The washboard effect has been observed in sawn spruce after both commercial and laboratory based preservative treatments and is of increasing economic importance. The effect is defined as a specific phenomenon at the wood surface. It is distinct from internal collapse in the wood, but it may occur in association with internal collapse damage. Experimental investigations on the causes of washboarding and preliminary results indicate that a combination of several factors induce the effect. At treating pressures below about 10 bar washboarding appears to develop with post treatment drying of the wood whereas with higher pressures it will occur during the impregnation cycle. Further research has been started to develop measures to minimise the problem.

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