Shingles are not a waterproofing material. When installed correctly, they do “shed” water. However, if installed on a flat roof, they would allow massive seepage. The way shingles work is by their overlapping design. As the previous layer is covered by the next it creates a stair step water shed design, disallowing seepage. This stair step design also has a vertical component to it. If each row is not offset correctly, water may enter the system.