This paper describes the background and principles behind an engineering design guideline for wood in outdoor above ground applications, i.e. use class 3 according to EN 335. The guideline has been developed in the European research project WoodExter and can be seen as a first prototype for a quantitative design tool in the area of wood durability. It is based on a defined limit state for onset of decay under a reference service life of 30 years. Onset of decay is defined as a state of fungal attack according to rating 1 in EN 252. The approach is to determine the climate exposure as a function of geographical location, local exposure conditions, sheltering, ground distance and detail solution. The exposure is then compared with the material resistance defined in five classes and the design output is either OK or NOT OK. The present version of the guideline covers applications for decking and cladding. The data included in the guideline have partly been estimated with the help of a dose-response model for decay, which was used here to derive relative measures of decay risk between different locations and between different detail solutions. Some other elements have however been estimated in a semi-subjective manner based on expert opinions as well as experience from field testing. The guideline has been verified by a number of reality checks, which show that the output from the tool agrees reasonably well with documented experience. The guideline has also been presented in a computerized Excel format, which makes practical use convenient. It is believed that many building professionals will appreciate a tool within the area of wood durability which has an approach similar to other design tasks in building projects. An advantage is that in applying the method the designer will go through a check list where he/she becomes aware of the importance of appropriate detailing solutions. In addition the user will have to think about the target service life as well as the consequences of non-performance in the design of a construction.

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