Straight run bitumen is widely used in the production of built-structure waterproofing systems, more particularly waterproofing membranes. In order to be able to obtain supplies of straight run bitumen wherever it is obtainable at favorable prices, it would have to be transported over relatively long distances. Since straight run bitumen is viscid at ambient temperature, it would have to be transported hot, which is not possible with conventional straight run bitumen. According to the invention, the straight run bitumen is admixed with a small proportion of at least one hydrocarbonaceous polymer. This solidifies straight run bitumen at ambient temperatures, making it transportable cold in the solid state.