Standing seam joins adjacent metal sheets with edges raised above the main roof surface. It can be closed traditionally or form a factory-made snap lock. This arrangement reduces the number of visible fastenings on the roof field, but does not in itself guarantee weathertightness. The outcome depends on the pitch, the substrate, thermal movement and the details at the eaves, ridge and penetrations. Each of these elements must be matched to the given system. Rainwater flows mainly over the fields between the seams. Raising the joint limits its direct exposure to the flowing stream. This does not mean the seam is completely isolated from water. During wind-driven rain, snow accumulation or ponding against an obstacle, conditions are more demanding than on a freely draining slope.