Published: 17 October 2026
In short
- The flap inside a backflow valve is lifted by water coming from the building and pressed into its seat by pressure coming from the street side.
- When the valve is closed, the building's own waste water cannot leave either, so lower-floor drains should be used sparingly during a downpour.
- Every drain below street level should sit on the inside of the valve, and the inspection cover must be reachable without breaking the floor.
- Paper, hair and grease caught on the flap's hinge or seat leave the valve slightly open, so regular cleaning is essential.
It is also called a non-return valve, a one-way valve or a flap valve. The mechanism is simple, but the protection it gives can be lost just as easily: a backflow valve fitted in the wrong place, or never opened up and checked, will not protect the basement in the first heavy rain, and the owner only finds out as the water rises.
How does the flap tell which way the water is going?
Most backflow valves used in sewers have a flap attached by a hinge along its top edge. The flap rests against the seat at the pipe opening under its own weight, and in some models with the help of a light spring or a slight slope. Flow from the building carries the flap along and lifts it, and once the water has passed, the flap drops back. When water rises on the street side, the pressure pushes the flap from the opposite direction, and the higher the pressure, the tighter the flap sits in its seat.
A closed valve has a side effect that is easy to overlook. For as long as the rain continues, the building's own water cannot get out into the sewer either, and it starts to collect on the inside of the valve. If washing machines are running and showers are on in the upper floors, this water will eventually rise from the lowest drain in the protected section. That is why using basement and ground-floor drains sparingly during a downpour is a sensible precaution even in buildings that have a valve.
Where on the line is the right place for a backflow valve?
The valve belongs on the run between the drains to be protected and the street connection. The aim is to bring every drain that sits below street level to the inside of the valve and to cut off the water coming from the sewer so that it cannot reach them. If the valve is fitted on a branch that lies outside this run, the returning water finds its way out through another drain.
In some buildings, only the basement drain line is collected on a separate branch and connected to the valve, while the water from the upper floors joins the line after the valve. The benefit is clear: when the valve closes, the upper floors can keep using their drains, because their water comes from above street level anyway. Whether this separation can be made in an existing building is found out by inspecting the line with a camera.
- Every floor drain, shower drain and toilet drain below street level stays on the inside of the valve.
- The inspection cover can be reached without stooping or stretching, and without breaking tiles or concrete.
- The flow arrow on the body points from the building towards the street.
- The slope continues after the valve, so water does not pool in front of the flap.
Which types of backflow valve will you come across?
The backflow valves found in home and apartment building plumbing fall broadly into a few groups. The differences lie in the number of flaps, where they are installed and how the inspection cover is built. Which one suits you depends on the pipe diameter, the place where the valve will go and the number of drains to be protected.
- Single-flap pipe type: added into a straight run, with a cover on top that opens for cleaning.
- Double-flap type: carries two separate flaps one after the other, so if one is held open by debris, the other can still close.
- Manually lockable type: lets you fix the flap in the closed position from outside with a lever, and is preferred for basements that go unused for long periods.
- In-drain type: a small flap or floating ball arrangement fitted inside the body of a floor drain, which protects only that one drain.
Why does a backflow valve that is never cleaned fail to stop backflow?
For the flap to close fully, nothing must be left between it and its seat. Toilet paper, wet wipe fibres, hair and grease that cools and hardens arrive with the waste water and stick first around the hinge, then to the edge where the flap sits. When the flap lands on this layer, a gap as thick as a finger is left, and water from the street side seeps in through it. Over time the seal also hardens, and the flap no longer presses against its seat as it used to.
How often it needs servicing depends on how the building is used. In homes, a check before the rainy season starts and after a major downpour is usually enough. In buildings with a restaurant or café on the ground floor, grease builds up quickly, so you need to look more often. If the valve is inside a manhole, do not open the cover and climb in, because sewer gas collects in a closed chamber.
- The inspection cover is opened, and the flap and seat are cleared of debris.
- The flap is moved by hand to see whether the hinge turns freely.
- If the seal is worn, cracked or hardened, it is replaced with a new one.
- After the cover is closed, water is run from the building to test the flow.
Installation and use mistakes that make a backflow valve useless
In buildings we visit for backflow complaints, the valve itself is often sound and the problem lies in how it was placed. The following situations are common, and each one either makes the valve ineffective or turns it into a new source of overflow.
- Fitting the valve on the building's main line, which traps the water from the upper floors in the basement throughout a downpour as well.
- Connecting the body the wrong way round, so the valve blocks the building's water and lets the street water through.
- Connecting the roof downpipe to the inside of the valve, so that when the valve closes, roof water is directed into the basement.
- Covering the inspection cover with tiles or screed.
- Fitting a valve to a single basement drain and leaving the shower and laundry drains on the same level unprotected.
- Never opening the cover after the valve has been fitted.
Does a backflow valve solve smell and pest problems?
A backflow valve is not a part designed to stop smells. What holds back the smell from drains is the water sitting in the trap. When this water seal dries out or the trap fails, the smell starts, and a backflow valve does not prevent it. The flap is also not completely airtight.
Expectations should be kept low on pests too. A flap that stays closed makes it somewhat harder for creatures trying to enter the line from the sewer, but the valve is not chosen for this purpose. If there is a constant smell or pests in the basement, first check the traps, the drain strainers and the possibility of a broken pipe.
