Slope of the floor near a linear drain – how to properly build and tile

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Grade of the floor with a linear drain – why it is the foundation of the entire work

When I watched the installation of the first linear drains in Slovakia years ago, the most common problem was not even the choice of the right product or the quality of the drain trap. The problem was always the same: incorrect floor slope. Tile layers who had successfully laid tiles in bathrooms with a central point drain for years simply transferred the old habit to the new technology – and the result was a tiled floor where water stood instead of draining away.

A linear drain works on a completely different principle than a classic central drain. While with a point drain the floor usually slopes from all sides toward one point (called a "pyramid slope"), with a linear drain the entire area of the shower corner must slope unidirectionally toward one wall where the drain lies. The entire geometry of the work is therefore different and requires different thinking, different preparatory masonry work, and a different approach to the tile laying itself.

In this article, I will cover the topic from the basics: why the slope is critical with a linear drain, what slope values are correct, how to make the base layer (mortar bed/underlayment), how to proceed with the tile laying itself, and where mistakes are commonly made in practice that later cause problems for years. If you are planning a bathroom renovation or building a new shower area, this is the article you should read before ordering any materials.

Basic principle: how a linear drain drains water

A linear shower drain is a long, narrow drainage channel typically placed along one wall (or less frequently along the entrance edge of the shower area). Its entire construction is designed so that water that falls anywhere on the shower surface reaches the drain by gravity – without any clogging, standing puddles, or water shadows.

For this to happen, the entire floor surface must be slightly inclined toward the drain. We are talking about a slope, which is a percentage value expressing the height change per unit of length. In practice, the slope is expressed either in percentages (e.g., 2 %), or as a ratio (e.g., 1:50), or simply in millimeters per meter (e.g., 20 mm/m).

From the diagram above, the difference is clear: with a point drain, water drains from all sides to one point, with the floor forming a slight cone shape. With a linear drain, water drains unidirectionally, in parallel streams, along the entire length of the drain. This allows the use of large tile formats (60×60 cm, 80×80 cm, even 120×60 cm) without the need to cut tiles into wedge shapes.

Correct slope value – how many millimeters per meter?

This is a question that leads to long discussions in practice. Technical standards (specifically STN 73 3450 and European EN 1253 for drainage elements) do not directly specify the minimum slope of the shower floor surface – this is a matter of the manufacturer's instructions for the drain and tiling recommendations. In practice, the following values have become established:

Minimum slope: 1.5 % (15 mm/m) – the absolute minimum that still ensures self-cleaning function. It may be sufficient on smooth porcelain tiles, but with rougher surfaces (so-called anti-slip textures), water drains worse and this minimum is insufficient.
Recommended slope: 2 % (20 mm/m) – the golden standard for regular shower areas. With a 90 cm length, this means a height difference of 18 mm between the furthest corner and the edge of the drain. It is almost imperceptible when walking, but water drains reliably.
Increased slope: 2.5 – 3 % (25 – 30 mm/m) – recommended for larger shower areas (120 cm and more), large-format tiles with thick grout lines, or anti-slip tiles with a rough surface texture.
Maximum practical slope: 3.5 – 4 % – at higher values, the slope becomes noticeable when standing and walking, which is unpleasant for many people. Above 4 %, it is not recommended for standard bathrooms.

Practical calculation: If your shower area has a depth of 100 cm and the drain is at the back wall, with a slope of 2 %, the front edge of the shower area (at the entrance) will be 20 mm higher than the edge of the drain. If the area is 120 cm deep, the difference will be 24 mm. You must take this into account already during the preparation of the base.

Preparation of the base: slope screed and its proper execution

The correct slope begins to be built even before the waterproofing. The most common method is so-called slope screed (cement screed with the addition of slope-forming material or manually shaped screed with continuous measurement). It is a cement screed applied to the load-bearing base, and its surface is shaped so that it slopes towards the location where the drain will be.

Step-by-step preparation of slope screed

1. Alignment and marking of the drain level: The first step is to precisely determine where the drain will be located and at what height. The drain must be installed so that its upper edge (edge of the grate) is at the same level or at most 1–2 mm lower than the surrounding floor. Ideally, the upper edge of the grate should match the upper edge of the floor – water will then smoothly flow from the floor surface into the drain without any step.

2. Determining the height of the screed at the entrance edge: Subtract the thickness of the tile (e.g., 10 mm) and the thickness of the adhesive mortar (6–8 mm) from the height level of the upper edge of the drain. This gives you the height of the finished screed at the drain. Towards the entrance edge of the corner, the screed must rise by the slope value. For a 90 cm deep corner with a 2 % slope, this is + 18 mm.

3. Installation of screed rails or formers: Before applying the screed, mark the slope rails (or plaster formers) that will serve as a guide for the correct height of the screed. Install the rails so that their upper edge exactly follows the desired slope. Check them with a spirit level with adjustable error (some spirit levels have a position for 1.5 or 2 % slope) or use a digital inclinometer.

4. Preparation of the mix and application: Slope screed is usually made from cement and sand in a ratio of 1:4 (dry method) or as cement mortar with a water factor of around 0.5. The consistency should be such that the screed can be pressed into a ball, from which water does not run out, but the ball retains its shape. Apply in layers of max. 30–40 mm and compact each layer.

5. Screed leveling and smoothing: Using a straightedge, level the screed according to the marked guides. The surface should be flat (without local depressions or bumps), but at the same time slope towards the drain across the entire area. This is the step where most workers make mistakes – they leave local depressions where water will later stand.

6. Technological break: Cement screed needs at least 3–5 days to sufficiently cure before further work. With larger thicknesses or lower temperatures, it may be necessary to wait up to 7–14 days. Hurrying this phase is one of the most common causes of later problems (cracking of the screed, poor adhesion of waterproofing).

Slope boards and ready-made slope systems – an alternative to screeds

In recent years, so-called slope boards (e.g., XPS boards with milled slope or polystyrene slope wedge kits) have become increasingly widespread. These products are manufactured with a predefined slope directly from the factory – simply place them on the base and you get a finished slope without mixing mortars. Their advantages are speed, simplicity, and significantly lower weight (important in the reconstruction of panel apartments, where ceiling load capacity is limited).

Slope boards are suitable especially when you do not want to add the large weight of a cement screed. Waterproofing and adhesive mortar are then applied directly on them. The disadvantage is the higher material cost and sensitivity to point loading (they may crack if the tile is improperly glued).

Waterproofing – do not rush and do not save

After the screed has cured (or after the slope boards are installed), the next step follows, which is often underestimated in practice: waterproofing. Every shower area without a bathtub must be reliably waterproofed, because water will directly fall onto the floor surface and run along the walls. Poor waterproofing means later moisture in the structure, mold behind the tiles, and cracked plaster in adjacent rooms.

The following specific rules apply to waterproofing for a linear drain:

Waterproofing must be extended onto the walls at least 20 cm above the floor (some experts recommend 30 cm). For showers without a curtain or door (walk-in design), consider a height of 60–100 cm.
In the corners (floor/wall), a waterproofing strip (sealing strip in corners) must be inserted, which compensates for micro-cracks from structural movements.
The drain must be integrated into the waterproofing layer – most quality drains including models from the SLEEK series are equipped with a flange or lip to which the waterproofing is attached. This point is critical: if the waterproofing does not follow the entire perimeter of the drain, water will precisely leak through this spot.
Apply waterproofing in at least two layers (first layer, mesh, second layer). The total thickness should not be less than 1.5 mm.
After applying the last layer, allow the waterproofing to fully cure (usually 24–48 hours depending on the product). Before continuing, perform a tightness test – cover the drain, fill the shower with 2–3 cm of water and wait 24 hours. If the water level has not dropped, the waterproofing is in order.

Tile installation with a linear drain – techniques and tile format selection

After waterproofing, tile installation comes next. This is the phase where the final look and functionality of the shower area are decided. With a linear drain, different rules apply than with a traditional bathroom.

Choosing tile format and tile orientation

One of the main advantages of a linear drain over a point drain is the ability to use large-format tiles without compromises. With a point drain, each tile must be cut "in a curved line" (angled cuts), because the slope radiates from all sides. With a linear drain, the slope is unidirectional and parallel – this means that tiles can be laid in rows perpendicular to the drain, with each row being a full (uncut) tile.

Recommendations for tile format selection:

Small formats (up to 20×20 cm): Can be laid almost without limitations, the slope is compensated by the adhesive. The risk of standing water is low, as the grout lines serve as natural drains.
Medium formats (30×30 to 60×60 cm): An ideal compromise. One tile must be laid with the correct slope, but it is still achievable without special tools.
Large formats (60×120 cm, 80×80 cm and more): Require an absolutely precise base. Any unevenness in the screed will manifest as "bouncing" or cracking of the tile. With these formats, it is essential to apply adhesive using a double-sided method (adhesive on the base and on the tile) and ensure 100% contact of adhesive with the tile.

The orientation of the tiles should be perpendicular to the axis of the drain (tiles run from the back wall towards the entrance). This ensures that the grout lines follow the direction of water flow, which helps the surface's water drainage capability.

Selection and application of tile adhesive

In shower areas without a bathtub, it is necessary to use waterproof flexible adhesive of class C2TE (or C2TES1/S2 for large formats). Do not rely on cheap basic adhesives of class C1 – they tend to peel off the base or the tiles during temperature fluctuations and constant contact with water.

Adhesive application must be thorough. For standard formats, a notched trowel with a tooth height of 6–8 mm is sufficient, for large formats 10–12 mm. Apply adhesive always in the direction of the slope (not crosswise), so that the trowel ridges do not create a barrier for water in the event of a joint overflow. For large-format tiles, apply adhesive also on the back of the tile – only this ensures 90–100 % coverage of the surface with adhesive, which is a requirement for wet areas (standard EN 12004).

Embedding the drain into the tiles – final height and coverage

The linear drain must be embedded so that its grate is in the plane of the surrounding tiles or at most 1–2 mm below them (never above them – that would create a step that people could trip over and water would pool behind the drain). Accurate height setting of the drain is therefore necessary to leave it until the end of the tiling, or to adjust the drain height first and then lay the tiles around it.

Most quality linear drains, including SLEEK 700 mm in black, have adjustable legs or adjustable sides, which allow you to fine-tune the drain height even after the rough tiling. This feature is particularly valuable – in practice, it is very difficult to guarantee that the screed has an exact height to the millimeter, and therefore the ability to fine-tune the drain is key.

After embedding the drain into its final position, the space between the drain body and the adjacent tiles (and wall) must be filled with silicone. Never use grouting compound for this joint! Hard grouting compound does not tolerate micro-movements between the drain and the structure and cracks over time – thus creating a path for moisture under the waterproofing. Sanitary silicone (neutral, with fungicide) is the right choice here.

Concrete working scenarios from practice

Scenario 1: Reconstruction of a panel apartment, concrete ceiling, limited height

This is the most common scenario we encounter. A panel apartment, a bathroom 180×180 cm, the original bathtub has been removed, leaving us with a bare concrete slab. The height space for the entire floor assembly is only 80–100 mm (so that the doors do not need to be shortened and the threshold does not need to be unnecessarily raised).

Solution: a sloped polystyrene board 40 mm thick (with a 2 % slope milled from the factory), waterproofing on it (double-layer, 1.5 mm), flexible adhesive on it (8 mm) and tiles (10 mm). Total thickness of the assembly: approx. 60–65 mm. The remaining 15–20 mm is sufficient to compensate for the unevenness of the original slab with foam or a thin-layer leveling screed.

This solution is suitable for SLEEK 600 mm in black – its depth of the drain channel is dimensioned for standard flows of shower fittings (up to 20 l/min), which is fully sufficient. The length of 600 mm covers more than 1/3 of the width of the shower area, which ensures reliable drainage even during intensive showering.

Scenario 2: New construction, walk-in shower 120×90 cm, large-format tiles 60×120 cm

Here, the floor assembly height can be planned from the beginning. Cement screed 50–60 mm with a 2 % slope, waterproofing with sealing tapes in the corners, C2TE S1 adhesive, 60×120 cm tiles oriented perpendicular to the drain (tiles run from the back wall to the entrance, the drain is at the back wall). Result: a visually clean surface, minimal grout lines, precise water drainage.

We recommend SLEEK 600 mm in gun grey for such a shower with white/light tiles, because the gun grey color creates an elegant contrast without appearing aggressive like black. Gun grey drains are also less noticeable in terms of limescale deposits – read more in the article Surface treatment of a shower drain: black vs. gun grey – which lasts longer.

Scenario 3: Small shower 80×80 cm, drain at the entrance

Less common, but functional solution: the drain not at the back wall, but directly at the entrance to the shower (e.g. at the transition from the bathroom). In this case, the slope runs from the back wall towards the entrance – the opposite of most installations. The principle is the same, only the direction. A shorter drain is sufficient for such a shower – SLEEK 500 mm in black or SLEEK 500 mm in gun grey are fully sufficient for an 80×80 cm area. Read more about choosing the right length in the article What length of a shower drain is suitable for my shower.

Most common mistakes with slope and tiling – and how to avoid them

After years of monitoring orders, we can identify several mistakes that repeat over and over:

Insufficient slope (less than 1.5 %): Water stands on the surface, does not drain. The solution is usually only to tear up the entire floor and re-tile it.
Uneven screed with local depressions: Even if the overall slope is correct, water stands in the depressions. Depressions are difficult to fix after tiling – they require grinding or cutting out the tiles.
Drain embedded too high: The grate protrudes above the tiles, water accumulates at it, a wet step forms. You need to lower the drain or grind the edge of the tile (difficult and aesthetically problematic).
Hard grouting compound between the drain and the tiles: After a few months, it cracks, a crack forms through which water penetrates under the waterproofing. Solution: mechanically remove the grouting compound, clean thoroughly, fill with sanitary silicone.
Missing sealing of the drain to the waterproofing: Water penetrates between the drain body and the tiling. This usually manifests itself after a longer time as moisture on the ceiling of the adjacent room.
Neglected penetration of the base: On absorbent bases (aerated concrete, old plaster) without penetration, the adhesive does not hold for long. Always penetrate with the appropriate preparation recommended by the adhesive manufacturer.

Scrubbing and final finishing – what should not be neglected

After the adhesive has hardened (at least 24 hours, 48 hours for larger formats), it is time for tiling. For shower enclosures without a bathtub, always use water- and mold-resistant tiling compound – ideally epoxy tiling compound or at least a cement-based tiling compound with a fungicidal additive. The grout must be filled completely to the full depth, without voids, because voids are places where dirt accumulates and mold grows.

Fill the joint between the drain and the floor (and all wall/floor corner joints) exclusively with sanitary silicone. Apply the silicone only after thoroughly cleaning and degreasing the surface (isopropyl alcohol works well). Apply the silicone evenly, smooth it with a wet finger or trowel, and let it cure for at least 24 hours before the first use.

For more information on regular cleaning and long-term maintenance of the drain, see the article Cleaning and maintenance of the linear shower drain SLEEK.

Final inspection – how to determine if the slope is correct

After completing the entire installation, tiling, and silicone application, it is advisable to perform a final functionality check. The procedure is simple: flood the floor of the shower area with water and observe how it behaves:

Water should reach the edge of the drain and start draining within 30–60 seconds.
Standing water should not remain on any part of the floor for more than 2–3 minutes (with the shower turned off).
Water flow through the drain grate should be smooth and without gurgling.
Check whether water is dripping from under the drain or in the corners – this would indicate a problem with waterproofing.

If water is pooling somewhere, identify the exact location and determine whether it is a local depression in the tile or a systemic slope issue. A small depression (up to 2–3 mm) can sometimes be corrected by grinding the edge of the tile or applying a waterproofing compound to level it. A larger issue unfortunately requires locally redoing the tile installation.

A detailed step-by-step installation procedure for the drain (including setting the trap and connecting to the waste system) can be found in the article Installation of a linear shower drain step by step.

Most frequently asked questions (FAQ)

What is the minimum slope required for the floor with a linear drain?

The minimum recommended slope is 1.5 % (15 mm per meter of length). In practice, it is safer to design a slope of 2 % (20 mm/m), as smaller deviations during the screed application are less noticeable. For anti-slip tiles with a rough surface, we recommend at least 2 %, ideally 2.5 %, as the rough surface slows down water flow.

Can I use large-format tiles 60×120 cm with a linear drain?

Yes, and this is one of the major advantages of a linear drain. Since the slope is unidirectional and parallel, the tiles do not need to be cut into beveled shapes. Orient the tiles perpendicular to the drain axis (rows run from the wall to the entrance) and ensure that the screed is perfectly flat without local depressions – with large formats, every unevenness will either cause the tile to flex or result in standing water.

Where exactly should the top edge of the drain grate be in relation to the surrounding tiles?

The top edge of the drain grate should be at the same level as the surrounding tiles, or at most 1–2 mm lower. If the drain is lower, water drains more easily, but visually it is hardly noticeable. If the grate is higher than the tiles, a step forms where water can collect – this is a non-functional solution that usually requires adjusting the height of the drain.

What to use to seal the joint between the drain and the tiles?

Exclusively sanitary silicone (not tiling compound!). Sanitary silicone is elastic, tolerates micro-movements of the structure, and contains fungicidal additives that prevent mold growth. Solid tiling compound in this joint will crack, allowing water to penetrate under the waterproofing. Apply silicone to a clean, degreased surface and let it cure for at least 24 hours before the first wet load.

How many layers of waterproofing should be applied under the tiles in a shower without a bathtub?

At least two layers of liquid waterproofing compound, with a waterproofing fleece or mesh pressed in between the first and second layer. In corners (floor/wall, wall/wall) a sealing tape is necessary before applying the waterproofing. The total thickness of the finished waterproofing should not be less than 1.5 mm. After application, we recommend a water tightness test for 24 hours before laying the tiles.

What if my shower is larger than the drain length – will water drain from the distant corners as well?

Yes, provided the slope is correctly done across the entire area. A linear drain functions as a full-surface water collector along its entire length, and if the floor is evenly sloped toward the drain, water from any part of the floor will reach the drain. For larger areas (e.g., a shower of 120×100 cm or more), we recommend a drain with a higher capacity – for example, SLEEK 700 mm, which has a longer drainage channel and thus a higher flow capacity. For more information on choosing the right drain, see the article How to choose a linear shower drain for a shower enclosure.

Conclusion: the slope is the foundation on which everything else stands

The correct slope of the floor with a linear drain is not a detail – it is the foundation on which the functionality of the entire shower enclosure depends for many years. No expensive tiles, no premium drain, or perfect tiling compound can replace the physical reality: if water has nowhere to flow, it will stand. Standing water in the shower means a wet surface, a slippery surface, mold growth, and gradual damage to the structure.

Investing in a properly prepared sloped screed, quality waterproofing, and careful tile installation will pay off many times over in the form of trouble-free operation and an aesthetically clean result. A linear drain itself – whether you choose SLEEK 600 mm in black or any other model from our range – is designed to function reliably for decades with proper installation. It all depends on what is underneath it.

If you are considering a walk-in shower solution without a bathtub and are interested in overall space planning, also read the article Integration of a linear drain into a walk-in shower without a bathtub – you will find a comprehensive view of the entire issue from design to final installation.

Do you have a question about this topic?

Can't decide or are dealing with a specific situation in your home? Write to us – we are happy to help.