5 hidden mistakes when waterproofing a bathroom (which are the most expensive) + checklist before gluing tiles

If there is one "invisible" part of bathroom renovation that decides whether you will have peace of mind or leaks, moisture and peeling tiles , it is the waterproofing under the tiles . It is not visible, but mistakes in it are expensive: demolition, replacement of materials and sometimes repairs to the neighbor below.

Below are the 5 most common (and most expensive) mistakes , plus a checklist and mini shopping list you can keep.

Before/After: what the problem looks like in real life

If done incorrectly, the following often occur: moisture around the shower area, darkening of joints in the corners, unpleasant odors, and "wet spots" on the wall or floor.
With the right system, the base remains protected and the risk of water penetrating behind the tiles and creating hidden problems is reduced .

Mistake #1: “Just the floor is enough” (missing critical areas on the walls)

Many people only waterproof the floor and 5–10 cm of the walls. This is often insufficient because the water actually reaches:

• the walls in the shower area (splashes and condensation),

• the area around the mixer/faucet,

• the area around the siphon/line siphon,

• corners and joints (where the structure "works").

⚠️ Don't do this: waterproof the floor only, without the wet area on the walls.
✅ Do this instead: treat the floor + walls in the wet area , and separately all critical areas (corners, siphons, outlets).

Mistake #2: Missing strips/cuffs at corners and pipe passages

Leaks rarely come from "the middle." Most often, the problem is in:

• corners and transitions (floor-wall, wall-wall),

• around the siphon ,

• around pipe passages .

System solutions for this are sealing strips for corners/joints and cuffs/pipe collars , which are installed between the layers of waterproofing.

⚠️ Don't do this: "smear directly into the corner" without a tape/cuff.
✅ Do this instead: use system components and integrate them between layers.

Mistake #3: Waterproofing on an unprepared base (dust, crumbling, residual moisture)

Manufacturers explicitly require that the substrate be sound, dry and clean , free of substances that interfere with adhesion (dust, grease, dirt).

⚠️ Don't do this: waterproofing over dusty/crumbling screed.
✅ Do this instead: clean, strengthen weak spots, and prime if the system requires it.

Mistake #4: Thin layer/gaps and hasty gluing (without drying)

With liquid waterproofing, "one coat" is almost always a recipe for trouble. For example, Ceresit CL 51 states that two coats are needed, with the second coat being applied after ~2 hours, and after about ~4 hours (depending on conditions) the base is ready for tiles.

In 2K cementitious systems, the times between coats may be longer (e.g. about ~6 hours at 20°C for SikaTop Seal-107 Elastik), with the second coat being applied perpendicularly after the first has hardened.

⚠️ Don't do this: "one layer and I'll stick it right away."
✅ Do this instead: work in a system (often 2 layers, crosswise) and follow the drying time from the technical sheet.

Mistake #5: There is no slope to the siphon (or there are “puddles”)

Even the best waterproofing suffers if water is retained . The CL 51 guidelines state that surfaces should have a minimum slope of 2% and be free of depressions.

⚠️ Don't do this: leave "pockets" and puddles around the siphon.
✅ Do this instead: check the slope (in the wet area) and do a test with some water - let it go to the siphon.

Checklist before gluing tiles (copy/save)

✅ 1) The base is solid (no crumbling/cavities/weak areas).
✅ 2) Cleaned and dust-free – free of dust/grease.
✅ 3) The shower area has a correct slope towards the siphon (no "puddles").
✅ 4) The primer (if needed) has been applied and dried.
✅ 5) The waterproofing is applied evenly (according to the system) and without gaps.
✅ 6) Corners/joints are reinforced with sealing tape (between layers).
✅ 7) There is a cuff/collar around the tubes (between the layers).
✅ 8) The detail around the siphon is made without "see-through" areas.
✅ 9) The drying time from the technical card has been observed (don't guess).
✅ 10) The correct adhesive/grouting materials for wet areas are prepared.

Mini shopping list

Materials: primer (if needed), waterproofing, corner/joint tape, pipe sleeves, tile adhesive, joint + sanitary silicone.
Tools: roller/brush, spatula/trowel, notched trowel, level, bucket/mixer, sponge.

What to buy from SIKO (quick list for a tiled bathroom)

To do the waterproofing "right", think in a system: base → waterproofing → details (strips/cuffs) → glue → joint → drainage.

• Waterproofing (liquid, cement, etc.)
• Tile adhesives (for wet areas, large format, etc.)
• Grouting compounds (water/mold resistance according to product)
• Siphons and accessories (floor siphons, sink/bathtub siphons, etc.)
• Linear siphons (if you are making a walk-in/shower area)
• Tile accessories (profiles, systems, etc. useful accessories)
• Detail example (waterproofing tape)