A mould resistance design (MRD) model was used to calculate the maximum duration of storage of timber outdoors under cover (protected from rain) before the onset of mould growth on the surface of wood. The prediction model is based on laboratory tests performed in constant and cyclic climates and were also confirmed against exposure tests in attics and crawl spaces. The prediction model presented is valid for Norway spruce wood, sawn and planed at a sawmill. Recommendations of maximum duration of storage were modelled for several locations in Sweden and a map dividing Sweden in risk zones for mould growth on timber was also produced. A field trial was set up at two locations in the Stockholm area to verify the prediction model. The results show reasonably good agreement between modelling and outdoor exposure testing at one site and less agreement at another. The question of risk assessment based on non-visible mould growth which can be observed with microscopy only or based on mould growth visible for the naked eye should be further examined.

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