Fallklappe

Fallklappe, also known as a backwater flap or backflow valve, is a passive device used in building drainage to prevent wastewater from flowing backward into a building. It is typically installed in the main drain line or a basement floor drain and functions as a one-way door: it opens to allow wastewater to leave the building and closes to block reverse flow when the public sewer backs up.

The device usually consists of a hinged or otherwise movable flap within a valve housing. In gravity-operated versions, the flap is kept closed by its weight and opens when wastewater flows toward the sewer. Spring-loaded variants use a spring to hold the flap closed, with opening triggered by forward flow. When backpressure or backflow occurs, the pressure from the sewer pushes the flap against its seat, sealing the drain.

Fallklappen should be sized and installed according to expected peak flow and local regulations. They are placed downstream of the building’s fixtures and connected to the main sewer line, with accessible access for inspection and cleaning. Regular maintenance is recommended to remove grease, debris, and sediment that can impede sealing or operation.

Benefits include reduced risk of basement flooding and protection against sewer gas intrusion with minimal active maintenance. Potential drawbacks are the risk of clogging from solids, wear of seals over time, and the need for periodic testing and cleaning to ensure reliable operation.

In many regions, backflow protection, including Fallklappen or Rückstauklappen, is required by building codes in flood-prone or densely populated areas.