Shower Membrane Waterproofing – The Definitive Guide for DIY’ers & Pros
The MOST IMPORTANT step in any shower remodel is waterproofing with a shower membrane.
In this post I, Steve of SKG Renovations (Steve’s Bio), will be using my extensive knowledge and experience in bathroom and shower remodelling to give you the whole story about shower membrane waterproofing.
I will explain the different types of shower waterproofing methods available, which ones I prefer and why.
Brief History of Shower Waterproofing
You might be wondering, “Is shower membrane waterproofing a new thing?” and “Why did the industry decide to abandon the old-school strategies?”.
Well those are good questions. And the answers to those questions provides a good background for this post about waterproofing products and strategies.
Shower waterproofing is not really a new thing. In the past (15+ years ago) it was assumed that the shower surround was relatively impermeable to water, and the shower base was the only place where any waterproofing was installed.
Older homes also were a lot more forgiving when it came to water penetration in and around the shower assembly.
Older homes were quite drafty with significant air leakage between the inside and exterior of home. Moisture that leaked out of the shower stall could often evaporate under these conditions.
This resulted in a far less comfortable home that was expensive to heat, but it also lessened the danger of mold growth, dry rot, and toxic air being trapped in your home.
Although adequate air movement helped a little, ultimately the problems could never be ignored because of the mistaken assumption that the tiled walls in a shower are essentially waterproof.
This was the flaw that ultimately caused all showers built in that era to fail regardless of the quality of the installation. Ceramic tile usually failed faster than porcelain because of higher absorption rates through the tile, but they all eventually failed.
Old School Shower Pan Liner
Because the shower floor is a horizontal surface always immersed in water, it is a very intuitive place to install some measures to prevent water from getting underneath and rotting your floor framing.
In the past, these measures usually involved the installation of a rubber or copper shower pan liner (or membrane) below a traditional mortar base to protect the subfloor beneath. This was considered a water in / water out system.
And this method is still used today by installers that subscribe to the “if it ain’t broke, no need to fix it” school of thought.
If installed correctly, this membrane did exactly what it was supposed to do; keep water from getting into the subfloor and wall base framing.
The only problem was that this method assumes that water penetration through and around the mortar base was acceptable as long as it eventually made its way towards the weep holes at the base of the drain. And I can tell you from personal experience that this assumption was not at all acceptable.
Since tile, grout and mortar was never designed to be waterproof, a significant amount of moisture always made it into and around the mortar base, pooling at the liner.
Even with the proper pre-slope below the liner, much of the water adhered to the porous mortar base so it always stayed significantly hydrated.
When this happens, the mortar base breaks down and mold starts to grow. I have seen this in countless demolitions, even if the subfloor had the proper slope and the weep holes were not blocked.
The result was always a rank, toxic smell from the bi-products of rubber and cement breaking down, and mold….. lots of mold. A seriously flawed design in my opinion.
In other words, It was a bad idea before, and it’s still a bad idea today.
Shower Wall Waterproofing
Before the waterproofing membrane systems were developed, there wasn’t really any consistently applied waterproofing strategies for the shower walls except for the shower pan liner which usually ran up the wall only about 1′ above the shower base.
Most building codes also required that a vapour retarder membrane (plastic sheet or building felt) be installed between the studs and the backer board on outside facing walls, to limit moisture transfer into the framing.
This code rule is still in place today in most parts of the world but it has always had serious limitations such as inconsistent coverage and varying degrees of moisture permeability.
The new problem is that this old system is incompatible with new shower waterproofing membrane systems and can cause major problems with condensation in your outside shower wall.
These old shower wall waterproofing methods were seriously flawed and needed to be replaced with the new membrane systems, not supplemented by them.
Shower nooks and niches – Most problematic
It is easy to see how shower nooks and niches could be the cause of major water infiltration issues in the showers of the past, when they still cause problems in some shower today.
A hole in the shower wall was always a risky undertaking and almost always resulted in water running into the shower wall no matter how careful the contractor was when building these problematic shower boxes.
There were no preformed ready to install tileable foam shower niches back then, and no shower waterproofing membranes, let alone quick to install finished shower niches. All these wall nooks, niches and soap trays had to be custom built by the framer or tiling contractor.
They had to use the materials available at the time, so wood studs framed it, tar paper “waterproofed” it, gypsum wall board lined it, and tiles were laid over it. At the time, tile and grout were considered to be relatively impermeable to water so it was not considered to be a problem.
This was a grossly mistaken assumption, but fairly common during that period. Needless to say, it didn’t take long for these assemblies to rot, causing the tiles to break off and water to get into the wall framing.
Fixing the Leaking Shower
The new shower waterproofing membrane technology has basically solved all of the problems of the past by placing the waterproofing layer directly beneath the tile layer and combining it with decoupling.
These technologies have truly revolutionized the entire tiling industry, including the way that tiled showers are constructed and waterproofed.
There are basically four main types of shower wall and floor membranes:
Sheet and liquid membranes, as well as the newer foam wall boards, foam shower bases and foam shower niches (in the foam wall boards category).
Each of these products have some similarities and some important variations that I will go over in detail.
Sheet membranes are also different from liquid waterproofing membranes in that they can also serve as uncoupling membranes. Before we get into the details of each type of waterproofing membrane, I will try to explain what uncoupling is all about.
Decoupling / Uncoupling Membrane Mystery
One of the additional benefits of the sheet style waterproofing shower membranes is that they can also serve as uncoupling membranes.
Not many people understand the term, or the purpose of uncoupling the tile assembly from the backer board and I can’t really blame them. There are some complex physics involved in these systems and frankly most tiling professionals don’t really have a clue either.
I will attempt to explain the basics of uncoupling in the following sections.
Crack Isolation Membranes
The explanation of crack isolation membranes goes a long way to also explain the advantage of the uncoupling membrane in the shower:
Most of you have probably seen tiled floors that have cracked along their grout lines, sometimes quite dramatically. This is often the result of a crack in the concrete floor beneath the tile.
When the tile is firmly attached to a concrete subfloor, the crack that forms in the subfloor will crack the tile assembly attached to it as well, usually separating along the grout lines. This horizontal movement can also cause tile delamination around the crack.
A crack isolation membrane is basically just a thicker and heftier version of a waterproofing membrane that can stretch horizontally sometimes as much as 1/8″.
This type of membrane can reduce the chance of grout line cracking in the example above, by floating the tile assembly (tile, grout, mortar) on the surface of the membrane so it can stretch horizontally as the floor cracks, instead of the tile assembly being pulled apart (explained more thoroughly in the next section).
Uncoupling Membranes in the Shower
Because the shower floor and shower walls are exposed to lots of moisture and wide temperature changes, this is also a place where grout line cracking can occur.
This can happen in the same way as the floor example above but to a much lesser degree. Without a uncoupling membrane, the bottom backer board layer can expand and contract at a different rate than the top tile layer, causing grout line cracking.
This movement can occur at the margins between two adjoining backer board panels, or most often in the corners where the wall meets the floor, and where the back wall meets the side walls.
Uncoupling can solve this problem by essentially disconnecting the top tile layer from the bottom backer board layer.
Uncoupling is accomplished by the mortar attaching not to the membrane sheet itself, but only to the fleece fibers (tiny plastic hairs) on each side of the sheet. This creates a tiny gap at the base of the fleece on each side of the membrane sheet.
This tiny gap allows the assembly on top side of the membrane to move a little. Not much, but it’s enough to allow the tile, grout and mortar to essentially float over the backer board while still able to carry a significant load.
Genius, right? Like I said, I wish I would have invented it!