8. BS EN 335 : - Hazard classes of wood and wood-based products against biological attack

(This standard, along with the voluntary European Standard ENV 1099 - Plywood : Biological durability: Guidance for the assessment of plywood in different hazard classes, replaces BS 6566: Part 7 Plywood - Specification for classification of resistance to fungal decay and wood borer attack.)

Part 1 : 1992 - Classification of hazard classes

Classifies the in-service environment for wood and wood-based panel products into 5 classes for the purposes of assessing the likelihood of different types of biological attack and need for preservative treatment.

Part 3 : 1996 - Application to wood-based panels

This gives guidance on the application of the hazard class system (defined in BS EN 335 Part 1) to wood-based panels. Plywood is covered specifically in section 4.2.

Annex A provides information for decision makers concerning general considerations related to the durability of wood components made of plywood and information on assigning hazard classes.

Hazard Class 1:- In this environment, plywood should have a moisture content no higher than that resulting from atmospheric conditions equivalent to 20° C and 65% relative humidity for the majority of their service life (12% mc or less). They are, therefore, regarded as being dry and thus the risk of attack by surface moulds or by staining or wood-destroying fungi is insignificant. Attack by wood-destroying insects, including termites, is possible but frequency and importance of this risk depends upon geographical region. Beetle attack can also depend upon the specie of veneer involved.

Hazard Class 2:- In this environment, the moisture content of the plywood can occasionally attain or exceed that resulting from exposure to an atmosphere of 20° C and a 90% relative humidity (24% mc approx.). Thus wood-destroying fungi could develop. Surface moulds and staining fungi could also grow. Risk of insect attack is similar to that for hazard class 1.

Hazard Class 3:- In this environment plywood can frequently have a moisture content above 20% and thus will be often liable to attack by wood-destroying fungi. The growth of surface moulds and staining fungi can also cause disfigurement of panels. Risk of insect attack is similar to that for hazard class 1.

Hazard Class 4:- Here plywood moisture content is permanently above 20% and thus liable to attack by wood-destroying fungi. Risk of insect attack above ground or to those parts in contact with water is similar to that of hazard class 1. Termite attack can also occur in certain geographical locations.

Hazard Class 5:- In this classification, plywood has a moisture content permanently above 20% and is wholly or partially submerged in salt water. Attack by marine organisms is the principal problem. Wood-destroying fungi are also liable to attack the plywood. Risk of insect attack to that portion above the water is similar to that of hazard class 1. Termites may also be a problem in certain geographical locations.

Table 1 in BS EN 335 Part 3 gives the general moisture levels for plywood and a summary of the biological agencies that can attack them in various hazard classes.