Floor drain trap is one of the most important elements of any drainage system installed in a garage, laundry room, boiler room, bathroom, or technical room. Its role is not only to drain water but, above all, to protect the interior from backflow of unpleasant odors from the sewer system – something we always want to avoid when installing drainage inside a building.
In practice, many industrial-type drains, especially those intended for outdoor or indoor installation, do not come with a factory-installed trap. In such drains, a trap is not fitted at the factory because it is not known which type of trap will work best in a given application.
There are different types of traps (e.g., dry trap, wet trap, bottle trap, bellows trap, etc.), and not every one will work in every location. For example, a wet trap built into a drain grate or linear drain will not work in a standard garage (water will evaporate quickly and unpleasant sewer odors will become noticeable), while a dry trap also requires a relatively large flow of water to open, so in a garage – except when washing the floor with a hose – it might essentially not work at all.
✅ Does this mean there's a problem? Not necessarily. In most cases, you can effectively create a floor drain trap yourself without having to replace the entire drain. This isn't any kind of DIY tinkering – these are methods that have been used in plumbing work for years, proven, highly effective, and at the same time simple and inexpensive.
☑️✅ We are talking about the installation stage – i.e., when you are just installing the linear drain and connecting it to the sewer system.
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Unpleasant odors appear when there is no water barrier between the sewer installation and the room. This is precisely the function of a trap.
In a classic wet trap, there is always a small amount of water that blocks the flow of sewer gases. If the drain does not have a trap or the water in it dries out, odors can freely enter the building interior.
A floor drain trap works on a very simple principle. A specially shaped section of the installation retains water, creating what is known as a water seal.
The most common solutions involve shaping the sewer pipe near the drain or just after the water inlet:
Below we present several visualizations that will certainly speak more clearly to the imagination and illustrate the possibilities of creating an odor-preventing trap on a pipe in a garage, warehouse, or terrace. This is merely a hypothetical visualization; one should take into account that in some cases (usually when pipe cross-sections are larger than DN50), instead of a bellows pipe, you will need to use sewer pipe elbows. The key aspect is the shape that allows water to be retained, which will act as a barrier against odors returning from the sewer.
A floor drain trap made from a bellows pipe connected directly under the grate – this is a U-type trap. The U shape creates a depression where water stands, forming a barrier against backflowing odors from the sewer.
The image above showed a "U" trap with a direct connection under the grate, but this solution is not always the best – for comparison, the next visualization will show a different solution.
☑️✅ Below is an odor-preventing trap in the form of the letter U without a direct connection to the grate. It is worth noting one important thing – this execution has a very significant advantage. As we know, water evaporates over time, even at very low temperatures. Therefore, its direct connection under the grate as shown in the image above – means a risk of water evaporation, which will result in the loss of the odor barrier. The method shown below eliminates the risk of water evaporation. Water still evaporates – that is an irreversible physical process – but the water vapor, having no direct outlet, will condense on the walls of the sewer pipe and return to the trap. This is therefore a better solution in places where water is encountered less frequently and is not replenished in the trap too often – e.g., garages or warehouses.
The image essentially explains why this type of trap is called a bellows trap.
Of course, the bellows pipe does not have to be connected close to the drain grate; the U shape can also be formed a bit further along the pipe. It also does not have to be made from a bellows pipe – you can make it from elbows or other hydraulic connections.
This is a slightly more elaborate solution, but still in the category of very simple ones. A W-shaped floor drain trap may be a solution when using a larger number of grates or linear drains, so it works well in large-area facilities. It is worth considering when installing drainage in a warehouse hall or production plant, but also on large commercial terraces (that's right – a terrace is an outdoor space, however, in places where people are directly present, traps are also used – a terrace with returning sewer odors will not be a pleasant place).
In many linear drains and drain grates, the manufacturer leaves only a simple DN50 or DN110 outlet without any additional odor-preventing protection. This is not due to negligence but to a practical aspect – namely, the need to choose the right type of trap depending on the specifics of the room, as we mentioned above.
✅ In such a situation, a trap can be made directly on the sewer installation.
The simplest solution is to use:
Yes, when correctly executed, there is no reason for concern – after all, living in a city, we are surrounded by various types of heating pipes, water pipes, and others where connections are used. On a daily basis, we are not even aware of it. Plumbing techniques are highly advanced nowadays, and there is no reason for concern, provided it is done correctly.
The second aspect is the fear of dirt getting into the trap – which could settle in the depression and over time weaken the trap's function as well as water collection. However, we are protected from this by a sediment trap, i.e., an element in the drain that protects the pipe from debris and waste entering it.
☑️✅ On the market, there are drain sets without sediment traps – we advise against using them.
If you care about the ability to clean the floor drain trap in the garage or warehouse at a later date, in case leaves, sand, etc. get into it – it is always possible to create an empty chamber for it in the floor, of small size, with some kind of cover – e.g., a tile. This is a simple and reliable solution.
One of the more popular solutions on the market, in the field of industrial linear drainage or garage drains, are Nordic Tec drains. They are a better solution than traditional concrete channels – a lighter but very durable solution, capable of withstanding car traffic or parking – made of stainless steel. They have a well-designed sediment trap – but they do not have a factory-installed trap, as the range of their applications is so wide that there is no certainty which trap a given user will specifically need.
In the case of these drains, we act exactly according to the patterns described above, using an external trap on the pipe. The graphics below are for illustrative purposes only – of course, we can place the floor trap a bit further away, at some distance from the drain grate – e.g., in a prepared chamber in the floor.
✅ Are there ready-made solutions available on the market for external floor traps?
Yes, the market for plumbing products and accessories, as the market usually does, responds to customer needs. Ready-made solutions are available – ready-made products in the form of U-type floor traps, less often also W-type. This is a good solution because you receive a finished product that only needs to be installed.
The advantage of ready-made solutions is that they often also have an additional top inlet that allows cleaning of the floor trap. Installation of such a device is also faster than constructing a bellows trap yourself.
However, ready-made floor traps also have certain disadvantages – a custom solution gives you the ability to size it specifically to your pipe layout and the space available in a given location. With a ready-made floor trap, unfortunately, you do not have this flexibility, and you have to adapt to the size of the device and find space for it.
Additionally, it is also worth mentioning – such solutions tend to be slightly more expensive than making your own bellows trap.
BLOG:
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Dry Trap vs. Wet Trap – What Is the Difference?
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How to Protect Floor Vents and Keep Debris Out of HVAC Ducts
By: Noahbenjamin On 2026-09-01This is a useful explanation of how floor drain traps work, especially the point about sediment and debris affecting the trap over time. Keeping drainage and vent areas protected from unwanted debris can make cleaning and maintenance much easier. For floor vents and HVAC ducts, a mesh solution can also help stop small objects and debris from falling inside: [XXX - moderated]