Creosote is widely used as a wood preservative for highway timber bridges in Norway. However, excessive creosote leaching at various highway timber bridge sites leads to a bad reputation for the use of creosote treated timber constructions and the use of wood in general. Macro- and micro anatomical factors such as amount of heartwood, annual ring width, annual ring orientation, ray- height and composition and resin canal area were investigated in order to classify seven timber bridges in Norway into leaching- and non-leaching bridges. With three anatomical factors a classification into leaching and non-leaching was possible for two discriminant categories based on observations on wood core samples and on entire bridges. The amount of heartwood content dominated the influencing factors, even obscuring the significance of other factors.

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