How a Shower Is Waterproofed, Step by Step

This is what actually happens when a shower is waterproofed properly. It is worth knowing, because it lets you tell whether a quote has allowed enough time for the work.

1. Substrate preparation

Everything starts here. The surface has to be sound, clean, dry and free of anything that will stop the membrane bonding.

Wall sheeting in a shower must be a suitable wet-area board, fixed and set correctly. Water-resistant is not the same as waterproof, and the sheeting is not the waterproofing. It is the thing the waterproofing goes on.

Any dust, old adhesive, loose screed or contamination gets removed. If the substrate is not right, nothing applied over it will be.

2. Falls to the waste

The shower floor has to fall to the waste so water leaves rather than sits. This is done in the screed before waterproofing.

A floor that ponds is a slow failure. Standing water will find the weakest point in any membrane eventually, and it will find it at the same spot every day for years.

3. Priming

The primer is what the membrane bonds to. Skipping it, or using the wrong one for the substrate, is a direct cause of adhesion failure.

When a membrane loses adhesion, water that gets past the grout travels underneath it and spreads. That is worse than no membrane, because the leak appears somewhere unrelated to where the water came in.

4. Bond breakers and reinforcement

Internal corners and the wall-to-floor junction get a bond breaker or reinforcing fabric before the membrane goes on.

The reason is movement. Buildings move, timber floors more than concrete. At a corner, a membrane bonded rigidly to both surfaces has nowhere to go and tears. A bond breaker lets it stretch across the junction instead.

This step is invisible in the finished bathroom and it is one of the first things dropped when someone is rushing.

5. Detailing the penetrations

Every hole through the membrane gets treated as a potential leak, because that is what it is. The floor waste, tap and mixer penetrations, and any fixing drilled into a wet wall.

Wastes are usually dressed with a collar or flange bedded into the membrane so there is no open edge around the drain.

6. The membrane coats

Multiple coats, in the direction and thickness the manufacturer specifies, with drying time between each.

The specified dry film thickness is reached across coats, not in one heavy pass. A single thick application skins over on the surface and stays uncured underneath. The differences between membrane systems are covered here.

7. The cure

Then nothing happens for a day or more while the membrane cures.

This is not padding in the schedule. A membrane that has not cured has not reached full adhesion, strength or elasticity, and tiling over it traps everything underneath. How long it takes and what changes it is worth reading if you are comparing quotes.

8. Flood test

Where practical, the waste is blocked and the base filled with water to confirm the membrane holds.

It is the only opportunity to find a problem while fixing it costs an afternoon rather than a full strip-out.

9. Tiling

Only now. With a compatible adhesive, because membrane, primer and adhesive have to work as a system.

Where it goes wrong

Almost every failure we strip out traces to steps 3, 4, 6 or 7: no primer, no reinforcement at the corners, not enough coats, or tiling before the cure. All four are invisible once the tiles are on and all four are caused by time pressure rather than lack of knowledge.

The requirements behind all of this are in AS 3740. If you would rather it was done to that standard the first time, this is what we do.