What makes the flap tile special is the striking bag-shaped flap on the left-hand side. When laid, this covers the raised edge of the adjacent tile, which means the roof tiles do not need a water recess. Flap tiles have been used on roofs since as long ago as the 11th century. Today, Jacobi-Walther is the only remaining manufacturer producing these clay roof tiles on an industrial scale. They are primarily used in listed buildings.