Water stained carpet in the hallway. Swollen and flaking bedroom wall. Musty odours near the bathroom. These are just some of the signs that a bathroom is leaking.

Potential Causes

Although the above and numerous more symptoms may indicate that a bathroom membrane system is defective or absent altogether, such symptoms are not exclusive to a leaking bathroom. There are a multitude of potential causes which need to be investigated and progressively eliminated. The cause may, for example, be related to the external cavity wall, or in the case of an apartment complex, be coming from above.

Even if a leak is undoubtedly coming from a bathroom, there are several potential causes of a leaking bathroom.

  • Leaking pipework
  • Failed waterproofing membrane
  • Leaking spindles/flick mixer
  • Inadequate bath lip detail
  • Failed grouting
  • Failed waterstop at entry door or at shower recess

So how could one determine if there is a shower leaking behind the wall, if waterproof membrane failure has occurred or if water is coming in through the roof and down a cavity wall, for example?


The first step to addressing a symptom or a number of symptoms is to accurately diagnose the source(s) of each symptom. In the instance of inadequate bathroom waterproofing, virtually all symptoms are moisture related. Accurate diagnosis usually requires systematic investigation and essentially a process of elimination.

The investigative process could involve a number of non-destructive or destructive techniques which may include:

  • Flood testing
  • Water simulation testing
  • Moisture detection, measurement and monitoring
  • Localised destructive investigations to expose construction detailing
  • Pipe pressure testing

In general, to accurately diagnose the source(s) of a symptom, each symptom should be investigated in isolation where possible. For example, if water damage is evident in a bedroom that is both situated next to a bathroom and has an exterior wall, the water damage could potentially be attributed to water ingress from the exterior of the building or could be due to a leaking bathroom or both. In such an instance, each possibility should be investigated in isolation.  

In some cases symptoms that are typically associated with failed or inadequate bathroom waterproof membrane may also be contributed to by moisture originating from outside the building, from a wet area on a storey above the habitable space of concern or a number of other origins. In order for the remediation to be successful, all leaks associated with the identified symptoms need to be adequately addressed, whether they are exclusive to the bathroom waterproofing or not.

Scope of Remedial Works

When compiling a scope of works, due consideration should be given to issues such as expectations upon completion of the works, the serviceable life of the bathroom (in context of the overall asset) and adding value to the finished product.

Remedial options may include only localised works sufficient to address the identified leaks. An example of such limited works could be localised sealing around tap spindles or the shower rose. In cases where the bathroom waterproof membrane is defective or non-existent, the solution may be to comprehensively (re)waterproof the bathroom.

Appropriate selection of a membrane system should take into account the layout of the bathroom, the substrate, drainage provisions, cost, building / substrate movement, compatibility with other products, the serviceable life of the bathroom, curing time restrictions, ease of application, acoustic requirements, bathroom finishes, bathroom use / access requirements (for example aged care or persons with mobility issues).

Selecting a bathroom waterproof membrane system should be undertaken on a case by case basis, and the above list of basic considerations is by no means exhaustive. Regardless of individual constraints / considerations, every bathroom membrane system should be:

  1. Designed with all relevant considerations in mind
  2. Correctly installed
  3. Capable of constant water saturation and exposure to household chemicals (eg. cleaning products, toiletries etc)
  4. Perform adequately for the serviceable life of the bathroom cognisant of anticipated structural movement
  5. Cared for / maintained throughout its serviceable life (to prevent fixture penetrations through tiles and underlying membrane, to assess integrity of membrane system in the instance of damage to tile floor etc)

It is important to remember that there is generally 4 lines of defence to water escaping from a bathroom:

  1. The first line of defence is adequate falls. Adequate falls allow water to escape easily and prevent saturation.
  2. Waterstops
  3. Tiles (considered by AS 3740 to be water-resistant)
  4. Waterproof membrane

Bathroom Waterproof Membrane Systems / Types

A number of waterproof membrane systems are available for bathrooms (and internal wet areas in general) and can be classified according to elongation at breaking stress into three generalised categories: rigid, flexible and elastomeric. Each membrane type could be further split into sub-types based on material composition, each with its own advantages and disadvantages. Alternatively, waterproof membranes can be broadly categorised as either sheet membranes or liquid applied membranes.

A number of proprietary bathroom waterproofing products / systems are available on the market today that are economical and can be installed without removing tiles. Such products are significantly cheaper to implement and do have their place on the market, and, like any other product or system, have their advantages, disadvantages and limitations. The specifier / designer should be aware of the limitations and expectations of any system and design / specify accordingly. Whilst manufacturers are constantly improving product performance through R & D, it is noted that there is no single product or type of membrane system that represents the best solution in all circumstances.

  • Sheet membrane systems: come in a wide array of sheet materials including PVC, polymer modified bitumen and rubber based products
  • Liquid applied membranes: examples include polyurethane, polyester, cementitious, bituminous, acrylic, chlorinated rubber, epoxies. They can comprise single component or two-part systems. They may include reinforcing mats that help increase the breaking stress of the membrane usually at the expense of decreased ductility (elongation at breaking stress).
  • Rigid / prefabricated shower tray: prefabricated shower trays are typically constructed of sanitary grade acrylic, stainless steel, copper and other materials that are usually pressed or moulded to form a single component construction that is highly resistant to leaks, mould and is coated with a slip resistant finish.

As described above, there are lots of steps involved after a suspect leak is identified. Firstly, a structured investigation process should be undertaken to determine the exact source(s) of the leak. There is a multitude of potential causes if you find a potential leak and a misdiagnosis can be very costly.

Once a diagnosis has been made, substantial consideration should be made to the design of a remedial solution. This remedial solution should not be a single approach in every case, but should be project-specific. A remedial design should account for such things as:

  • the building type and construction
  • age of the building
  • use case (e.g. residential vs. boarding house vs. commercial, etc.)
  • reactivity of founding soils
  • anticipated building movement, deflection and creep

Every building is unique, and thats why a one-size-fits-all approach when it comes to a repair strategy just won't do.