Bathroom and Shower Waterproofing Guide

Which bathroom surfaces are waterproofed and to what height, how corners, pipes and drains are detailed, and the test to run before tiling.

A bathroom leak rarely shows itself immediately. Water seeps slowly into the structure through grout lines, the edge of a silicone bead or a pipe penetration, and by the time a stain appears on the neighbour's ceiling downstairs, the problem has been going on for months. Bathroom waterproofing is a thin layer hidden beneath the tiles, but the life of the whole room depends on it.

Tiles do not leak — joints do

Ceramic or porcelain tiles are water-resistant in themselves, but the grout between them is porous and develops micro-cracks over time. Silicone joints also age over the years. So treating the tiles as the waterproofing is a mistake: tiles are the finish, and the layer that holds back water lies beneath them.

Bathrooms mostly use liquid-applied waterproofing: polymer-cement slurries or elastic liquid membranes designed for use under tiles. Some systems use thin sheet membranes. The key condition is compatibility with the tile adhesive — this is stated in the manufacturer's system description.

The substrate matters too. If a plasterboard partition or boxing is built in a bathroom, moisture-resistant board or cement-based board is chosen, but even these do not replace waterproofing — they simply tolerate damp better. In older buildings, the floor base is often uneven and weak after demolition; it is levelled with a screed and cracks are repaired before waterproofing. The best membrane laid on a weak base will crack along with it.

Floors: the whole area and the turn-up

The bathroom floor is waterproofed across its entire area. Treating only the area under the bath or shower is not enough, because water splashes onto the floor, gets spilled during cleaning, and sometimes a washing machine or connector hose leaks.

The floor waterproofing is also turned up onto the walls so water cannot enter the structure along the corner. Outside the shower zone this upstand is typically around 15–20 cm, but the actual figure is set by the system used and the design requirements. The waterproofing is not interrupted at the doorway; the bathroom floor level and falls are set so water cannot escape into the corridor.

Walls: the splash zones

Waterproofing every wall surface is not always necessary, but areas where water falls directly must be covered:

the walls of a shower enclosure or walk-in shower — at least up to splash height, usually above the level of the shower head;
walls around the bath — above the bath rim, and higher if a hand shower is used;
behind the washbasin — across the splash zone;
plasterboard partitions and boxings — with particular care, as they sit in a wet zone.

If the room has a steam cabin or a heavily used rain shower, it makes sense to treat the walls to full height, sometimes up to the ceiling.

Corners, drains and pipe penetrations

The weak points of waterproofing are not flat surfaces but junctions. Special elastic tape is bonded into wall-floor and wall-wall corners; it stops the layer tearing with small movements of the building. Prefabricated collars are used where mixer, shower head and drainage pipes pass through.

The shower drain or linear drain needs particular attention. Models with a clamping flange or a dedicated collar are chosen so that any water that seeps around the drain is still led into it. In a level-access shower, the floor fall towards the drain must be set accurately at the screed stage; the waterproofing is applied onto that sloped surface.

If a bath is installed, the joint between bath and wall must not be left to silicone alone. The waterproofing is carried down to below the bath rim, and an elastic sealing tape or profile is fitted over the joint.

Application and testing

For a sound result the sequence is:

the substrate must be dry, sound, dust-free and primed;
corner tapes and collars go in first;
the waterproofing is applied in the number of coats the manufacturer specifies, each coat after the previous one has dried;
tiling does not start until the coats have fully cured.

Before any tiles go down, it pays to flood-test the floor: the drain is temporarily plugged, a small amount of water is poured onto the floor and left for a period, then the ceiling below and adjoining walls are checked for damp. This simple check catches a hidden defect at the cheapest possible stage. The test procedure should follow the requirements of the system used.