Step-by-step installation of a backflow valve
Step-by-step installation of a backflow valve – complete technical guide
A backflow valve is a simple but extremely important component of every sewer installation in houses with a basement, cellar or facilities located below the level of the public sewer. When the sewer is overloaded by a storm, heavy rain or the main collector is clogged, water starts to flow back – and without a valve it ends up directly in your cellar, bathroom or on the floor of the boiler room. From my own experience, I know that people call for help after damage has occurred, not before. That is why in this article we will look at the entire installation in the smallest detail – from preparing the site through selecting the right part to the final verification of functionality.
If you are still considering a purchase and are not sure which type or diameter you need, I recommend first reading the articles How to choose the right backflow valve – diameter, type and location and Intermediate vs. end sewer valve: what is the difference and where to use each, which you can also find in our Knowledge Center. Here I assume that you have already decided on the type and size of the valve and we can get straight to the point.
What you need before starting the work – list of tools and materials
Good preparation is half the success. Installing a valve into an existing sewer without sufficient preparation most often means that you will realize that you are missing some reducer or sealant, and you will waste half a day going to the store. Collect everything in advance.
Tools
- Angle grinder or plastic pipe cutter (for cutting existing pipe)
- Tape measure, pencil, spirit level
- Drill or hammer drill (if anchoring the shaft into concrete is needed)
- Screwdriver, pliers, installation wrench
- Container for collecting residual water in the pipe
- Cloth and cleaning agent (degreaser for PVC/PP)
- Protective gloves and goggles
- Lighting – work in a shaft or under the floor requires good visibility
Materials
- The backflow valve itself – for example, intermediate backflow valve DN 110 for drainage from a shower or sink, intermediate backflow valve DN 125 for a bathroom branch, or intermediate backflow valve DN 200 for the main house drain
- Sewer socket pipes in the appropriate DN for connection – have at least 2× 0.5 m as a reserve
- Seals (rubber gaskets) – usually included in the valve package, but check
- Sewer glue for PVC or degreaser + primer (depends on the pipe material)
- Shaft and cover – if installing the valve in a floor shaft, you need a corresponding cover
- Concrete or base mortar (in the case of built-in installation)
Where exactly to install the valve – choosing the installation location
This is a decision that will affect the rest of the work, and a mistake in this phase is very difficult to correct. The only rule applies: the valve must be installed on the sewer pipe inside the building, before the point where the pipe passes through the foundation or enters the outdoor environment. Installation outside the building is possible, but complicates maintenance – and this is precisely about the access for service.
In practice, it looks like this: if you have a basement and a drain from a washing machine, a shower or a floor trap siphon in it, these drain pipes usually converge into one main pipe that runs through the wall to the inspection shaft and further to the public sewer. You install the valve on this common pipe, still before the transition through the wall, from inside the basement. The reason is simple: if the valve one day requires cleaning or replacement of the seal, you must have convenient access to it without digging outside.
If it is a new construction or a reconstruction with a new pipe layout, the ideal location is an intermediate installation into a shaft placed under the floor, or into an inspection shaft in the basement floor. We will get to the shafts later – including shaft cover 315 or shaft cover 400, which form the final cover of the entire node.
Minimum distances and pipe slope
The valve must be installed on a pipe with the correct slope – at least 1 % (1 cm per each meter of length), recommended 2 % for DN 110 and 110 to 160. For DN 200, 1 % is sufficient. The slope is important not only for the flow of waste water, but also for the proper function of the flap inside the valve: if the pipe were level or even sloped in the opposite direction, the flap would not fall back on its own and the valve could remain partially open. In practice, I have seen installations where the valve "did not work" – in reality, the pipe had no slope and the flap did not close by its own weight.
Shutting off and preparing the drainage section
Before any intervention in the drainage pipe, you must stop the flow. This means: inform all house residents that they must not use the toilet, sinks, shower or washing machine during the work. Estimate the working time and add a reserve – a typical installation of a flap into an existing pipe takes 2 to 4 hours, including waiting for the adhesive to dry. In a single-family house with one bathroom this is usually not a problem, but in an apartment building it needs to be coordinated.
If the pipe runs in a concrete floor or is covered by a ceiling, you will have to cut access. When cutting into concrete, always use a diamond disc – it provides a much cleaner cut and less vibration than a saw, which is also appreciated by the surrounding pipes. Before cutting the pipe, place a container under the cut location – the remaining standing water in the pipe will immediately flow out when it is opened.
Cutting the existing pipe
A through-type flap (and this is exactly the type we are talking about) is inserted into a section of pipe so that you cut out a piece of the same length as the flap body, plus tolerance for inserting into the sockets (usually 5 to 8 cm on each side). Before cutting, measure the total mounting length of the flap – this information is always found in the manufacturer’s technical documentation. For example, a through-type flap DN 110 has a typical mounting length of about 400–450 mm, a flap DN 200 can be up to 600 mm.
Mark the cut locations with a pencil, check the slope with a spirit level and measure twice. Cut perpendicular to the pipe axis – a slanted cut will make sealing difficult and leakage is likely. Remove burrs from the cut edge with a scraper or a filed file.
Installation of a through-type flap – step by step
Below I describe the installation with a socket (flange) joint, which is the most common case for plastic drainage pipes PP or PVC. If you have an older house with stone or cast iron drainage, you will need transition fittings – sleeves or couplings with rubber seals for different materials.
Step 1 – Cleaning and preparing the surfaces
Clean the pipe ends and the flap sockets thoroughly with a cleaner for PVC/PP (most commonly sold as Tangit Cleaner or a similar product). The surface must be absolutely dry, clean, free of dust, grease or old sealing residue. This is a step that people often underestimate – and it is precisely here that most leaks after installation occur.
Step 2 – Checking the flap seals
Before installation, open the flap’s service cover and check the condition of the flap (the flap inside). The seal must be free of cuts, cracks or deformations. Lightly grease the rubber sleeves on the inlet and outlet of the flap with vaseline (not motor oil!) – this will make insertion easier and prolong the life of the rubber.
Step 3 – Inserting into the pipe
Insert the flap evenly into both sides of the pipe. Pay attention to the orientation – there is an arrow on the flap body indicating the normal flow direction (= from the equipment to the public sewer). If you turn the flap the wrong way, water from the household will not drain and the appliances will stop working. This is the most common installation mistake by beginners – always check the arrow before final insertion.
If the space is tight and you cannot easily insert the flap, use an installation wedge or gently tap the flap body with a rubber mallet. Strong blows with a metal hammer are prohibited – they can damage the socket or the sealing body.
Step 4 – Securing the position
A through-type flap, when properly inserted, holds in the pipe by the friction of the rubber seals – it is usually not necessary to glue it. Some types have a flange or console for anchoring to the wall or floor. If the flap has such a feature, always use it – in the case of backflow, a significant force acts on the flap and without anchoring, it may shift.
Check with a spirit level that the flap lies in the pipe axis and is not offset. An offset causes uneven sealing and shortens the life of the flap.
Step 5 – Checking functionality before covering
Before you backfill the pipe or embed the shaft, test the flap manually. Open the service cover and with your finger/rod, slightly open and close the flap – it must move freely and return to the closed position by gravity after release. If it stays open or moves more difficultly, check if anything is blocking the pivot axis.
Then have someone run water into the household appliances and monitor the flow through the flap – water must flow smoothly towards the sewer and the flap must open easily. After stopping the flow, the flap must close. If the flap does not close or water seeps around the seals, stop immediately and look for the cause (see also the article Common problems with check valves – why the flap leaks or gets stuck).
Installation in a manhole – detailed procedure
In many new buildings and during renovations, the backflow valve is installed directly into the floor manhole. This is an elegant solution – the valve is protected from mechanical damage, is always accessible through the manhole cover, and the whole unit looks neatly integrated. Finally, the manhole is covered with a lid.
For a manhole with a diameter of 315 mm, the 315 manhole cover is suitable, while for a larger manhole cross-section, use the 400 manhole cover. The size selection depends on the diameter and shape of the pipe that is led into the manhole – for DN 200, you will need a larger manhole and thus a larger cover. This topic is discussed in more detail in the article 315 or 400 manhole cover – how to choose the right size?
Installation procedure for the manhole with the valve
Insert or place the manhole into a pre-prepared recess in the basement floor. The bottom of the manhole must be aligned with the pipe axis – manholes are usually sold as cylindrical elements into which the pipe with the valve is inserted from the side. The connection between the manhole and the pipe must be sealed – use rubber gaskets or seals recommended by the manhole manufacturer.
Install the valve into the pipe (according to steps 1 to 5 described above), route the pipe through the manhole so that the service cover of the valve points upwards – towards the manhole cover. After installing the valve, pour concrete or masonry mortar around the manhole, align the top edge of the manhole with the finished floor level, and fit the cover. The cover must be removable without tools (at most a screw closure) – for regular valve maintenance.
Special situations and complications you may encounter
Old house with clay sewer pipes
If your house is older than 40–50 years, it is very likely that your sewer system is made of clay (glazed clay, grey-brown surface, cement mortar joints). You cannot directly insert a plastic valve into such a pipe without an adapter. You will need a round transition sleeve (clay/PVC or clay/PP) – these are sold as expansion sleeves with rubber seals on both sides. Be careful, as clay pipes are not always round – in old manually laid pipes, the diameter may be distorted and the rubber sleeve may not seal properly. In such a case, I recommend replacing a short section of the pipe with a new PVC section with a neck.
Low ceiling height in the basement – limited working space
Some basements have only 180–190 cm ceiling height and tight spaces that do not allow for comfortable work. In such a case, consider cutting the pipe into shorter sections and installing the valve in parts – first install one side of the pipe with the valve, then connect the other side. A running valve usually allows this thanks to neck clamps.
Multiple sewer branches running in parallel
If you have two or more separate sewer branches in the basement (for example, one from the toilet and another from the washing machine in the utility room), you must install a separate valve for each branch – or first combine them into one discharge pipe and install one valve on the combined discharge. The second option is cheaper and easier to maintain, but only if it is hydraulically possible (sufficient slope of both branches to the point of connection).
Combination with a floor drain or grease trap
If you have a floor drain in the basement floor (which is very common in boiler rooms and utility rooms), the valve must be installed after this drain – that is, on the discharge pipe to which the drain leads. Otherwise, the backflow from the sewer could rise through the floor drain as well, even if the valve is installed elsewhere.
Requirements of standards and building regulations
In Slovakia, the installation of a backflow sewer valve is regulated by the standard STN EN 13564 (backflow devices for sewerage) and related regulations. The valve must be installed in such a way that it is always accessible for inspection, cleaning, and replacement of parts – this is a legal requirement, not just good practice. Covering the valve without access is an error that can cause problems during commissioning or in the event of an insurance incident.
In some towns and cities, the installation of a backflow valve is even mandatory for all new connections and during renovations. More information about this obligation can be found in the article Protecting your home from sewer backflow – when is the valve mandatory? in our Knowledge Centre.
Valves of class I (according to EN 13564) provide basic protection and are suitable for typical households. For objects with a higher risk (for example, those located close to a sewer with a history of overflow), valves of class II are recommended – with an additional manual closure in addition to the automatic flap. Always check which protection class the valve provides when selecting it.
Maintenance of the flap after installation – what and when to do
Installation is only the beginning. A backflow flap requires regular inspection – at least once a year, twice a year is better (spring and autumn, before and after the period of heavy rainfall). The inspection takes 5 minutes and prevents major damage. A detailed cleaning and inspection procedure can be found in the article Maintenance and cleaning of a sewer backflow flap.
Short version: open the service cover, check the condition of the flap and the seal, remove any deposits or dirt from the flap seat (this is the most common cause of leakage – a small stone or a piece of toilet paper caught on the flap seat). Apply vaseline to the seals, tighten the cover properly and record the date of inspection.
Most frequently asked questions (FAQ)
Can I install a backflow flap myself, or do I need a licensed plumber?
From a technical point of view, installing a backflow flap in an existing plastic pipe is not complicated – a skilled DIY enthusiast with basic tools can manage it. Slovak law does not require a license for this specific type of installation. Nevertheless, if you have doubts about the diameter, type of flap or the condition of the existing pipe, I recommend consulting a plumber – you will pay for advice, not for unnecessary work.
How do I find out what DN size flap I need?
Measure the outer diameter of the existing pipe. For sewer PVC/PP pipes, the following applies: outer diameter 110 mm = DN 110, outer diameter 125 mm = DN 125, outer diameter 200 mm = DN 200. Never measure the inner diameter – sewer pipes are marked according to the outer dimension. For more information on selecting the diameter, see the article What DN size do I need for a sewer backflow flap? and the comparison Backflow flap DN 110 vs. DN 160 vs. DN 200 – usage comparison.
The flap is leaking slightly after installation – what to do?
The most common cause is dirt on the flap seat (a small stone, sand grain or dirt caught during installation). Open the service cover, clean the flap seat and check the condition of the rubber seal. If the seal is damaged or deformed, replace it – replacement seals are commonly available and very affordable. If the leakage persists, it may be due to an incorrectly inserted flap or damaged neck seal – in this case, remove the flap and reinstall it with new seals.
Can I install the flap vertically (on a vertical pipe)?
It depends on the specific type of flap. Most standard flaps are designed for horizontal installation or installation with a slope of up to 15°. In vertical installation, the flap would remain permanently open (gravity would not lift it) or, on the contrary, would constantly close and block the outflow. Always read the technical specifications of the specific product – some special types of flaps are designed specifically for vertical installation.
What if I don’t have access to the pipe in the basement – can the flap be installed externally?
Yes, the flap can be installed on the external pipe before it enters the building or in an inspection chamber outside. The disadvantage is reduced accessibility for service (especially in winter) and a higher risk of freezing for flaps with metal parts. If access and frost protection conditions are met, external installation is a fully functional alternative. Internal installation is always preferred.
Is it necessary to inform the insurance company or the network operator after installing the flap?
It is not necessary to inform the insurance company about the flap installation, but I recommend keeping the purchase receipt and photo documentation of the installation. In the case of an insurance incident caused by backflow (flood, property damage), you will be able to prove that you have taken preventive measures. You do not need to inform the public sewer network operator about your own installation, but if you are working directly on the connection or in the public network inspection chamber, you need the network operator’s consent.
Conclusion – most important installation rules at a glance
Installation of a sewer backflow flap is not rocket science, but it requires precision, correct selection of components and thorough preparation. The most common mistakes I see in practice are these: the flap is installed in the wrong direction, the pipe slope is insufficient, lubrication is neglected during installation – and worst of all – the flap is covered without any access. Any of these problems can cause the flap to simply not work in a critical moment.
If you have selected the correct type and size of flap (a standard flap is the standard choice for typical basement installations), you pay attention to the slope, flow direction and service access – the rest is just physical work. The result will be reliable protection for your home for decades, because a quality plastic flap will last 20 years or more without problems with regular maintenance.
The full range of standard flaps for common household sizes can be found in the category PE fittings, drainage, flaps – from the smaller flap DN 110 through the medium flap DN 125 to the large flap DN 200 for the main household connection.
Do you have a question about this topic?
Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.