Bathroom Waterproofing: Why Tanking Is Non-Negotiable

What Tanking Is

Tanking is the application of a waterproof membrane — a continuous barrier material — to the floor and walls of a wet room or bathroom before tiles are laid. The membrane sits between the building substrate (plasterboard, concrete screed, masonry, or timber) and the tile adhesive bed, creating a waterproof layer that prevents water from reaching the structure behind and beneath it.

The concept is straightforward: tiles and grout are water resistant, not waterproof. In normal use, water will eventually penetrate grout joints — particularly as grout ages, cracks, or is cleaned with abrasive products. Silicone joints at the base of shower enclosures and along bath edges fail over time as the silicone deteriorates. If the substrate behind the tiles is not waterproofed, this water penetrates into the wall structure and begins to cause damage that may not be visible for months or years.

Tanking eliminates this risk by ensuring that any water that penetrates the tile and grout layer encounters a continuous waterproof barrier that returns it to the room surface (where it drains) rather than allowing it to enter the building fabric.

Why It Matters in London Flats

In a London terraced house, a bathroom leak behind tiles is serious but at least self-contained — the damage is to your own property. In a London flat — and the majority of London's housing stock is flats and conversions — a bathroom leak from the first floor means water penetrating into the structure of the building and appearing in the ceiling of the flat below.

The consequences are disproportionate: a small, slow leak behind an un-tanked shower in a first-floor flat can destroy the plasterwork and ceiling of the ground-floor flat below over several months. By the time it is discovered, the damage may be £3,000–£8,000 to remediate — including stripping the bathroom above to trace and repair the source, replastering ceilings below, redecoration, and potential damage to contents. In a leasehold block, this creates disputes between leaseholders, insurance claims, potential litigation, and a severely deteriorated relationship between neighbours.

For London flat owners undertaking any bathroom renovation, tanking is not a premium addition — it is basic protection against a disproportionately costly risk.

Tanking Materials: What Is Used

Slurry-Type Tanking Membrane

A cement-based waterproof slurry — products such as Ardex, Mapei, or BAL waterproof membranes — is applied by brush or roller to the prepared surface in two coats. The material is similar in application to a thick paint; once cured it creates a rigid, waterproof cement-based layer bonded to the substrate. Slurry tanking is suitable for walls and floors, bonds well to tile adhesive applied over it, and is the most common professional tanking method for bathroom renovations in the UK.

Application requires the substrate to be clean, structurally sound, and free from loose material. Existing tile adhesive residue must be removed. New plasterboard must be taped at joints and primed. The slurry is applied, allowed to cure (typically four to six hours between coats), and the second coat applied at right angles to the first to ensure coverage of any pinholes.

Sheet Membrane (Self-Adhesive or Fleece-Bonded)

Sheet membranes — such as Schluter Kerdi, Laticrete Hydro Ban Sheet, or Wedi board — are applied as panels bonded to the substrate using a polymer-modified tile adhesive or purpose-made adhesive. Sheet membranes are particularly effective in corners and transitions, where a slurry membrane requires careful application and fabric tape to reinforce the joint. Some sheet membrane systems (notably Wedi) are structural panels that provide both waterproofing and substrate in one product — they replace plasterboard and tanking as a combined element.

Sheet membranes are faster to install than slurry systems (no waiting for multiple coats to cure) and provide reliable, consistent waterproofing at joints and corners. They are more expensive in material cost than slurry systems but reduce labour time.

Critical Areas That Must Be Tanked

Not every wall in a bathroom necessarily requires full tanking — the requirement relates to areas that are subject to direct water contact. The following areas must be tanked without exception:

• Shower enclosure — full height: The entire shower enclosure from floor to ceiling must be tanked. Not halfway up the wall, not to the top of the tiles — full height. Shower spray creates high-humidity conditions throughout the enclosure, and water vapour condenses on the upper walls and ceiling where it is outside the direct spray zone but still exposed to moisture.
• Bathroom floor: The entire floor area — not just the shower tray area — should be tanked. In a wet room design (level access shower with no tray), full floor tanking is essential. In a bathroom with a shower tray, full floor tanking provides insurance against tray overflow and general bathroom splash.
• 300mm above the bath rim: The wall area from bath-rim height up to 300mm above it is in the splash zone and receives direct water contact. This zone must be tanked on all walls adjacent to the bath. Some contractors tank full height on all walls as a standard practice.
• Corner joints: Corner joints between walls, and between walls and floor, are the highest-risk points in any tanking system. These joints must be reinforced with fabric tape embedded in the slurry, or with appropriate corner membrane strips for sheet systems. A tanked wall that meets an un-reinforced corner joint has a failure point at every corner.

What Happens Without Tanking

The failure mode of an un-tanked bathroom is predictable and follows a consistent pattern:

1. Grout in the shower enclosure begins to crack or deteriorate, typically within two to five years of installation depending on cleaning practices and water hardness (London's very hard water accelerates grout degradation).
2. The silicone bead at the shower tray perimeter begins to lift or crack, creating gaps that allow water penetration.
3. Water penetrates behind tiles and into the plasterboard or masonry substrate. Plasterboard saturates rapidly — it is not a moisture-resistant material in prolonged contact with water despite being labelled "moisture-resistant".
4. Tiles begin to lose adhesion — the wet adhesive bond fails progressively. Tile drop-off follows.
5. Mould appears — first on the grout surface, then behind tiles as the saturated substrate supports mould growth.
6. In a flat, water tracking through the saturated structure appears in the ceiling of the flat below.

Re-tiling an un-tanked bathroom that has reached this state requires stripping all tiles, removing saturated or damaged plasterboard, treating mould, drying the substrate (potentially with dehumidifiers), and starting from scratch. The cost is higher than a properly tanked installation would have been from the outset.

Choosing a Contractor: What to Confirm

When appointing a tiler or bathroom contractor for a renovation that includes a wet area:

• Ask specifically which tanking product they use and ask to see the product data sheet or packaging. "I use a waterproof membrane" is not sufficient — confirm the specific product and that it is a certified bathroom tanking product, not a painted sealant or general damp-proof compound.
• Confirm that the shower enclosure will be tanked to full ceiling height, not to the top of the tiles.
• Confirm that corner joints will be reinforced with fabric tape or corner strips, not just covered by the slurry alone.
• Ask whether tanking is included in the quote or is an additional cost — some bathroom contractors quote for tiling and treat tanking as a separate item. Confirm the full scope in writing.

A contractor who is reluctant to discuss their tanking approach in detail, or who dismisses the importance of tanking, should be treated with caution. Tanking is an established, well-documented installation requirement — any competent bathroom tiler should discuss it confidently.

Cost

Tanking materials for a standard bathroom shower enclosure and floor: approximately £80–£150 in materials (slurry membrane system). Labour to apply the tanking as part of a bathroom renovation: approximately £150–£250 additional to the tiling cost. Some contractors include tanking in their standard tiling quote; others quote it separately. Total additional cost of proper tanking over un-tanked installation: £200–£400 — a small premium relative to the cost of remedying a tanking failure in a London flat, which can run to several thousand pounds and significant neighbour dispute.