The short answer: Waterproofing slurry is applied in two to three thin layers, crosswise, never in one thick one. The substrate has to be sound, clean and – on strongly absorbent surfaces – pre-wetted, otherwise it draws the mixing water out of the cement and the slurry does not set. Wait 4 to 12 hours between the layers depending on the product. All corners and connections get a sealing tape laid into the fresh first layer – that is where waterproofing almost always fails first. After application, protect from sun, draughts, rain and frost for three days.
What you need
- Waterproofing slurry, rigid or flexible depending on the substrate and load
- Primer or bonding bridge for absorbent and smooth substrates
- Sealing tape for corners and edges, sealing collars for pipe penetrations and floor drains
- Clean mortar buckets, at least two
- Drill or mixer with helical paddle, 400–600 rpm
- Flat brush or ceiling brush, short-pile paint roller, smoothing trowel
- Folding rule, spirit level, trowel for the cove
- Gloves and safety glasses – fresh waterproofing slurry is strongly alkaline
Step 1: check and prepare the substrate
The substrate decides the result, not the product. It must be sound, dimensionally stable, clean, free of release agents and sufficiently level.
The three checks beforehand
Scratch test: Draw a screwdriver across the surface. If sand trickles out or a deep trace remains, the substrate is not sound. Tap test: Tap with the hammer handle – spots that sound hollow have come away and have to go. Wipe test: Run your hand over the surface. If dust is left behind, it must be cleaned more thoroughly.
Old plaster, paint coats, sinter layers, efflorescence and cement film are removed mechanically – wire brush, sanding disc or, on larger areas, pressure washer. Close cracks and spalls beforehand with repair mortar, fill joints in masonry flush. Edges and corners are rounded off: a cove with a radius of about 4 cm prevents the waterproofing from being drawn over a sharp edge and becoming thin in the process.
Strongly absorbent substrates such as sand-lime brick, aerated concrete or old plaster are pre-wetted until the surface is matt-damp – dark, but without standing water. Smooth, non-absorbent surfaces such as concrete get a primer instead.
Step 2: mix correctly
This is where most mistakes arise, and they show only weeks later.
Always put the powder into the water, never the other way round – otherwise lumps form that no longer dissolve. The amount of water is on the sack and is not a recommendation but a requirement: too much water lowers the strength and makes the slurry run off vertical surfaces, too little makes it unworkable.
Mix lump-free with the mixer for two to three minutes, then let it mature for about five minutes and stir briefly again. This maturing time is readily skipped, but it ensures that the additives swell completely. Afterwards do not adjust with water again, even if the mass stiffens.
Mix only as much as you will also use within the working time – usually 30 to 60 minutes. Slurry that has stiffened is not revived but disposed of.
Step 3: brush, roll or trowel?
All three methods are permissible, but they suit different situations.
| Method | Suitable for | Advantage | Disadvantage |
|---|---|---|---|
| Flat brush or brush | rough substrates, masonry, first layer | presses the mass into pores and joints | Texture stays visible |
| Paint roller | smooth surfaces, large wall areas | fast, even thickness | fills pores less well |
| Smoothing trowel | floors, surfaces beneath tiles | greatest layer thickness, smooth result | more laborious, air pockets possible |
In practice the combination proves its worth: work the first layer in with the flat brush generously into the substrate so that it makes contact everywhere. Then the second layer with roller or trowel, to reach the required layer thickness reliably.
Step 4: lay sealing tape and collars
Before the surface is tackled, the weak points are secured. This is the work step that decides between success and water damage.
Corners between wall and floor, internal wall corners, connections to bathtub or shower tray and all movement joints get a sealing tape. For this the slurry is applied generously in the area of the tape, the tape is laid into the fresh mass and pressed on free of bubbles. Important: leave a small loop in the corner so that the tape can take up movement and is not under tension. Slurry is then drawn over the tape once more.
Pipe penetrations for mixer taps get wall sealing collars, floor drains floor sealing collars – laid into the fresh mass on the same principle and worked over. Overlaps of tapes with each other at least 5 cm.
Step 5: waterproof the surfaces
The first layer is applied generously and covering. After the intermediate drying – 4 to 12 hours depending on product and temperature, the surface must no longer rub off – the second layer follows crosswise to the first. This change of direction ensures that flaws and thin zones of the first layer are closed reliably.
Many products are coloured, so that the layers differ in colour. That is no gimmick: it is the only simple way to recognise a forgotten spot before it disappears under tiles.
For a complete room the sequence applies: walls first, then floor – so that you do not walk through the fresh floor waterproofing. And if tiling follows: set the wall tiles first, then waterproof the floor. A fallen tile fragment is enough to puncture a finished floor waterproofing.
Step 6: protect and let dry
Freshly applied waterproofing slurry has to set slowly. Direct sun, draughts, heating air or frost in the first days lead to shrinkage cracks – the slurry then dries at the surface before the cement inside could set.
Outdoors you cover the surface with foil or fleece. In enclosed rooms windows stay closed for the first 24 hours. The waterproofing must be protected from rain at least until it is rain-resistant – 4 to 24 hours depending on the product.
The waterproofing is loadable after one to three days. Only then is it tiled, backfilled or exposed to water.
Waterproofing slurry outdoors: plinth and facade
Outdoors two particular features are added. At the plinth the waterproofing is carried up from the ground to at least 30 cm above ground level and overlaps the below-ground waterproofing by at least 10 cm. Because frost and splash water act here, a flexible slurry belongs at this spot.
On the facade waterproofing slurry is not a standard build-up but a remediation measure against masonry that is exposed to driving rain and has become damp. What is decisive is that the wall can dry out towards the inside – a wall waterproofed on both sides never dries out. With exposed masonry and historic facades this decision belongs in expert hands.
Application temperature outdoors as indoors: between 5 and 30 °C, at night too. If frost is forecast within the next three days, no waterproofing is done.
Before or after the plaster?
The waterproofing lies beneath everything that comes afterwards – it is the water-bearing level, not the appearance. Indoors, therefore, first waterproof, then tile. At the plinth first waterproof, then apply the plinth plaster.
On existing plaster a waterproofing slurry can be applied only if it is firm, sound and mineral. Old, hollow or painted plaster has to come off. No waterproofing slurry belongs on gypsum plaster – gypsum and cement are not permanently compatible.
Rigid or flexible – which waterproofing slurry to take?
Before the first work step comes the choice of product, and it has only one criterion: can the substrate move?
A rigid mineral waterproofing slurry is purely cement-bound. It is compression-resistant, inexpensive and vapour-permeable, but does not bridge cracks. It belongs on solid masonry that does not settle, on concrete and in tanks – that is, wherever nothing will certainly move any more.
A flexible waterproofing slurry additionally contains a plastic dispersion, either as a second component or already in the powder. Depending on the product it bridges cracks of over a millimetre and tolerates temperature changes. It belongs on balcony and terrace, on screed, on young masonry and on everything exposed to the weather.
In case of doubt, decide on the flexible variant. The extra price is in the range of a few euros per square metre – the effort of restoring a cracked waterproofing under a finished covering is of a quite different order.
The substrates in detail
| Substrate | Suitable | What to watch for |
|---|---|---|
| Concrete, reinforced concrete | yes | Remove formwork skin and release agent residue, blast off sinter layer |
| Sand-lime brick, clay brick | yes | Fill joints flush, wet thoroughly beforehand |
| Aerated concrete | yes | very strongly absorbent, pre-wet several times or prime |
| Cement plaster, cement screed | yes | sufficiently old and dry, screed ready for covering |
| Anhydrite and calcium sulphate screed | conditionally | only with primer, sulphate and cement are poorly compatible |
| Gypsum plaster, plasterboard | no | Cement and gypsum react, expansive compounds form |
| Old tiles | conditionally | not absorbent, only with bonding bridge with quartz sand content |
| Old coating, emulsion paint | no | remove completely, otherwise the waterproofing comes away with the paint |
| Bitumen | no | mineral slurry does not adhere to bitumen |
| Wood, wood-based panels | no | moves too much, use a cement-bound building board instead |
Indoors or outdoors – the differences
Indoors you are mostly working against splash water from use. The build-up is thinner, the waterproofing is tiled over directly, and the connections to bathtub, drain and pipe penetrations are the real effort. Do not ventilate vigorously in the first 24 hours – draught dries the surface faster than the cement can set.
Outdoors weathering is added. Direct sun on a freshly applied surface leads to shrinkage cracks, rain in the first hours washes the slurry out, frost destroys the structure. Cover the surface with fleece or foil and, on hot days, work early in the morning or in the evening. A look at the weather forecast for the next three days is part of the preparation.
Underground waterproofing slurry reaches its limit. Against pressing water and mechanical load from the backfill the bitumen thick coating is the more robust system – the comparison is in our guide Waterproofing slurry or bitumen.
Checking the waterproofing before it disappears
As soon as it has been tiled or backfilled, the waterproofing is inaccessible for good. Take half an hour beforehand:
Visual inspection in raking light. Shine a lamp flat across the surface. Thin spots, roller marks and flaws become visible that disappear under frontal lighting. Layers set off in colour help additionally.
Feel over corners and penetrations. Touch every sealing tape, every collar: does the fabric sit free of bubbles, is it worked over on all sides, is it not under tension?
Measure the heights again. Was work carried up to the planned upper edge everywhere? A missing strip of ten centimetres at the wrong spot makes the rest ineffective.
With level-access showers a flood test is also possible: close the drain, dam the surface up a few centimetres high and leave it for 24 hours. The effort is worthwhile for every waterproofing that lies above an occupied room.
How much do you need?
Usage is around 1.5 kg of dry mortar per square metre and millimetre of layer thickness. For the usual 2 mm total thickness that is about 3 kg/m², with rough masonry rather 3.5 to 4 kg/m². Allow a 10 to 15 percent surcharge for waste and substrate profile.
A 25 kg sack is therefore enough for about 7 to 8 m² in two layers. When ordering, better take a sack too many than too few: an interrupted waterproofing with a day's pause between the areas is a predetermined breaking point.
The most frequent mistakes
The most frequent and most expensive shortcut. Slurry applied too thick shrinks unevenly, cracks and comes away again after weeks to months.
The substrate draws the mixing water out, the cement cannot set. The slurry looks finished, but can later be rubbed off as powder.
Corners move. A tape laid taut tears at the first settlement, a missing tape lets water straight into the construction.
When the mass stiffens, the working time is over. Additional water destroys the structure – the slurry becomes porous and no longer tight.
The surface feels dry after hours, but is cured through only after days. Anyone who carries on too early tears the waterproofing off with it.
Frequently asked questions
How is waterproofing slurry applied – brushing or trowelling?
Both are right. The first layer is best worked into the substrate generously with a flat brush or brush, the second with roller or smoothing trowel. On floors and beneath tiles the trowel has the advantage, because it reaches the layer thickness reliably.
How many layers of waterproofing slurry are necessary?
Always at least two, applied crosswise. With a rough substrate or increased load three. A single thick layer is not a permissible alternative.
How long does waterproofing slurry have to dry between the layers?
Depending on product and temperature 4 to 12 hours. The first layer must have set far enough that it is no longer softened when reworked and does not rub off.
Can waterproofing slurry be applied to tiles?
Only with a bonding bridge. Glazed tiles are not absorbent, the slurry does not bond there. The tiles must sit firmly, be degreased thoroughly and be pretreated with a suitable primer. For wet rooms with existing tiling a liquid membrane is often the more practical choice.
Is waterproofing slurry frost-resistant?
Cured yes, during application and in the first days no. At temperatures below 5 °C or frost forecast within three days, no work is done.
Waterproofing slurry or bitumen – which is better?
That depends on the component, not on quality. If tile or plaster is to follow, waterproofing slurry belongs there; if the surface disappears underground, bitumen is the standard solution. The detailed comparison is in our guide Waterproofing slurry or bitumen.
Can you tile onto waterproofing slurry?
Yes, after complete curing in a thin bed with a tile adhesive of class C2. Usage depends on tile format and substrate – the values are in our overview of tile adhesive usage.
