The outdoor field trial has always been regarded as the ultimate test of performance for assessing the effectiveness of a wood preservative. For this reason, the Building Research Establishment (BRE) has invested much effort in such trials over many years. Emphasis in early trials was on ground contact tests which were the basis both for assessing the likely performance of a preservative in protecting posts and poles, and for classifying a timber's natural resistance to fungal decay. When preservation of timber in the building and construction industry assumed greater importance, the emphasis was placed on the development of out-of-ground field trials to provide data on the likely performance of exterior joinery work in buildings. For these tests, the exposure conditions provided a less severe hazard for out-of-ground timber than for wood permanently in contact with the ground. Out-of-ground trials have been running at BRE since 1967 (Orsler and Smith, 1993) initially using T-joints and L-joints and later the window joinery test rig was established to provide conditions close to those in buildings (Purslow, 1975). All the early trials were assessed only by visual assessment for decay. Later still, small L-joints with a coating (the methodology which was used as the basis for EN 330) were used for a comprehensive study to establish the pattern of colonisation by microorganisms and associated changes to the wood. It was established that the basidiomycete fungi ultimately responsible for the decay are the climax of a colonisation sequence of a range of types of fungi plus bacteria (Carey, 1980). In an attempt to give early indications of long term performance, colonisation by basidiomycetes and changes in the porosity of the wood during exposure have been used as the basis for assessing performance relative to that of a reference preservative (1.0% tri n-butyltin oxide). Hyphen joints were introduced (Orsler and Holland, 1993) for studies of preservative distribution.

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