It is customary to vent the main soil pipe by carrying it through the roof of the building, and leaving the end open. This is shown in Fig. 74. On gravel roofs which drain toward the center, the soil pipe is sometimes stopped on a level with the roof, and serves as a rain leader. In other cases the roof water may be led to the soil pipe in the cellar. If the latter method is used, the water should pass through a deep trap before connecting with the drain. These arrangements tend both to flush out the soil pipe and trap and prevent the accumulation of sediment.