Waterproof Membrane Defects and Leaking Showers are so Commmon, It Begs The Question, Why Do Membranes Fail so Often?
A new Australian Standard, 3740:2021 was released in 2021 in an apparent attemp to address to the widespread and growing problem of defects in waterproof membranes installed in building wet-areas such as bathrooms and laundries.
To understand why waterproof membranes fail, it is first important to understand how waterproof membranes are installed.
The short answer is that modern waterproof membranes are a ‘paint-on’ liquid material, similar to thick paint. Most are one-part polyurethane coatings that dry to become impervious to water penetration but remaining flexible enough to withstand minor movement of floor or wall elements. Commonly found in wet areas like bathrooms, laundries, toilets and behind basins. Other important areas for waterproofing in buildings include roofs, retaining walls, basement walls and planter boxes (for the purpose of this post we will focus on domestic bathrooms). \
Some waterproof membranes contain additives/fibres that improve the ‘strenght’ of the coating. Correct application processes are CRITICAL to achieve a membrane that will last the test of time.
The best way to understand membranes is with illustrations. A good drawing is worth a thousand words to a good builder. The illustrations help to explain the relationship between different building materials in relation to the waterproof membrane and determines the sequence of construction. The drawings also represent areas that are commonly defective.
Waterproof membranes are ‘invisible’ once they are installed, so it is most instructive to look at illustrations.
All waterproof membrane installation must be done by suitably qualified and licensed tradespeople. All waterproofing services should come with a limited warranty, usually 6yrs. If part of a larger renovation or new construction, the building certifying company will inspect the membrane before installing tiles. Whilst this kind of oversight is good, these kinds of visual inspections are unlikely to detect issues in a brand new structure. Furthermore, damage to the membrane is commonly caused by subsequent trades people.
Leaking shower recess causes damage under carpets
Membrane section – level shower floor
Leaking shower drain under floor, subfloor view
Membrane section – hobbed shower
Tile Bedding or Screed Goes Under the Floor Tiles to Create Gradient / Falls to Drain
Before we explain the membrane, it is important to understand how tiling is installed in modern bathrooms. Regardless of the type of structure (full bick and concrete or timber frames with sheeting) the tilers will install whats called a ‘Screed’ or layer of sand and cement topping on the floor in the shower and across the bathroom floor, stopping in the doorway. This layer is approximately 20-40mm thick and is intended to create ‘gradient’ or slope towards the floor drains within the bathroom. These gradients (and drawing details) actually form part of the construction standard (NCC) and the recent updates to the Australian Standard AS3740:2021. This screed is sometimes referred to as a ‘mortar’, ’tile bedding’ or simply ‘bed’. Its all the same thing and relates to getting the levels of floor tile correct. Once this screed has hardened the tiles are glued directly (sometimes with minor level adustments in the glue).
The Membrane Can Go ABOVE or BELOW the Tile Bedding, But Both is Best
This significance here is that the waterproof membrane can be installed ABOVE or BELOW the tile screed. The waterproofing code illustrates both options and leaves it up to the Builder. There are Pro’s and Con’s to both approaches, with both options together providing the most leak resiliant option long term, in our opinion.
Most common option – Where the waterproof membrane is installed first onto the floor substrate (before the tile bedding). This is usually moisure resistant chipboard timber sheet floors in 2 storey project homes. Because the floor substrate has no slope or gradient, (remember this comes with the tile bedding) there exists the possibility of water to ‘sit around’ under the tiles in the bedding. not fully draining away. The advantage of this sequence means the delicate membrane is protected during tiling (by the bedding on top). Membranes are delicate and can be easily damaged by careless tradespeople such as tilers, who usually have a couple of days work to install the tiles.
The other membrane sequence option is install the membrane over the hardened tile bedding/screed. This has the obvious advantage of gradient and sending water to surface drains. However, the membrane is more exposed and vulnerable to damage during tiling or potential future tile deterioration.
A good tiler can use a ‘waterproofing agent’ added to to the tile bedding for an additional layer of protection. The more the better.
Tile Bedding or Screed Goes Under the Floor Tiles to Create Gradient to Drain
Before we explain the membrane, it is important to understand how tiling is installed in modern bathrooms. Regardless of the type of structure (full bick and concrete or timber frames with sheeting) the tilers will install whats called a ‘Screed’ or layer of sand and cement topping on the floor in the shower and across the bathroom floor, stopping in the doorway. This layer is approximately 20-40mm thick and is intended to create ‘gradient’ or slope towards the floor drains within the bathroom. These gradients (and drawing details) actually form part of the construction standard (NCC) and the recent updates to the Australian Standard AS3740:2021. This screed is sometimes referred to as a ‘mortar’, ’tile bedding’ or simply ‘bed’. Its all the same thing and relates to getting the levels of floor tile correct. Once this screed has hardened the tiles are glued directly (sometimes with minor level adustments in the glue).
The Membrane Can Go ABOVE or BELOW the Tile Bedding, But Both is Best
This significance here is that the waterproof membrane can be installed ABOVE or BELOW the tile screed. The waterproofing code illustrates both options and leaves it up to the Builder. There are Pro’s and Con’s to both approaches, with both options together providing the most leak resiliant option long term, in our opinion.
Most common option – Where the waterproof membrane is installed first onto the floor substrate (before the tile bedding). This is usually moisure resistant chipboard timber sheet floors in 2 storey project homes. Because the floor substrate has no slope or gradient, (remember this comes with the tile bedding) there exists the possibility of water to ‘sit around’ under the tiles in the bedding. not fully draining away. The advantage of this sequence means the delicate membrane is protected during tiling (by the bedding on top). Membranes are delicate and can be easily damaged by careless tradespeople such as tilers, who usually have a couple of days work to install the tiles.
The other membrane sequence option is install the membrane over the hardened tile bedding/screed. This has the obvious advantage of gradient and sending water to surface drains. However, the membrane is more exposed and vulnerable to damage during tiling or potential future tile deterioration.
Detail of waterproof membrane below the tile bedding in a ‘step-down’ shower floor. This is most common type of installation as the membrane is protected from damage during tiling. One disadvantage relates to the flat floor, water has the opportunity to ‘sit-around’ under the bedding. Membranes with gradients will be more efficient at drainage.
Detail of how the waterproofing should continue fully into the drainage flange which connects to the waste pipe. The drainage pipe is a common location for membrane failure.
The above detail is an ‘accessability shower’ with no shower screen, hob or floor level change.
A 3D illustration of the ‘water stop’ floor angle required in the bathroom door threshold. This acts like a ‘water tank’ prevent water penetration into the surrounding elements.
