This engineering design guideline is intended for wood in outdoor above ground applications, i.e. use class 3 according to EN 335. It is based on a prescribed limit state for onset of decay during 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, distance to ground and design of details. 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 derived with the help of a dose-response model for decay, which was used to derive relative measures of decay risk between different locations and between different detail solutions. Other elements in the guideline have 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 of real buildings, which show that the output from the tool agrees well with documented experience. The guideline has also been presented in a computerized Excel format, which makes practical use more 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 design and detailing solutions. In addition the user will have to consider the target service life as well as the consequences of non-performance in the design of a construction.