Pool waterproofing: what the membrane does and why it matters before re-tiling

Two lines of defence

A tiled concrete pool holds water in two ways. The first is the structure: a reinforced concrete shell designed and built to be watertight. The second is a waterproofing membrane applied to the inside of the shell before the tiles go on.

Mapei's technical service puts the order plainly: only watertight reinforced concrete pool shells are suitable for receiving tiles, and the shell should pass a watertightness test before tiling starts. The surfaces are then levelled and waterproofed. ARDEX UK says British Standards require swimming pools to be watertight structures, and that where a structure is not watertight, a waterproofing product must be applied over the entire area at the correct thickness, or there could be leaks that are notoriously hard to find once the installation is complete.

A membrane protects a sound shell, bridges small movements and stops water travelling behind the tiles. It cannot make up for a shell that leaks.

What the membrane is

Pool membranes are usually liquid-applied. Mapei names flexible two-component cementitious membranes, applied in two coats to a total thickness of at least 2 mm, with a glass fibre mesh or fabric embedded between the coats where the pool is subject to high loads or movement. Construction Chemicals, an Australian manufacturer, specifies an acrylic membrane in two coats to a final thickness of 1.5 mm. The products differ, but the principle is the same: a continuous, flexible film built up to a specified thickness.

The details decide whether it works:

• Corners and junctions. Mapei and ARDEX both call for reinforcing tape at the joints between walls and floor.
• Penetrations. Skimmers, inlets, lights and pipes pass through the shell. Mapei lists them, together with corners, edges and cold joints, among the critical areas in pool waterproofing.
• Weather and curing. Construction Chemicals tells installers not to apply its products in direct sunlight or hot, windy conditions, and allows a minimum of seven days at 23 °C and 50 per cent relative humidity before filling. In a UAE summer that narrows the working window.
• Compatible adhesive. Over a membrane, Construction Chemicals specifies flexible adhesives, and Mapei expects adhesives and grouts suited to contact with chlorinated water.

What an internal membrane is not designed for

Construction Chemicals warns against using its internal membrane on inground concrete pools subject to negative or hydrostatic back pressure, meaning groundwater pushing from outside. Mapei offers a separate cementitious mortar for concrete under negative and positive hydraulic pressure. If your pool sits in high groundwater, the specialist has to design for it rather than reuse a standard internal system.

Mapei's waterproofing product manager also makes a point that applies to every renovation: cracks in an existing structure may be a sign of settlement, so look into them carefully before any waterproofing work.

Signs the waterproofing may have failed

No single symptom proves it. These signs point towards the shell or membrane rather than the plumbing, and justify an investigation:

• the pool loses more water than evaporation explains, and the loss continues with the pump switched off (see why is my pool losing water)
• pressure tests on the pipework pass, but the pool still loses water
• tiles sound hollow, lift or fall off across an area rather than one or two at a time
• damp patches appear on walls next to the pool, in the plant room or, for rooftop and podium pools, in the space below
• grout keeps cracking or washing out along the same lines

A leak detection specialist can rule out the pipework and fittings first, so that the shell is only opened up when the evidence points there.

Why it matters before re-tiling

Re-tiling is the moment the membrane can be reached. Once new tiles are bonded, the only way back to it is to remove them again. The ARDEX warning about leaks being hard to find after installation is the practical reason to deal with waterproofing during a renovation rather than after it.

Before agreeing to a re-tiling quote, ask the contractor to answer in writing:

1. Will the old tiles and adhesive be removed down to the shell?
2. Will the shell be inspected and tested for watertightness before new work starts?
3. How will cracks found in the shell be assessed and repaired?
4. Which membrane will be used, in how many coats and to what thickness?
5. How will corners, skimmers, inlets, lights and pipe penetrations be detailed?
6. How long will the membrane cure before tiling, and before filling?
7. Which adhesive and grout will go over it, and are they compatible with it?

Put the answers into the quote itself. Our guide to warranties and written quotes covers what else to ask for.