Protection of the house against sewer backflow - when is a flap required?

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Protection of the house against backflow of sewage – when is a check valve mandatory and when is it just sensible?

A sewage backup is one of the most unpleasant malfunctions that can affect a single-family house or an apartment building. It is not just an aesthetic issue – wastewater brought back by reverse flow contains faecal bacteria, chemicals and organic waste that contaminates floors, walls, insulation and sometimes even structural elements to such an extent that remediation costs more than the entire renovation of the bathroom. Despite this, the installation of a backflow valve on the sewage system is still underestimated – many homeowners only learn about it after experiencing their first incident.

In this article, we will look at the topic comprehensively: we will explain where backflow occurs, which technical standards and building regulations govern the obligation to install a valve, in which situations the valve is legally mandatory and in which it is voluntary but practically irreplaceable. We will also go through how the valves work, how to choose them correctly and what risks arise if they are neglected.

What is sewage backflow and where does it come from?

Backflow is a phenomenon in which sewage water instead of flowing towards the wastewater treatment plant starts flowing in the opposite direction – from the public sewage system into the connection and from there into the household fixtures. It can manifest as gurgling in the traps, sudden water rising in the shower or toilet, or – in the worst case – water directly flowing out of the floor drains.

The causes vary, but the most common ones include:

Heavy rain and local flooding – the sewage network is designed for a certain maximum flow; during extreme rainfall, it quickly overflows and the pressure is transferred to the connections
Blockage of the network – fat deposits, solid waste or construction material in the network cause the water to seek a way back
Reconstruction and cleaning of the network – during pressure cleaning of the public sewage system, short-term pressure surges occur
Inappropriate elevation of the connection – a connection placed below the maximum water level in the sewage system is prone to backflow
Combined sewage (sewage + rainwater) – in older areas with a combined sewer system, the risk of backflow during rain is particularly high

From a geographical perspective, areas with shallow sewage laying, lowland locations and entire city districts built before the 1980s, where the dimensions of the networks do not match today's load, are more vulnerable. However, even new houses are at risk if their sewage connection is located at an elevation below the so-called control level of backflow.

Control level of backflow – a key term from the standard

The Slovak standard STN EN 12056, which governs the gravity drainage of buildings, as well as the decree No. 532/2002 Coll. (on general technical requirements for construction) use the term control level of backflow (CLB). It refers to the highest assumed water level in the sewage network that can occur during maximum network loading.

This number is determined by the sewage network operator (usually a municipal company or a water and sewage company). It is usually given in meters above sea level and must be part of the technical conditions of connection, which you receive when applying for connection to the public sewage system.

The rule is simple: if any extraction points in the building (traps, drains, toilet bowls) are located below the control level of backflow, you must protect them with a backflow valve. And this is not a recommendation – it is a legal obligation arising from the cited decree and related standards.

When is a backflow valve mandatory – specific situations

Despite the legislation being clear, in practice we often encounter the fact that many builders and designers ignore this rule or interpret it loosely. Here is an overview of situations in which the valve is mandatory and in which it is at least strongly recommended:

Mandatory cases (by law and standards)

Floor drains, traps, and toilet bowls in basements or on the ground floor below the RHV level – if the height of the floor drain is below the control level of the swelling of your sewerage, each such connection point must be protected. In practice, this means practically every basement in buildings with a combined sewer system.
Apartment buildings with drainage of basement areas – if the apartment building has shared basements, laundries, or boiler rooms connected to the sewerage below the RHV level, the building manager is required to ensure the installation of the valves.
New buildings with a basement – during the construction process, the designer is required to assess the height of the RHV and design protection. If the project does not include a valve and the equipment is below the RHV, the building does not meet technical requirements.
Wastewater pumping station below the level of the gravity discharge – when the basement is deeper and gravity discharge is not possible, it is solved by a pumping station; a backflow valve on the pump's discharge pipe is part of the basic equipment here.

Cases where the valve is not mandatory, but is essential

Ground-floor house in a high-risk area – if all the equipment is above the RHV level, the law does not require a valve, but in construction in flood-prone areas or in places with historically overloaded sewerage, the investment in a valve is incomparably cheaper than one renovation.
Older house after renovation – during insulation and renovation, the drainage method is often changed, and new equipment may end up lower than the original. This is surprisingly often neglected.
Cottage with year-round use – cottages are sometimes located in low-lying areas and have a simple sewerage system without any protection.

Where in the building is the backflow valve installed?

The backflow valve can be installed in two places – directly on the pipe inside the building (after the trap of an individual device or on the collection pipe before the exit from the building) or in a manhole on the connection outside before connecting to the public network. Each placement has its advantages and limitations.

Valve inside the building – placing it in a technical room or in the basement floor allows easy access for inspection and cleaning. The valve here protects specific sections of the internal installation. This is a common solution in the renovation of older buildings, where it is not possible to interfere with the external connection.

Valve in the manhole on the connection – this is the preferred solution for new buildings. The valve is installed in a manhole, to which access is provided via a cover. It protects the entire internal installation of the building at once. The manhole must be accessible and properly covered – for this purpose, for example, manhole cover 315 or manhole cover 400 can be used, depending on the diameter of the installed manhole.

Types of backflow valves – through and end types

In practice, we mainly encounter two basic structural types: through and end valves. Their difference is not only structural, but it directly affects where and how they can be used.

Through valve is inserted into a section of pipe – it has the same inlet and outlet, so it continues the pipe route without changing direction. The closing element (valve or float) is located inside the body and opens during normal flow towards the network; in the case of any swelling or back pressure, it automatically closes. This type is most common when installed on horizontal sections of the collection pipe or in manholes.

Specific products from this group that you will find in the range include, for example, sewage backflow valve through DN 110 suitable for standard household connections, sewage backflow valve through DN 125 for stronger profiles and higher loads, and sewage backflow valve through DN 200 for main connections of apartment buildings or industrial applications.

End valve is mounted at the end of the pipe – for example, at the outlet into a manhole or a reservoir. It is not suitable for insertion into a pipe section, as it has no neck on one side.

Choosing the right type is only part of the decision – just as important is the diameter (DN), material, and method of connection. For more information, see the article How to choose the right backwater valve – diameter, type, and location and the article Intercepting vs. End-of-line backwater valve: what is the difference and where to use each.

How the valve works – technical principle

The principle of a backwater valve is simple and durable: inside the body is a movable closing element – most often a rotating flap (disc) on an axis or a floating ball. During normal flow from the building to the network, the flow opens the flap and water drains smoothly. As soon as the flow stops or the direction reverses (back pressure from the sewer), the closing element closes under its own weight or water pressure and prevents further inflow of sewage into the building.

Modern valves also have an emergency manual closure – a dual-flap design or a screw closure, which allows the pipe to be manually closed, for example, in the event of a predicted flood risk or during network cleaning. This is especially valuable in areas where floods occur repeatedly.

A crucial condition for proper operation is installation on an horizontal or slightly sloped section of the pipe and ensuring access for regular inspection. A valve embedded in concrete without any inspection opening is practically useless from an operational point of view – over time it can become clogged with solid waste and stop sealing properly, while the owner may not be aware of it for several years.

Concrete examples from practice – what actually happens

Over the years of practice in the field of sewage and domestic installation, certain types of situations repeat with such regularity that they can be considered typical. Here are a few of them:

Case 1: New two-generation family villa with a basement

The owner built a modern house with a full basement – a game room, sauna, laundry room, and bathroom. The project was approved, the house was commissioned. Two years later, during the first major downpour in July, water came out of the floor drains in the basement and the toilet – the entire basement was flooded with 10 cm of sewage. Looking back: the HVAC designer did not even check, no valves were designed. The total damage, including furniture and floor coverings, exceeded 25,000 euros. Installing a valve would have cost less than 400 euros.

Case 2: Apartment building, basement areas

The manager of an 80s apartment building was dealing with tenant complaints – during heavy rain, the basement repeatedly filled with sewage. Inspection revealed that the drainage inlet in the basement was directly connected to the common sewer system without any protection. The manager installed an intercepting valve DN 200 into an existing inspection shaft and replaced the original cover with a new, durable one – specifically manhole cover 400. Since then, no incidents.

Case 3: Ground-floor house in an area without flood occurrence

The owner of a ground-floor house in a flat terrain argued that floods had never occurred there and that a valve was unnecessary. The problem arose differently – a neighboring developer began building a new apartment complex and during excavation work damaged the sewer. A pressure jet cleaning vehicle caused a surge that pushed water into three neighboring connections. Lesson: reverse flow is not caused only by floods.

Case 4: Renovation of an older cottage

The owner renovated a cottage for year-round living, adding a bathroom in the basement. The original cottage had only a simple septic tank, the new connection was connected to the municipal sewer. No one asked about the HVAC – and the municipality did not issue technical conditions with this information (which is itself a mistake). After two years – a flood from the network during an autumn long-lasting rain.

Valve sizing – which diameter DN to choose?

Choosing the correct diameter is essential. The valve must have the same clear diameter as the pipe it is mounted on – otherwise it will be either restrictive (too small) or not seal properly (too large). In practice, the following dimensions are used for family homes:

DN 110 – standard diameter for a home connection in a typical family home with 1–2 bathrooms; suitable for horizontal collection pipes inside the building and for smaller profile connections
DN 125 – less common, but occurs in older buildings or in combinations of old and new systems; also in some foreign projects
DN 160 – for larger family homes with a greater number of fixtures or basements with multiple drains
DN 200 – standard diameter for apartment buildings, commercial and industrial buildings; occasionally for family homes (larger connection, special situation)

A more detailed comparison of dimensions can be found in the article Backwater valve DN 110 vs. DN 160 vs. DN 200 – comparison of use and also in What diameter DN do I need for a backwater valve?

Installation – where and how to install the flap correctly

Installing a flap is not technically complex, but errors during the process can have fatal consequences. A few essential rules:

Flap orientation – the flap body is directionally oriented and must be installed in the correct direction (the arrow on the flap body must point in the direction of normal flow, i.e., from the building to the network)
Horizontal position – the flap should be installed on a horizontal or slightly sloped section; most standard flaps do not function properly on vertical pipes
Access for inspection – the flap must be accessible; if it is in a manhole, the manhole cover must be functional and easily openable; for internal installation, ensure an inspection opening in the floor or wall
Pipe cleanliness before installation – clean the pipe before installing the flap; dirt and grease can settle under the sealing surfaces and the flap will not seal properly from the start
Pipe slope – maintain a minimum slope of 1–2 % (10–20 mm per standard meter) to prevent sediment from accumulating under the flap

The complete installation procedure, including connection to socket welds and leak testing, is described in the separate article Installation of a backflow flap step by step.

Inspections and maintenance – what not to forget

A backflow flap is not a one-time investment that requires no further care. It is a mechanical device with moving parts that must be regularly checked. The frequency depends on the load – for a standard family house, an inspection once every 6–12 months is sufficient; for an apartment building or a property with higher load, ideally every 3–6 months.

The following should be checked during an inspection:

whether the closing element moves freely and is not stuck or coated with fat deposits
whether the sealing is not worn or cracked
whether the inflow to the manhole is not clogged with sediment
whether the manhole cover – for example, manhole cover 315 – fits properly and is not damaged

The most common faults and their causes are described in the article Common faults of backflow flaps – why the flap does not seal or gets stuck, and the complete cleaning and inspection procedure can be found in Maintenance and cleaning of a backflow flap.

Legislation and standards – summary

For those who want to have the legal framework clearly summarized in one place, here is a brief overview of the key regulations governing the topic in the conditions of Slovakia:

Act No. 442/2002 Coll. on public water supply and public sewerage – regulates the conditions for connection and the obligations of users; it also establishes the right of the sewerage operator to require protection of the network against backflow effects
Decree No. 532/2002 Coll. – sets general technical requirements for construction; directly states the obligation to protect equipment below the flood level using closable devices (backflow flaps)
STN EN 12056 (parts 1–5) – harmonized European standard for gravity drainage of buildings; defines terms, sizing, installation, and protection conditions
STN EN 13564 – standard directly for backflow devices (flaps) on sewerage; sets performance classes, testing methods, and material requirements
Technical conditions of the sewerage operator – each operator (e.g., BVS, VSS, ...) issues its own connection conditions, where the flood level must be specified

It is important to know that failure to comply with these requirements has not only technical, but also legal consequences: in the case of an insured event (flooding from the sewerage), the insurance company may refuse to pay out precisely because of the absence of legal protection. And this is even if you were not aware of the obligation to install a flap.

Most frequently asked questions (FAQ)

I have a ground-floor house without a basement – do I need a backflow flap?

The legal obligation applies to devices located below the flood level (RHV), which is determined by the sewerage operator. If all the consumption points in your ground-floor house are above this level, a flap is not required. Nevertheless, I recommend finding out the RHV from the operator – it is not always clear exactly where this level lies. In areas prone to flooding or with overloaded sewerage, a flap is a reasonable preventive investment, even without a legal obligation.

Can I install the flap myself, or must a professional company do it?

Installation of a backflow flap on the sewerage does not require special authorization, unlike, for example, gas installations. An experienced DIY enthusiast can install it on an accessible horizontal pipe themselves. However, when installing it in a manhole on the connection line, when embedding it in the floor, or when it is necessary to intervene in the existing pipe using couplings, it is better to call a plumber – not for legal reasons, but to ensure the work is done with the correct slope, tightness, and inspection access.

My flap keeps getting clogged. What am I doing wrong?

The most common cause is solid waste, paper towels, or damp rags that get caught on the flap's closing element and prevent it from opening fully. A secondary cause may be insufficient pipe slope before the flap (sand and grease settle). The solution is to increase the cleaning frequency, check the pipe slope, and educate the household about what should go into the toilet. Details can be found in the article Common faults of backflow flaps – why the flap does not seal or gets stuck.

What is the difference between a DN 110 and DN 125 flap? Are they interchangeable?

No, they are not interchangeable. DN 110 and DN 125 are different internal pipe diameters, and the flap must exactly match the diameter of the pipe it is installed on. Most domestic connections in Slovakia are DN 110; DN 125 is less common and occurs in older installations or special layouts. Always measure the existing pipe before ordering – the outer diameter of a DN 110 PVC pipe is 110 mm, but the internal diameter (the opening through which water flows) is smaller. Flaps are sized according to the internal diameter (DN).

How long does a backflow flap last?

With proper installation, regular inspection and cleaning (at least once a year), a quality PVC flap can last 15–25 years. The sealing (rubber membrane or O-rings) is a wear part that can usually be replaced without replacing the entire body. Aggressive chemical environments (strong cleaning agents) and neglected maintenance – a clogged flap that cannot open fully – significantly shorten its lifespan.

Do I need a separate flap for each siphon in the basement, or is one on the main pipe sufficient?

From a practical and economic perspective, it is more advantageous to install one flap on the collection (main waste) pipe before the exit from the building or in the inspection chamber on the connection line. This flap protects the entire internal installation at once. Individual flaps for each siphon are unnecessarily expensive and difficult to maintain – they are used only when it is not possible to provide protection by intervening in the main pipe (for example, when the main pipe is embedded in concrete without inspection access).

Conclusion – a small investment, great protection

A backflow sewer flap is a simple, inexpensive, and reliable component whose absence can lead to damages in the tens of thousands of euros. The obligation to install it for equipment below the flood level is not just a legislative formality – it is based on real physical risks that are becoming increasingly relevant in the era of climate extremes.

The first step is to find out where the flood level is in your area – the sewerage operator will inform you in writing. The second step is to choose the right flap (type, diameter, material) and install it in the correct location with access for inspection. Help with selection can be found in the article How to choose the right sewer backflow flap – running flap vs. end flap, and the full range, including running flaps of various dimensions, can be found directly in the category PE fittings, drainage, flaps.

Do you have a question on this topic?

Having trouble making a decision or dealing with a specific situation in your home? Write to us – we are happy to help.