Waterproofing is one of the renovation stages that remains completely hidden beneath the tiles, yet it largely determines whether mold appears a year later at the junction with the downstairs neighbors' ceiling or whether tiles begin to detach due to constant moisture in the screed. The question of how to waterproof a bathroom under tiles is especially relevant for areas with high humidity, such as shower zones, floors around the bathtub, and areas adjoining risers. Below, we will go through the process step by step, from choosing the material to common installation mistakes.

Why Waterproofing Is Necessary and Where It Is Essential

A bathroom is exposed to water every day — from splashes during a shower to condensation and accidental leaks. Without a waterproofing layer, moisture gradually penetrates through the tile joints into the screed and floor slab, which can lead to deterioration of the substrate, mold growth and, in the worst case, flooding the neighbors below.

The areas that must be waterproofed include the entire bathroom floor; walls to a height of at least 20–30 cm from the floor, and up to 180–200 cm in the shower area; areas around risers, pipes and plumbing connections; as well as corners and joints between the floor and walls, where the risk of leakage is particularly high.

How to Waterproof a Bathroom Floor Under Tiles: Choosing the Material

Before starting the work, it is important to choose the right type of waterproofing product, as this determines both the application method and the final reliability of the coating.

• Liquid-applied waterproofing (polymer-cement or bitumen-polymer mastics) — applied with a brush or roller in several layers to create a flexible, seamless coating. It is one of the most common options for household renovations due to its ease of application.
• Roll waterproofing — used less frequently in apartments and more often on large areas or in combination with liquid-applied products to provide additional protection for joints.
• Penetrating waterproofing — a compound that fills the pores of concrete and crystallizes within its structure, increasing the water resistance of the substrate itself. It works well as an additional layer before applying liquid waterproofing.

For most apartment bathrooms, ready-to-use polymer-cement waterproofing materials remain the optimal solution. They are compatible with subsequent tile installation and do not require complicated application equipment.

How to Waterproof a Bathroom Under Tiles Yourself: Step-by-Step Instructions

The process can be divided into several consecutive stages.

1. Prepare the substrate. Clean the surface of dust, old coatings and grease stains. Cracks and uneven areas in the screed should be repaired in advance using a suitable repair compound.
2. Apply primer. Before applying the waterproofing, treat the substrate with a deep-penetrating primer. This improves adhesion and reduces the absorbency of the surface.
3. Seal corners and joints. Use a special sealing tape at floor-to-wall junctions and in corners. These areas most often become the source of leaks when they are not treated thoroughly enough.
4. Apply the first coat. Apply the waterproofing compound evenly with a brush or roller, including areas around pipes and plumbing outlets.
5. Apply the second coat. Once the first coat has dried according to the time specified by the manufacturer on the packaging, apply the second coat perpendicular to the first. This helps prevent missed areas and improves the overall integrity of the coating.
6. Check for leaks. After the waterproofing has completely dried, it is recommended to perform a water test by applying a small amount of water and checking whether any moisture appears outside the treated area.
7. Install the tiles. Tiles should only be installed after the waterproofing has completely dried, using an adhesive that is compatible with the treated substrate.

How to Waterproof a Bathroom Under Tiles Yourself: Common Mistakes

Even when high-quality materials are used, the result may be unreliable if the application technology is not followed correctly. The most common mistakes include:

• Applying one coat instead of two. A single-layer coating often contains micropores and does not provide complete watertightness, so a second coat is essential.