What is tanking and do I need it behind bathroom tiles?

Almost every failed shower wall is the same story: water passes slowly through the grout, soaks the board behind, and years later the tiles come off in sheets. Tanking exists to interrupt that path, and it costs a fraction of what putting it right costs.

What you'll need

• Tanking kit — primer, liquid membrane or sheet, and reinforcing tape
• A brush or roller
• Time to let each coat cure

Step by step

1. Understand what tiles actually do.Glazed tile is impervious, but grout is not, and neither is the adhesive bed. Water passes slowly through the joints. On a bathroom wall that dries out; in a shower it never does.
2. Decide where you actually need it.Inside the shower enclosure and for at least a couple of hundred millimetres beyond, and around a bath used as a shower. A splashback behind a basin generally does not need it.
3. Let the substrate decide the urgency.On plasterboard or timber-based board, tanking is not optional in a wet area. On dense cement backer board it is still recommended. On solid render it is prudent.
4. Prime first, always.Every system starts with a primer suited to the board. Skipping it is the commonest cause of a membrane peeling away later.
5. Reinforce every corner and junction.Bed the system's tape into the first coat at internal corners, along the tray or bath junction, and around pipe penetrations. Corners move and are where failures start.
6. Seal around pipes and fixings.Every hole through the membrane is a potential path. Use the kit's pipe collars, or dress the tape carefully around each penetration.
7. Apply the stated number of coats.Liquid membranes are usually two coats at right angles, with a curing interval. One thick coat is not equivalent and often stays soft underneath.
8. Let it cure before tiling.Follow the kit — usually several hours to overnight. Tiling onto uncured membrane pulls it off the wall.
9. Keep the sealant joints as part of the system.The membrane stops water in the wall; the silicone at the tray stops it getting behind the tile edge in the first place. Both matter.
10. Know what you cannot retrofit.Tanking goes on before tiling. On an existing tiled shower you cannot add it without stripping the tiles, so the practical answer there is keeping the grout and sealant sound.

Tips

• Tanking a shower area costs a small fraction of retiling it. It is the cheapest insurance in a bathroom refit.
• Buy a complete system — primer, membrane and tape from one manufacturer. Mixing brands voids the compatibility they have tested.
• Photograph the tanked wall before tiling. It is worth having evidence it was done.

Common mistakes

• Tiling straight onto plasterboard in a shower — Plasterboard has a paper face and a gypsum core, both of which soften when wet. Once damp, it loses the ability to hold the tile weight and the wall fails.
• Skipping the reinforcing tape at corners — Corners are where movement concentrates. A liquid membrane alone cracks there, and a crack in the membrane makes the rest of it decorative.

If it doesn't work

Questions people ask