Waterproof Shower Walls Before Tiling

Do I need to waterproof shower walls before tiling?

Tile is, for the most part – waterproof. Natural stones no, but porcelain tiles, for the most part, will not absorb water. Ceramic tiles installed on the walls will shed most water. Grout, on the other hand, sucks. Literally. Grout is sand that is glued together with Portland cement (with some exceptions – epoxy grout, non-sanded grout, etc..). Therefore it acts like sand and absorbs water as it flows across the surface of the grout. This means that when building your shower, you have to consider what the water will do when it passes through your grout and onto the wall behind.

The standard shower is one of the wettest places on earth with the equivalent of 2,500 inches of rainfall a year. This means that properly waterproofing a shower is not just a recommendation; it is essential in protecting your home or commercial structure from the effects of water. Excessive water migration into wall cavities can cause expensive structural damage and invite mould growth which leads to illness and an unsafe environment.

Before even starting the process of waterproofing, it is important to prepare the substrate. First, make sure that the foundation is solid. The floor and frame must be built to industry standards. Then carefully plan drain placement to ensure proper water removal. Besides, check for level and pitch to create adequate drainage.

Contrary to popular belief, ceramic tile and grout, by themselves are not waterproof. Water can penetrate through cement-based grout and work its way through the substrate. To prevent water damage, you must install a waterproof membrane just below the tile bonding mortar as close to the tile as possible. There are two types of waterproof membranes, sheet and liquid applied options.

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Sheet Membranes

Sheet membranes are plastic or elastomeric sheets that have to be bonded to the substrate before tile installation. Some sheets have an adhesive backing and others require separate adhesives or cement mortar. Pressure-sensitive adhesive-backed membranes almost always require a primer be applied to the substrate. The primer may take an hour or more to dry. Other membranes must be bonded using a cement-based thin-set mortar which also takes time to cure.

When installing a sheet membrane, it is important to remember to treat and seal the seams where the sheets come together to maintain waterproofing. Pay close attention to all transitions and changes of the plane to create a continuous waterproof membrane. Prefabricated sheet membranes are typically not as fast and easy to install as advertised. All components (membrane, preformed corners, bonding mortar, primer and sealant) must be gathered before installation can begin and the sheets must be cut. Sheet membranes are also more costly and require more labour and time.

Waterproof Shower Walls Before Tiling 1

Liquid Membranes

Liquid membranes eliminate much of the hassle of installing prefabricated sheet membranes. There is no need for primers, no seams to treat and no worries about transitions up walls and through corners. RedGard Waterproofing and Crack Prevention Membrane is a ready-to-use, liquid-applied elastomeric membrane that creates a continuous waterproofing barrier with outstanding adhesion. It can also be used to effectively vapour-proof steam rooms and gang showers. RedGard can be applied by brushing, troweling, rolling and spraying. To assure a continuous film with no voids on the floor, apply a second coat over the first. Embed a thin fibreglass mesh into the membrane for changes of plane and gaps 1/8″ or greater for the best performance.

No Waterproofing: Will this cause shower leaks?

Shower leaks.

The last thing anyone wants when they build a new shower. And yet I hear the following questions quite often:

  • Is cement board waterproof?
  • The tile and grout will keep the most of the water out.
  • My contractor didn’t put any waterproofing in our shower. Will this cause shower leaks?
  • We didn’t know we were supposed to have a shower moisture barrier. Should we start over?

What inspired this post was that recently, on the John Bridge forum, a member had asked a simple enough question: Is a moisture barrier required? He had hired a contractor to install his shower, and the contractor said that it wasn’t necessary. But as more information came out-and photos were posted- it was obvious that there were going to be some shower leaks. I would encourage you to click the link and skim over the post.

This might be news to some but let’s get a couple of things out in the open:

Cement board is not waterproof. Hardibacker is not waterproof. Tile and grout are not waterproof.

Cement board will not fall apart when exposed to water. But like a concrete walkway, it can soak up water. It can also dry out and be perfectly fine.

What is a moisture barrier?

A moisture barrier is essentially a waterproof barrier. Its purpose is to keep moisture from getting through the backer board and into the wall. Acceptable forms of waterproof barriers for a shower are:

  • Four mil plastic sheet behind the tile backer board
  • liquid waterproofing on the surface of the tile backer board
  • a waterproof sheet membrane on the surface of the tile backer board

Shower Waterproofing Crash Course

I have a hard time calling a four mil plastic sheet behind backer board a waterproof membrane. Why? Because you attach this membrane by stapling it to the studs then puncture it further by nailing (or screwing) backer board in front of it. But it is an acceptable, and minimal, way to keep moisture from getting into a wall cavity.

So here’s where I draw the distinction between a moisture barrier and a waterproofing membrane:

  • Waterproofing for vertical walls is a moisture barrier
  • Horizontal surfaces, even those that slope, need a waterproofing membrane.

Moisture Barrier vs Waterproofing

Now I’m not the only one that makes a distinction between waterproofing vertical and horizontal surfaces. Custom Building Products requires two coats of their Redgard liquid waterproofing on all waterproof surfaces. However, they also have this technical bulletin which says that only one coat is needed on the vertical surfaces of the backer board.

It’s like: All bourbons are whiskey, but not all whiskey is bourbon. Waterproofing can serve as a moisture barrier, but a moisture barrier doesn’t necessarily qualify as a waterproofing barrier.

A moisture barrier is all that’s needed to keep water out of the water cavity. But more extensive waterproofing is required for the horizontal surfaces if you want to prevent a shower from leaking.

Where you are likely to have shower leaks

The horizontal surfaces in a shower usually include the shower floor, half walls, benches, curbs, and recessed niches. These need special attention to ensure that they don’t leak. Acceptable ways of waterproofing these types of surfaces are two coats of liquid waterproofing, a waterproof sheet membrane, or the many different foam waterproof products that are available now. They make foam recessed niches, corner benches, floating benches, curbs and shower pans.

Do you need to waterproof shower walls before tiling?

Before 1960 the average house didn’t have any “showers stalls”, only bathtubs with surround walls. Those walls were built with concrete plaster just as the rest of the house. On top of that plaster was added a second ½” layer of concrete plaster called “mud bed” in which the tiles were embedded and on the edges finished off with a rounded piece of tile called mud cap (hence the name).

The disadvantage of the concrete plaster though, it soaked in the moisture through the grout lines. Once inside the wall, the wood frames soaked up the moisture from the wet concrete and through lack of fresh air started to deteriorate the studs, also known as stud rot.

Later on, when drywall or gypsum boards were generally recognized, the so-called “green board” replaced the plaster, with the promise of being mould and water-resistant.

Little did they know (or much did they ignore) that the green, the water-resistant paper layer is a cellulose product and mould thrives on cellulose! As soon as the water penetrated the water-resistant coating, the mould had a free for all, feeding of the paper and the gypsum.

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Why you need to waterproof a shower for tiling

In our opinion and most peoples automatic assumption the most important building concept of any bathroom is that it does what it’s designed for and keeps the water where it is meant to go and more importantly where it isn’t meant to go. How a bathroom shower is built, what materials are used and how it is waterproofed effects this. Regardless of whether the walls are to be wet panels or tiled, whether for a shower tray or a bath with a shower, the panels or the backer boards the tiles adhere to must sit on top of the edge of the bath or tray. Only due to special circumstances such as wet rooms do the panels drop down beside the tray continuing to the floor. Wet rooms use a special sealant tape to make a waterproof seal between wall and floor which I’ll cover later. Save money by cutting out the cost of waterproofing a bathroom, and it will cost you a lot more than the waterproofing would have. 

Tile Backer Boards

If tiles are used, then they must have the appropriate backer board to adhere to. There are dedicated tile backer boards on the market; however, the most commonly used material is plasterboard. The reason for this is the material price. Customers are often working to a budget. A sheet of 1200*2400mm plasterboard. Dedicated tile backer board for a quarter of the amount of the plasterboard, i.e. 600*1200mm, double the price, so the same quantity as a sheet of 1200*2400mm plasterboard is on average four times the price. When doing an entire bathroom, this cost adds up and is often not foreseen by potential customers. Whether ordinary plasterboard or moisture-resistant plasterboard is used without waterproofing, it is not a suitable material to use in a wet environment. Tile grout is not waterproof, and neither are a lot of tile adhesives even if proper tile installation methods are used. Water will reach the plasterboard and soak it, leading to its eventual failure. Fit heavy tiles as well bearing in mind plasterboard can only support 32kg/m2 which includes the tile, grout and adhesive, and you have an accident waiting to happen. Imagine the weight of a 300*600mm 10mm thick porcelain tile falling off the wall onto you when in the shower. Plasterboard must be waterproofed before tiling. It is very common to see bathroom fitters have used the green paper-faced moisture resistant plasterboard. It is only water-resistant, not waterproof.

The dedicated tile backer boards are undoubtedly superior products to use for a wet environment. There are generally two different materials used for tile backer board, cement board and insulated foam board, and both are capable of supporting between 50-200kg of weight of material depending upon the different manufacturer’s product.

How to build a shower correctly.

So now that I have covered the different materials for tile boards, I’ll first cover the wrong way to build a shower, whether tiled or wet panelled with the exception of wetrooms. This is the quickest way to build. Put the tile backer board either plasterboard, cement board etc. on the walls or install the wet wall panels. Then push or lower the bath or shower tray into position alongside the panels or boards so that they continue down to the floor. If further corners are being cut, don’t even put in a timber frame to support the bath. Flood the gap between the tray and the panels with silicone seal then if tiling is required tile without applying waterproofing compound. Finish off with grouting and siliconing. Sooner or later, this will fail. Not only will the wall get damaged if timber but due to flexing and movement you’ll get water escape to the floor. If this does happen over time, you’ll certainly get wet rot and possibly dry rot. Unfortunately, we see this regularly when going to view potential repair jobs and there is little that can be done pending how bad the damage is aside from redoing the silicone on a regular basis or ripping the whole shower area out and starting again.  

The right way to do it. If a bath is going to be installed, even a steel bath will flex a little with the weight of water and a person. An acrylic bath will flex far more. So a timber frame must be built that is completely solid so that there is no flexing. The reason for this is as follows. When done right, the shower tray or bath is installed first. The wet panels or tile backer boards are then installed so that the bottom of the boards lips down onto the top of the bath or shower tray. There must be about a 2-3mm gap between the bottom of the board and top of the bath or shower tray. If wet panels are installed the gap at the bottom is flooded with silicone seal. If tiling is to be done, then the boards must be waterproofed. 

If there is any flexing of the tray or bath, then the silicone seal or the waterproofing will fail at the joint of the tray or bath. This is why for both fittings, the timber frame must be built to stop any flexing. Shower trays must normally be solidly bedded into cement mortar or better yet tile adhesive. Done this way your new bathroom should last for year after year without any problems.

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How to waterproof a shower using tanking kits

First, let me clear up the difference between waterproofing kits and tanking kits. They both do the same, and waterproofing is the name generally used for products more commonly targeted at the DIY market, tanking kits more often at the trade because they are more professional products, slightly harder to work with but quicker drying and give better results in my opinion especially when waterproofing plasterboard for shower tiling.

Before using either the preparation work to the wall must be done. First sweep and hoover up any dust. You will be provided with whichever manufacturers joint sealing tape. Apply this over the walls between the board joints and in the corners. 

Do this all around the bath. Then apply the tape so that the edge of one side sits no further onto the bath than the pencil line, whilst the other side fixes to the wall. Apply the bath side first, push the tape hard into the corner between bath and panel using a scraper or other flat-sided tool to create a good 90-degree corner and then fit the wall. There should be no slope in the tape when it transitions from board to bath.

Tanking scrim tape

These kits are messy, so I strongly advise you also do the following. Apply cheap masking tape around the perimeter of the top of the bath or shower tray along the pencil line against the edge of the jointing tape. It should sit beside it, not on top of it with a 1mm clearance between the two. Get some plastic sheets and cover the bath or tray holding it in place with more masking tape.

Once this is done if the kit supplies a primer, apply the primer to the wall and allow it to dry fully. You are then ready to apply the compound. Wear cheap disposable clothes, gloves and eye protection.

If a bathroom floor is to be tiled, it needs to be waterproofed if it’s a timber build. This will prevent the subfloor where timber-based from rotting. You can apply any mixed up tanking compound to the floor material whether timber or cement board laid on top of the timber floor around the shower area when doing the walls. This is a cheap way of doing it. The alternative method is the use of decoupling membrane, which I mentioned on the tiling preparation page. Most of them are waterproof, and if needed for what they are designed for then, the bonus is the waterproofing is also done. They are however expensive in comparison to liquid tanking so if not required for decoupling purposes not the ideal way to do the job. You should be aware that the manufacturers of luxury vinyl often state that their products are waterproof. They are not; water will still seep between the joints. However, you will only get significant quantities of water through these joints if you were to step straight out the shower and deluge the floor with pools of standing water.

Taking the extra time to waterproof a bathroom is probably the single largest factor that can result in vastly different prices quoted for a new bathroom and just because a large well-known company are doing the job does not mean they will do the work. Some companies even go so far as to give a description and time frame of the work claiming to turn a bathroom around in just five days. You will see nothing mentioned about waterproofing because of the time and extra cost, which makes the quote noncompetitive. You end up with a bathroom that whilst on the surface will probably look good it may only last five years. Done right, it should last 25 years.

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