The paper describes the development of a method to measure the relative bridging properties of solid biofuels over openings. The system consists of a silo, with a bottom made of rubber mats, which can be rolled on a drum, so that a gap is created. Over the gap, a "bridge" of fuel particles is formed. When the gap is wide enough, the bridge collapses and the distance between the drums is measured. This number is a measure of the relative bridging property of that specific fuel. General results are given for a variety of solid biofuels. The bridging property of solid biofuels is mainly controlled by the shape and the size of the particles. If the fuel contains many hooked or long and thin particles, the bridging tendency is high. The influence of the moisture content is not as pronounced, but exists. In further articles, specific results will be discussed.