How to Waterproof a Bathroom, Stage by Stage

Waterproofing is the part of a bathroom nobody sees once it is finished, and the part that decides whether the room lasts. Tiles and grout are not waterproof on their own — water passes through grout lines and hairline cracks. What stops it reaching the frame, the slab or the ceiling below is a continuous membrane underneath the tiles, installed in the right order and given time to cure.

This guide walks through how a bathroom is waterproofed, stage by stage, so you understand what should be happening and what questions to ask. It is not a licence to skip the professional.

First, the rules in NSW

In New South Wales, waterproofing of wet areas is licensed work. It should be carried out by a waterproofer holding the appropriate licence, who issues a certificate once the job is complete. Across Australia, the National Construction Code sets the performance requirements for wet areas and references AS 3740, the Australian Standard for waterproofing domestic wet areas, which covers what must be treated and how.

If you are thinking about doing it yourself, check the rules first. Whether an owner can do their own waterproofing depends on the job and on owner-builder requirements, and uncertified work can create problems with home insurance, with a future sale, and with any warranty on the renovation. NSW Fair Trading or a licensed professional can tell you what applies to your situation. You can also check any tradesperson's licence on the NSW public register before you sign anything.

Which areas get waterproofed

A bathroom is divided into wet zones. The shower floor and walls get the most protection, because they take direct water every day. Areas around a bath, the rest of the floor and the junctions between floor and walls are treated as well, to degrees set by the standard. As one example, shower walls are typically treated to at least 1800 mm above the floor. Laundries and toilets have their own requirements.

Many renovators treat the entire bathroom floor and run the membrane up the walls at every junction, even where the minimum would allow less. It is good practice, particularly upstairs, on timber floors or in older homes where the structure moves more.

Stage 1: a sound substrate

Waterproofing is only as good as what it is bonded to. The walls need to be sheeted with materials suitable for wet areas, fixed at the right spacing, with joints set. Floors need to be flat, clean, dry and solid. On timber floors that often means fibre cement sheeting or a compressed sheet laid over the joists; on a slab it means removing old adhesive and grinding back high spots.

This is also when the falls are formed. The shower floor has to slope towards the waste so water drains rather than pooling. Falls are usually built with a sand and cement screed. Getting them right before the membrane goes on is far easier than trying to correct them with tile adhesive afterwards.

Stage 2: primer

Most liquid membranes need a primer applied to the substrate first. It seals porous surfaces, controls dust and helps the membrane bond. The primer has to be the type the membrane manufacturer specifies and has to dry before the next step.

Stage 3: bond breakers and flashing at every junction

Buildings move. Timber shrinks and swells, slabs shift slightly, and walls and floors move independently. Where two surfaces meet — the internal corners of the shower, the junction between wall and floor, around the bath — a rigid membrane bonded to both sides would tear.

A bond breaker stops that. It is a bead of flexible sealant, or a tape, run into the internal corner so the membrane can stretch across the joint rather than being locked to both sides. Reinforcing tape or fabric is often bedded into the first coat across these junctions for extra strength. This detail is where a lot of failed bathrooms went wrong.

Stage 4: puddle flanges and penetrations

Every floor waste gets a puddle flange — a collar that sits around the drain so the membrane can be dressed down into it and water that gets under the tiles still finds its way out. Linear drains have their own flange or channel detail.

Tap bodies, shower arms and any other pipes that pass through the membrane are sealed with purpose-made collars or sealant so the membrane stays continuous around them. A shower niche is also a break in the wall and needs every internal corner treated the same way as the rest of the shower.

Stage 5: the membrane coats

The membrane is then applied, usually by brush or roller, in at least the number of coats the manufacturer specifies and to the stated thickness. A second coat is often applied at right angles to the first to avoid thin spots. Sheet membranes are another option in some situations and have their own lapping and sealing details.

What you want to see is an even, unbroken coat with no pinholes, bubbles or bare patches, taken up the walls to the required height and over the upturns at the doorway.

Stage 6: curing time

Every membrane has a drying time between coats and a curing time before it can be tiled or flood tested. That time depends on the product, the thickness applied, and the temperature and humidity on the day. Cold, damp weather slows it down.

This is the stage most likely to be rushed on a tight program. Tiling over a membrane that has not cured can trap moisture, damage the film and void the product's warranty. A good program allows for it.

Stage 7: inspection, flood test and certificate

Before tiling, the membrane should be inspected. Some waterproofers carry out a flood test — blocking the waste and holding water in the shower area for a period to check for leaks. It is not required on every job, but it is a sensible check, especially upstairs.

The licensed waterproofer then issues a certificate confirming the work was done to the standard. Keep it. It is the document insurers, certifiers and future buyers will ask for.

Stage 8: protecting it during tiling

The membrane is only protected once it is covered. Tilers should use an adhesive compatible with the membrane, avoid dragging tools or ladders across it, and never fix anything through it. Internal corners and changes of plane are finished with flexible sealant, not grout, so the movement joint below carries through to the surface.

Signs waterproofing has failed

In an existing bathroom, these are the usual warnings:

• Swollen skirting boards or door frames just outside the bathroom.
• Stains, bubbling paint or mould on the ceiling below an upstairs bathroom.
• Loose or drummy floor tiles.
• Grout that cracks in the same place however often it is repaired.
• A musty smell or mould that returns after cleaning.

A failed membrane generally cannot be fixed from above the tiles. Sealing grout or re-siliconing can slow a leak, but the lasting fix is to strip the area back and waterproof it properly.

Questions to ask before work starts

• Who is doing the waterproofing, and what licence do they hold?
• Which areas will be treated, and to what heights?
• How is each junction, waste and penetration detailed?
• How long will the membrane cure before tiling?
• Will you issue a certificate, and will there be a flood test?

Getting it done properly

If you are renovating, waterproofing should be a named line in your quote, done by a licensed waterproofer and certified. Our waterproofing and tiling page explains how we handle it on every job, and a full bathroom renovation includes it as standard. For what a renovation involves in money terms, see our bathroom renovation cost guide. If you are in the Sutherland Shire, we can measure your bathroom and give you a fixed written quote.