Investigating the Causes of a Leaking Bathroom

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.

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:

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.

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:

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

What screed bed thickness is required by Australian Standards?

AS 3958.1 Ceramic tiles, Part 1: Guide to the installation of ceramic tiles stipulates a minimum screed thicknesses of 15 mm for bonded screeds and 40 mm for unbonded screeds. However, AS 4654.2 Waterproofing membranes for external above-ground use, Part 2: Design and installation states that an unbonded screed should be of a minimum thickness of 50 mm with lightweight mesh reinforcing.

By comparison, BS 8204 stipulates 25 mm for bonded screeds and 50 mm for unbonded screeds.

These thicknesses should be used as guide only. Many factors are required to be considered in order to determine the most appropriate screed/tile bed thickness. Each project is going to demand a unique screed thickness to ensure suitability, performance and durability based on the application, function and service conditions of the screed.

What is the fall requirement for a balcony?

AS 4654.2 Waterproofing Membranes for External Above-Ground Use stipulates a minimum fall requirement of 1:100.

What is the fall requirement for a bathroom floor?

AS 3740 Waterproofing of Domestic Wet Areas stipulates a minimum fall of 1:100 for general bathroom floor areas. Additionally, the floor shall allow all surface water to drain without ponding except residual water remaining due to surface tension.

What is the Australian Standard fall requirement for a shower floor?

AS 3740 Waterproofing of Domestic Wet Areas stipulates that where vertical separation (e.g. shower screen, hob, step-down, water stop) is provided, the fall to the floor waste is to be 1:100.

Where no vertical separation is provided, the fall shall be a minimum of 1:80.

How do I calculate finished floor levels with falls?

  1. First work out what the minimum thickness of the screed bed needs to be.
  2. Next check that the base slab is level by using a laser level. If the slab IS level, skip the next step.
  3. If the slab is not level, use a laser level to find the difference in height of the drainage outlet and the floor at each perimeter wall.
  4. Measure the distance from the outlet to the furthest wall.
  5. Work out what fall you are trying to achieve. 1:100 is the minimum required by AS 3740 for bathroom floors outside the shower, and 1:100 is also required by AS 4654.2 for external balcony decks.
  6. Use the following formula to calculate the height of the finished floor level (H) at the walls. Once calculated, measure up the wall and mark the height.

H = L x r – d + s + t


H = height of finished floor level at walls
L = length distance from outlet to wall.
r = ratio required to be achieved
d = difference in height of slab at wall relative to at outlet.
s = minimum thickness of screed bed
t = thickness of tile & adhesive

Example as follows:
Let’s say the distance from outlet to wall is 6 m, convert this to mm, L = 6000 mm.
r = 1:100
Let’s say the base slab at the wall is 5 mm lower than at the drainage outlet, as determined used a laser level so d = -5
s = 60 mm
t = 8 mm

H = (6000) x (1/100) – (-5) + 60 + 8

H = 133 mm

What vertical termination detail for a waterproofing membrane is required by Australian Standards?

AS 4654.2 provides 3 options for vertical termination of a membrane:

  1. Chase a 15 mm deep reglet into the wall and turn the membrane inside. Insert an ‘over-flashing’ embedded in silicone to cover the top of the membrane (liquid and fully bonded sheet membranes. The bottom of the flashing must be at least 10 mm from finished floor level.
  2. Use a pressure seal and sealant (sheet membranes. The bottom of the pressure seal must be at least 10 mm from finished floor level.
  3. Terminate under overhang of cavity flashing (any membrane). The bottom of the cavity flashing must be at least 10 mm from finished floor level.

In situations with a parapet wall, the membrane is required to be applied up, over and down the other side of the parapet wall. See also, What is the Australian Standard requirement for vertical termination height of external waterproofing?

What vertical termination height for a waterproofing membrane is required by Australian Standards?

AS 4654.2 stipulates the requirement for upward vertical termination height of an external waterproofing membrane. The height requirement ranges from 40 mm to 180 mm depending on the building’s wind class. Note that this is the height required above finished floor level.

Wind Class
Regions A & B (non-cyclonic)
AS 4055
Wind class
Regions C & D (cyclonic)
AS 4055
Ultimate limit state (Vh,u)
AS/NZS 1170.2
Termination Height (mm)
N1 - 34 40
N2 - 40 50
N3 C1 50 70
N4 C2 61 100
N5 C3 74 150
N6 C4 86 180

Remember the quoted termination height is the height above finished floor level.

A building’s wind classification (AS 4055) can be calculated in accordance with AS 1170.2 which sets out 4 primary factors for determining a building’s wind class determination:

  1. Geographic wind speed region
  2. Terrain category
  3. Topography
  4. Shielding

A guide to calculating wind class can be found here. Calculations should be undertaken by an engineer familiar with AS 1170.2.

If you are going to have falls in your finished floor, then you will need to work out the required falls and calculate the finished floor levels prior to working out where your membrane will terminate.

How long does concrete need to cure before waterproofing?

Traditionally, a concrete slab would need to cure for about 28 days before waterproofing application can commence. This allows time for the concrete to dry and dispel water vapour and any excess bleed water which can compromise the bond of the membrane to the substrate and cause membrane blistering/delamination.

However, the curing time for concrete before waterproofing membrane application depends on the waterproofing product being used and varies from 24 hours to 28 days.

Many membranes on the market today can be applied after just 7 days of curing as they are designed to tolerate damp substrates and allow water vapour diffusion to dispel water vapour from the concrete which continues to cure.

Further to this, there are special-purpose moisture barriers (usually epoxies such as ARDEX’s WPM 300 HydrEpoxy) which are suitable for very damp substrates and can be applied 24 hours after concrete placement. These epoxy products are usually designed to accommodate over-coating with most waterproofing and decorative coating to allow waterproofing to continue over the freshly poured concrete.

This is a game-changer for expediting the waterproofing process and reducing construction time along the critical path.

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