Step-by-step installation of floor drain: slope, position, connection

We are still preparing products.

But you can look at other categories.

Installation of a floor drain step by step: slope, position, connection

A floor drain is now a standard component of modern barrier-free shower enclosures, walk-in showers, and generally any bathroom where the architect or customer wants a clean line without a prominent drain element in the center of the floor. However, the installation of a floor drain is not as trivial a matter as it may seem when looking at the finished result. Most of the problems I encounter in practice – standing water, water seepage under the tile, bad odors from the trap, cracked grout around the drain – originate directly from the installation, or from mistakes made even before the first tile was laid. In this article, I will describe the entire process from the first measurement to the final adjustment of the edge, including specific numbers, tolerances, and practical experiences from real projects.

What affects the installation before you even start

Before you even pick up the first tool, you need to be clear on a few things. The first and most important is the route of the waste pipe. A floor drain, unlike a floor drain outlet, only drains at one point – usually at the end of the drain, less often in the middle (in symmetrical models). If the designer or plumber has forgotten this fact and designed a vertical waste pipe in the center of the shower (a typical position for a classic drain), you may have a problem. A drain and a drain outlet are not interchangeable without interference with the piping.

The second thing is the thickness of the floor structure. A standard floor drain requires a minimum height of 90 to 110 mm in the structure (from the load-bearing layer to the finished surface), some slim models can manage with 65 mm, but these are exceptions. If you are renovating an old apartment bathroom with the original concrete screed and limited space, you may run into insufficient depth. Always measure it before you choose a specific product.

The third thing is the width of the shower and the associated length of the drain. Typical drain dimensions are 650, 750, 850 or 1000 mm. There are also longer variants, but they are less common. If you have a 900 mm wide shower, it does not mean you need a 900 mm drain – it depends on whether the drain runs from wall to wall (in which case 750 or 850 mm is usually sufficient, the rest is taken up by profiles, edges and grout), or only part of the width. More on choosing dimensions can also be found in other topics of our Knowledge Center, specifically in the article Floor drain vs. floor drain outlet: what is better for a shower enclosure?

Choosing the position of the drain: next to the wall, or freely in the space?

The position of the drain significantly affects not only the aesthetics, but also the technical solution of the slope. There are three basic configurations:

Drain next to the wall – the simplest technical solution in terms of slope, because the entire floor slopes in one direction towards the drain. The shower looks more airy, the installation of the edge to the wall is standard.
Drain in the middle of the enclosure – aesthetically interesting, but technically demanding, because the floor must slope from both sides. A so-called saddle slope is created.
Drain next to the opposite wall (opposite the entrance to the shower) – less common, but functional. Water naturally flows from the standing area to the drain.

In practice, the drain next to the wall clearly prevails, and for a good reason: it is the simplest and most reliable method. The drain is embedded so that one side aligns with the wall and the edge (if adjustable) closes the gap between the body of the drain and the wall tile. If you are considering this configuration, I recommend looking at the product Flexible – Floor drain with edge for perforated grid and with adjustable edge to the wall, length 850 mm, which has an adjustable edge specifically designed for this situation.

Correct slope of the floor: numbers you must follow

Slope is one of the most underestimated aspects of installing a floor drain. In practice, I have seen bathrooms where the customer complained about standing water after years, and when we measured the slope, it turned out to be 0.3 % – which is simply too little. Standards (STN and European EN) recommend a minimum slope of the floor in wet rooms of 1.5 %, ideally 2 %. What does this mean in practice?

For a 90 cm wide shower and a slope of 2 %, the height difference between the highest and lowest point of the floor is 18 mm.
For a 120 cm wide shower and a slope of 1.5 %, it is 18 mm.
For an 80 cm wide shower and a slope of 2 %, it is 16 mm.

These numbers are relatively small, which is good – you won't get a "channel", but at the same time, those 16–18 mm difference can easily be lost due to inaccuracies in the screed, tiling or drain adjustment. Therefore, it is crucial to have a laser level or at least a long spirit level (at least 100 cm) and to actually check the slope repeatedly during the entire implementation.

If you are doing a saddle slope (drain in the middle), the same principle applies, only it is measured from both sides. In such a case, I recommend using a symmetrical drain, for example floor drain double-sided 750 mm DOUBLE9 FIT and GO, stainless steel, which is structurally designed precisely for this configuration and allows central connection of the drain.

Step 1 – Substrate Preparation and Rough Work

Installation of a floor drain always starts with demolition (in renovations) or preparation of a clean concrete slab (in new constructions). In a renovation, you must remove the old screed, tiles, and especially the old waterproofing down to the load-bearing concrete. No temporary solutions, no gluing the drain to an existing screed – always start from scratch.

Next, mark the location of the waste pipe. Basic rule: the waste pipe must be installed before you proceed with the slope. The waste pipe route must be laid with sufficient slope, at least 1 % (2 % is recommended) along the entire length to the vertical waste or to the inspection chamber. The same principles apply to connecting the drain to the waste pipe as with a classic trap – more about diameters can be found in the article What drain diameter do I need: DN32, DN40 or DN50?

Common floor drains connect to DN40 or DN50. DN40 is sufficient for a smaller shower area (up to about 4–5 m²), DN50 is recommended for larger areas or if the drain is longer than 1 000 mm. Most quality drains come with a reducer that allows both diameters.

Step 2 – Embedding the Drain into the First Screed Layer

Before embedding the drain, mix the first layer of the sloping screed (so-called rough screed). Its consistency should be firmer – when you press it in your hand, it should retain its shape. Too fluid a screed will not slope well and during embedding of the drain, it can settle unevenly.

Now set the height of the drain feet. Most modern drains have adjustable feet at the bottom with a range of 60 to 120 mm (from the bottom of the body to the floor). Set them so that the top edge of the grate (not the top edge of the drain body, but the grate itself after it is mounted!) is at the level of the future finished floor. This is a key point – if the grate protrudes even 2 mm above the floor, water will not drain properly and you will also trip on it.

Height adjustment procedure:

Determine the total thickness of the layers above the screed: waterproofing (1–2 mm), adhesive mortar (5–8 mm), tiling (8–12 mm).
Overall, count on 15–22 mm from the top edge of the screed to the finished surface.
Set the drain so that its top edge is 15–22 mm lower than the finished floor at the highest point of the room.
Check with a spirit level that the drain is horizontal (this is important to ensure water drains evenly along the entire length).

After setting the height, pour the rough screed around the drain and let it set. At this stage, the drain is not yet permanently embedded – it serves only as a template for sloping. Many plumbers prefer to fix the drain with plaster at several points, which allows for quick hardening and stabilization before pouring the screed.

Step 3 – Screed Sloping and Inspection

This is the most important phase of the entire installation. Once the drain is fixed at the correct height, start sloping the screed. The simplest method is to use guides (concrete runners), which you install along the walls at the correct height, and you level the screed with a straightedge along these guides.

A slope of 2 % means that for every 100 cm of horizontal distance, you descend 20 mm. Specific inspection procedure:

Place a laser or spirit level on the top edge of the drain grate.
Measure the height at a distance of 50 cm from the drain – it should be 10 mm higher (at a 2 % slope).
At a distance of 100 cm, you should have 20 mm higher.
These values refer to the rough screed – after tiling, the surface will be parallel, so it is checked again after the tiles are laid.

Let the screed dry sufficiently. A traditional cement screed should dry for at least 4 weeks before applying waterproofing. For anhydrite or quick-drying screed, shorten the drying time according to the manufacturer's instructions, but never drastically shorten the drying – a wet screed under waterproofing is a direct source of problems.

Step 4 – Waterproofing: the Foundation That Is Often Forgotten (But Shouldn’t Be)

Waterproofing in a shower with a floor drain has two parts: floor waterproofing and wall waterproofing up to at least 200 mm above the floor (2 000 mm is recommended for fully wet walls). In the area near the drain, a special rule applies: waterproofing must be connected to the drain using a sealing strip (butyl or special tape, which is produced by various manufacturers for use with their drains).

A technical detail that is often underestimated in practice: sealing in the corners (wall-floor) must be done with a corner sealing strip, not just painted with liquid membrane. Liquid membrane (acrylic or polymer-modified) is applied in two layers, each after the previous one has dried, and the minimum thickness of the finished layer is 1 mm (you usually achieve this with two coats of 0.5 mm each). Always use reinforcing tape in corners and at transitions to the drain.

Sealing strip around the drain: most modern drains have a direct strip or groove on the body of the device to which the sealing strip is attached. The strip is glued to the drain body, then the liquid membrane is applied over the strip and onto the screed around it. This creates a continuous waterproof layer without interruptions. I recommend verifying this solution with each specific purchase – not every cheap drain includes or at least offers a waterproofing connection solution in its packaging or catalog.

Step 5 – Connection to Drain Pipe

After waterproofing, the connection of the siphon to the drain pipe follows. Most floor drains have an integrated siphon directly in the body, but there are also variants where the siphon is separate and mounted under the drain. In both cases, the following applies:

The joint must be sealed – either with a rubber gasket (O-ring) or with a specific sealing material from the manufacturer. Never add Teflon tape where the manufacturer does not anticipate its use.
The drain pipe under the drain must be long enough to make the joint accessible (at least minimally) in case of need. If it is completely embedded without a maintenance opening, any repair will require demolition.
The flow capacity of the siphon must match the diameter of the outlet. For DN40, a standard flow of 0.5 l/s is assumed, for DN50 it is around 0.9 l/s.

When connecting the drain to the drain pipe, also consider the height of the water seal in the siphon. The minimum height of the water seal must be 50 mm (according to EN 1253). If the water seal is too small, the siphon will release odors and – in extreme cases – also waste from the sewer into the bathroom. More on this issue can be found in the article Siphon smells or water stands: causes and solutions.

If you also need to drain other equipment in the bathroom, for example a bathtub, you may also find interesting the bathtub siphon with automatic overflow, chrome – an elegant solution for bathtubs, where you want an automatic overflow function without a manual stopper.

Step 6 – Tiling and Final Setting of the Edge

Tiling is carried out only after the complete drying of the waterproofing (at least 24 hours for most liquid membranes, up to 48 hours for some products). When tiling, there are several rules specific to the drain:

The tile is laid up to the grate of the drain (or to the edge, if the drain has an edge), not under it. The gap between the tile and the drain must be filled with silicone (not with grout – grout is not sufficiently elastic and cracks in a wet environment). The color of the silicone is chosen to match the color of the grout, resulting in a uniform appearance.

Adjustable edge to the wall: if your drain has an adjustable edge (see, for example, the solution at Flexible drains with 850 mm length), it is installed and adjusted only after the tiles are laid. You push or pull the edge according to the gap that formed between the drain body and the wall tile. A properly adjusted edge is pressed against the tile (without silicone underneath it – silicone only on the side of the wall, where the edge meets the wall tile). The edge covers the grout and at the same time directs water that runs down the wall directly into the drain.

Important detail: when tiling, check the slope not only towards the drain, but also along the drain. If the drain is tilted, water accumulates at one end and the outlet at the other end remains dry – which can in extreme cases cause the water seal in the siphon to dry out and result in odors.

Step 7 – Installation of the Grate and Final Tests

After completing the tiling and grouting (at least 24 hours before the test), install the grate and perform the final test. The test is simple: fill the drain with water and observe:

Does the water drain quickly (not standing for a long time)?
Does moisture appear around the drain on the walls or in the corners?
Is the grate level with the tile, or at most 1–2 mm below the tile level?
After 30 minutes from the end of the test: are there any visible wet spots under the drain (on the ceiling slab of the lower floor or in the structure)?

When choosing a grate, make sure it is always replaceable – the grate is the only replaceable component that is in contact with the public and dirt. A stainless steel grate (304 or 316L) will last for decades, galvanized or chrome-plated brass is cheaper, but in an aggressive bathroom environment (hard water, cleaning agents) it corrodes faster. More on material selection can be found in the article Stainless steel, brass or plastic: what to choose for drainage equipment in the bathroom?

If you are considering a more classic solution instead of a drain, there is still on the market the floor drain with chrome grid 12×12 cm, DN 40, brass – a compact solution for smaller showers, where a drain is not architecturally necessary.

Typical mistakes from practice and how to avoid them

Over the years of work with bathroom renovations, I have repeatedly seen these mistakes, so I mention them separately here:

Mistake No. 1: The drain is installed too high. The grate protrudes above the tile, water flows to the drain, but hits the edge of the grate and runs back. The solution always requires chiseling out – no other method exists.

Mistake No. 2: The drain is not horizontal along its axis. If one end of the drain is 5 mm lower, water accumulates at that end. Drainage is only where the siphon is – so water stands at the other end. Check the horizontal alignment of the drain with a cross level.

Mistake No. 3: Missing sealing at the connection of the waterproofing to the drain. The liquid membrane is only applied to the screed around the drain without attaching the tape to the drain body. After some time (1–3 years), the membrane begins to separate from the metal drain body and water penetrates into the structure. Always use sealing tape on the drain body.

Error No. 4: Drainage pipe diameter is too small or the slope of the drain is insufficient. Water drains, but slowly. With a strong flow from the shower, the channel is temporarily filled. The correct choice of DN is essential – consult before purchasing, not after.

Error No. 5: Silicone instead of grout, or grout instead of silicone. Silicone should be used only in elastic joints (trough-tile, wall-floor corners). Grout belongs in the regular joints between tiles. If you use grout at the trough, it will crack within a year and water will penetrate into the substructure.

Error No. 6: Forgotten water trap. A trap without water will dry out if the shower is not used for a long time. In some cheap models, the water trap is too shallow. The result is bad odors. The solution is to always pour water into it after a long absence, or to purchase a trap with a mechanical anti-odor flap.

Special situations: renovations and limited construction height

One of the most challenging situations is the renovation of an old panel apartment bathroom, where the existing drain is located in the floor (vertical drain) and the new HS board is only 60–80 mm thick. In such a case, you have limited options:

Choose a slim trough with a construction height of at least 55 mm (available, but expensive).
Use a special pre-mounted frame that allows installation without cutting the ceiling board.
Alternatively, a compromise in the form of a floor drain instead of a trough – a drain has lower construction requirements.

If you are dealing with old panel bathrooms, consult a plumber before purchasing a trough. Returning a product because it does not fit into the structure is unnecessary work and cost.

In new constructions, the situation is simpler – the construction height is planned with a reserve and the problem of small thickness does not arise. Nevertheless, I recommend explicitly checking the project documentation and confirming with the plumber exactly where the drain will be located before ordering the trough.

Maintenance of the trough after installation

A floor trough requires regular but simple maintenance. The grate can be removed (in most models it simply lifts up) and cleaned under warm water. The inside of the trough should be rinsed at least once a month with a stream of warm water to prevent the buildup of soap and hair deposits. For a clogged trough, the cleaning procedure is described in the article How to clean a clogged trap without chemicals.

If your trough has a removable trap (cassette trap), I recommend removing and cleaning it every six months. Chemical cleaning agents (strong alkaline gels) should be used only occasionally and according to the manufacturer's instructions – long-term use can damage the seals.

Most frequently asked questions (FAQ)

Do I have to completely remove the old screed when installing a floor trough, or is a cut-out groove enough?

For renovations, the recommendation is clear: the entire old screed should be removed down to the load-bearing concrete, not only in the area of the trough, but across the entire shower area. The reason is simple – you need continuous waterproofing on a stable base, and you also need to regrade the entire floor. A cut-out groove is sufficient only if the original screed is in perfect condition, has the correct slope, and the waterproofing beneath it is functional – which is very rare in old bathrooms.

What is the difference between an adjustable and a fixed flange on a trough at the wall?

A fixed flange has a fixed width and is suitable when the tile thickness and the accuracy of the trough installation are clearly defined in advance. An adjustable flange (for example, in the Flexible trough with 850 mm) can be extended or retracted within a range of about 10–20 mm, which allows for tolerance during tiling. In practice, an adjustable flange is more convenient for tilers, as they do not have to hit the exact millimeter. More on this topic can be found in the article Adjustable flange on a floor trough: when do I need it and how does it work?

Can I install a floor trough in a bathtub instead of a shower?

No, a trough is designed for floor applications where access is from above. Bathtubs use bathtub traps – either an automatic version (for example, automatic bathtub trap with overflow, chrome), or a CLICK/CLACK system (for example, CLICK/CLACK bathtub trap with removable drain). Bathtub traps handle the drain through the bottom drain of the tub and also integrate the overflow. Installation of bathtub traps is described in more detail in the article Installation of a bathtub trap with an automatic or CLICK/CLACK system.

How long do I have to wait after installing a trough before I can use the shower?

The minimum waiting time depends on the waterproofing and the grout. Liquid membrane (waterproofing) hardens in 24–48 hours. Adhesive mortar under the tiles needs at least 24 hours. Grouting compound and silicone usually need 24–72 hours. In practice, I recommend waiting at least 72 hours after the last intervention (grouting and sealing) before using the shower for the first time. With some products (fast-drying silicone, epoxy grout), the time may be shorter – always read the manufacturer's instructions for the specific materials used.

What should I do if after completing the installation I notice that water is pooling near the trough?

First, check whether the trap or the grate is clogged – dirt can be brought in from the construction process. If the trap is clean and the water still pools, the problem is either insufficient slope (you need to measure it), or the trough is installed too high and the grate forms a barrier. A shallow slope (under 1 %) in longer troughs can only be resolved by redoing the floor. If the grate is high, check whether it is properly pressed into the trough body – sometimes grates are height-adjustable and simply need to be pressed in deeper.

Is a trough with a central drain or a side drain better?

It depends on the location of the drain in the floor. A side drain (at the end of the trough) is standard for troughs installed along the wall – the drain pipe runs along the wall, which is structurally cleaner. A central drain is used for symmetrical troughs (for example, DOUBLE9 FIT and GO double-sided trough 750 mm) or for troughs mounted in the center of the room. If you are designing from scratch, I recommend the standard solution with a side drain – it is less prone to errors in slope execution.

Conclusion

The installation of a floor trough may seem simple at first glance, but the details determine whether the bathroom will be trouble-free for decades or whether problems such as dampness, bad odors, and standing water will appear within a year. The key points that everyone planning a renovation or new shower should take from this article are: a correct slope of at least 1.5 % (ideally 2 %), a horizontal position of the trough along its axis, high-quality waterproofing with a tight connection to the trough body, and a correctly dimensioned trap and drain. If you manage these four things, the rest is just a matter of careful tiling and grouting.

If you are still deciding whether to choose a trough or a classic drain, see the comparison in the article Floor trough vs. floor drain: which is better for a shower area?, or visit the entire category drainage technology, where you will find a complete range of troughs, drains, and traps.

Do you have a question about this topic?

Still undecided or dealing with a specific situation in your home? Write to us – we are happy to help.