To limit the probability of the absorption of bitumen by porous roof sheathing or the draining of bitumen through gaps between sheets of roof sheathing, which could lead to an insufficient thickness of bitumen to achieve the required degree of watertightness and adhesion of the roofing membrane, which could lead to water penetrating directly to roof decking or sheathing, which could lead to deterioration, which could lead to the structural failure of roofs or elements protected by roofs, which could lead to harm to persons.