A hydrophobisation composition, which includes a hydrophobisation extract obtained by extracting outer bark of birch in an organic and/or polar solvent and which includes at least 30 weight-% betulinol. Also disclosed is a method for hydrophobisation of a surface, which includes steps of applying a layer of hydrophobisation composition containing betulinol on a surface of a substrate, and drying and heat-treating the layer of hydrophobisation composition. A substrate having a surface which is hydrophobised by using the method is also disclosed.