Wet rooms, balconies & swimming pools

Waterproofing a balcony over existing tiles — without removal

Last updated: 26 October 2026

Solutions & Applications · Liquid-applied membranes

This is the most frequently asked question and the most dangerous answer. Laying a screed over tiles does indeed solve the problem in some cases — whilst in others it causes damage to the flat below. The difference is determined before the first tub is purchased.

First, the right question: why is it leaking?

The coating seals the surface. If water enters from the surface, it is repelled. If water is already inside the structure, it is not repelled — it is trapped.

Cause of leakage	Can this be resolved by applying a coating?
Porous or cracked tile joints	Yes — typical case
No waterproofing beneath the tiles, but the underlay is intact	Yes, provided the substrate is dry
Cracked or hollow tiles	No — they are removed and replaced first
Saturated sub-base or cement screed	No — sealing traps moisture
Incorrect or missing specifications	Not on its own — water will pool
Failure at the point of the window frame or guttering	Only by addressing the detail, not the surface

ARGYNET Note

The coating does not dry out what is already wet — it simply blocks its path upwards.

On a balcony that has been leaking for years, the cement screed beneath the tiles is often saturated. Until now, some of this moisture has evaporated through the joints. By sealing the surface, the only available route left is downwards — that is, the ceiling of the flat below.

The result is paradoxical yet commonplace: the leak gets worse after waterproofing, and the owner cannot understand why. The moisture has not been added — it simply has nowhere else to go.

Before any coating is applied: check for hollow tiles by tapping and measure the moisture content of the substrate. This is free of charge and determines whether the solution is feasible. At ARGYNET, we draw up specifications after diagnosis, not before.

The inspection that costs nothing

Tile tapping test: using a metal object, tap the surface systematically. A hollow sound indicates a loose tile — remove it, clean the substrate and replace it. Any covering applied over a hollow tile will crack at the first sign of movement.
Moisture measurement: using a moisture meter, at multiple points and particularly at lower levels. A surface that appears dry does not necessarily mean that the substrate is dry.
Check for water ingress: use a hose. Where water pools, this is recorded — and a decision is made as to whether it should be rectified or whether the material must be able to withstand standing water.
Inspection of expansion and perimeter joints: cracks running in a straight line indicate movement, not deterioration — and will reappear over the new layer if not addressed.
Checking the frame clearance: how many millimetres are available beneath the balcony door — this determines whether new paving can be laid or only a liquid-applied coating is possible.

Two architectural styles — and the choice of materials stems from them

	A. Exposed membrane	B. Membrane beneath new paving
Added thickness	Minimum — a few millimetres	Important: membrane + adhesive + tile
Material	Polyurethane or polyurea — UV-resistant and walkable	Elastomeric cementitious compound — suitable for tile adhesive
Final layout	The membrane itself — its colour and texture are the finish	Tile
When to choose	When there is no height, or when speed and low cost are the priority	When height is a factor and a specific aesthetic is required
Please note	Slip resistance and UV resistance — the membrane is the flooring	The cementitious product must explicitly state that it is suitable for tile bonding

The choice is not an aesthetic one but a matter of dimensions: if there are ten millimetres beneath the balcony door, the second option is out of the question. Conversely, where there is sufficient headroom, the new tiling produces a result that the owner perceives as a refurbishment rather than a repair.

Adhesion to polished tiles

This is the second area where the project falls short. Glazed tiles are non-absorbent and smooth — in other words, the worst possible surface for adhesion. Without proper preparation, even the best membrane will peel off in sheets.

Deep cleaning: grease, soap, limescale and cleaning product residues are completely removed.
Mechanical roughening where required — a grinder or diamond wheel gives the surface the profile it lacks.
Suitable primer: polyurethane primer for polyurethane membranes, water-based epoxy primer for cementitious substrates and polyurea. The primer is selected on the basis of the membrane, not the substrate.

The details that make all the difference

Perimeter drains: curved floor-to-wall joint using repair mortar. The right angle of 90° is the point at which each liquid-applied coating naturally tapers off.
Vertical extension: the membrane is extended up the parapet and along the walls, terminating above the level that water can reach — never at the same level as the floor.
Balcony door threshold: the most common point of entry. The membrane terminates beneath the door frame and is sealed; it does not stop in front of it.
Gutter: it is reinforced locally and the membrane is inserted into the outlet so that water does not pass along the side.
Movement joints: these are bridged with tape or an elastic strip that allows movement — the membrane must not be rigidly fixed across them.

Stagnant water

If the specifications are incorrect and are not rectified, the material must be selected on that basis. Not all liquid-applied coatings can withstand the permanent presence of water — some gradually hydrolyse.

Where there are areas that do not drain, the question in the tender is specific: does the product data sheet specify resistance to standing water? If not, either the slope must be corrected by local levelling, or a material capable of withstanding it must be selected.