnated with wood preservatives, for example, copper-containing wood preservatives can be used for demanding applications such as farm buildings. Organic solvent preservatives can also be used for plywood which is to have a paint/wood stain finish applied.

Insect attack of plywood in service is infrequent in exterior situations in the UK and is not considered further here. Having identified which hazard class is appropriate, BS EN 350 should be consulted to identify which species of timber has adequate inherent decay resistance (natural durability) or, if not, what level of preservative treatment/factory applied protective coating/finish would be appropriate. Guidance can be found in DD ENV 1099 on which durability classes for the veneers used in a plywood under certain hazard classes require treatment.

However, it can be virtually impossible to determine the constituent species of many types of plywood, particularly of tropical origin and, in any case, no plywood specification totally excludes sapwood, which is always rated in the lowest durability class. As a result, the guidance falls between preservative treatment ‘usually being necessary’ or ‘not usually being necessary’ in Hazard Classes 2 to 5 respectively. If preservative treatment is specified, the only official guidance that exists at present is the UK National Annex to DD ENV 1099 (issued as an amendment to the 1998 edition, in April 1999 as a replacement for BS 6566-7).

In practice, with good design, installation and maintenance, decay in plywood used externally is relatively uncommon, despite the fact that it may well contain sapwood.

Marine grade plywood to BS 1088 can be manufactured from inherently durable timber species, although it should be noted that other non-durable species such as ‘gaboon’ can also be used if correctly marked. Even if a marine plywood is made from a durable species, constant wetting and drying and exposure to the elements can lead to surface checking and roughness. The timber will also weather to a grey colour unless a coating or finish is applied.

The final choice, however, must be based on individual assessment of the exposure condition, risk of failure in service and intended service life.

Biological durability of wood-based panel products

Service class 2 stipulates very similar in-service conditions to hazard class 2. However service class 3 is characterised by material moisture contents higher than that of service class 2 which requires hazard classes 3, 4 and 5 to be considered. As shown in Table 2, wood-based panel products used in hazard classes 2 to 5 are likely to be subjected to biological attack. For construction purposes some wood-based panel product types may be suitable in these situations and others may not. Table 4 shows the suitability of different types of panel products with respect to their biological durability in hazard classes 2 to 5. The guidance for plywood is taken from DD ENV 1099.

Table 4 Suitability of panel products with respect to biological durability for use in hazard classes 2 to 5.

Wood-based panel type Hazard Class 2 Hazard Class 3 Hazard Class 4 Hazard Class 5
Plywood species durability class 1 U U U U
Plywood species durability class 2 U U UT UT
Plywood species durability class 3 U UT TU TU
Plywood species durability class 4 UT TU T T
Plywood species durability class 5 TU TU T T
OSB/3 UT# - - -
OSB/4 UT# - - -
MDF.H UT# - - -
MDF.HLS* UT# - - -
HB.H UT# - - -
HB.HLA1 UT# - - -
HB.E UT# TU# - -
HB.HLA2 UT# - - -
MBL.H UT# - - -
MBH.HLS1* UT# - - -
MBH.HLS2* UT# - - -
MBL.E UT# TU# - -
SB.H UT# - - -
SB.HLS UT# - - -
SB.E UT# TU# - -
P3 UT# - - -
P5 UT# - - -
P7 UT# - - -
Cement Bonded Particleboard Class 1 U U U U
Cement Bonded Particleboard Class 2 U U U U
Exterior MDF (commercially available but not defined in a BS or EN product standard) UT# TU# - -

U - untreated

UT - left untreated is normally sufficient but, under certain end uses, treatment can be advisable

TU - treatment is normally advisable but, in certain end uses, the panel may be left untreated

T - treatment necessary

# - for particleboards and fibreboards treatment is often difficult or impossible, therefore a properly maintained coating may be the only way to achieve the desired result.

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