Alsatian traditional houses, as in Riquewihr, have roofs with a fairly steep gradient (40 to 60°). They are covered with flat terracotta tiles of low thickness that are relatively fragile. The slope softens down with the core. It is a rafter attached to the frame of the roof that evacuates rainwater. The saddleback is the oldest and most common form of Alsatian roof. Its tiles are often rounded in the shape of a beaver tail, hence their name biwerschwànz. Some are decorated with lucky charms or religious motifs, such as crosses or hearts planted with nails reminding Christ's Passion.