Waterproofing tanking a shower before tiling

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 on 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. The photos below show the consequences of not building a bathroom properly and not waterproofing the shower area. This property is only 3 years old and is a prime example of the ever increasingly common problems we are finding in poorly built new builds. The home has an inner timber frame which had water damage.

Bathroom rip out in progress

Notice the MDF skirting along the wall below the bath? It was also used as the bath support batten. MDF swells when wet as this and loses all its strength. It was doing nothing to support the bath. It should never have been used. Black mould growing on the plasterboard, which was barely supporting tiles above.

Damp proofing failure

Notice the plastic sheeting on the right of the photo which is meant to act as a damp proofer to prevent moisture penetrating to the timber frame. None was installed to the left hand side.

Mould and rot on OSB

Where the damp proofing sheets had been installed it had been done poorly. Moisture had soaked through the plasterboard and insulation and resulted in mould and rot to the base of the timber frame and OSB used to clad the exterior of the inner frame.

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 material price. Customers are often working to a budget. A sheet of 1200*2400mm plasterboard costs roughly £7. 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 the 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 resistance 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 prior to tiling. It is very common to see bathroom fitters have simply 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 2 different materials used for tile backer board, cement board and insulated foam board, both are capable of supporting between 50-200kg of weight of material depending upon the different manufacturers product. The three major UK manufacturers of cement board are BAL Board, Hardieboard and No more ply. All types of cement boards unlike plasterboard can get soaking wet and be fine suffering no damage themselves. However cement boards are porous and allows the penetration of water through to whatever they are secured to. If this is a timber stud wall then the wall will eventually get rot. Cement boards need to be waterproofed all over and at the joints. The foam core backer boards are waterproof so they do not need waterproofed all over like the cement board however the joints between the boards still need waterproofing.

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 paneled 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 re doing 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 absolutely 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. You can see below in the photos the before and after of the waterproofing of plasterboard.

Furthermore, when the waterproofing is applied to the boards the compound itself must lip onto the bath. You can see this in the photo below. The compound lips onto the bath for a distance equal to the thickness of the tile.

This is then hidden once tiling and siliconing is complete.

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 bath fitting 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.

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, 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 which ever manufacturers joint sealing tape. Apply this over the walls between the board joints and in the corners. It also needs to be applied to the top of the bath or tray as shown in the photo above. The easiest way to do this is to take one tile, sit it on the bath against the board and use a pencil to mark the front of the tile so you know how far out the tile will sit as shown below.

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 to the wall. There should be no slope in the tape when it transitions from board to bath.

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.

Waterproofing kits such as the Mapei kit come pre mixed all you need do is prime the walls with the primer provided and apply the compound to the wall. Wait a few hours for one coat to dry and then apply a second. Tanking kits such as BAL TANK IT and ARDEX WPC come as 2 separate components a latex based liquid and cement based powder and have to be mixed before use. You need a power drill and paddle mixer to do this, do not try to simply stir the mix manually. Beware that the pot life of this stuff is about 30-40 mins on average before it sets so much that it is unworkable. Get everything ready so that once mixed up you're ready to go. You need to apply two coats of it therefore you need to half the quantity of the materials. For the tanking the mix must be mixed up to a lump free consistency which is achieved by using the correct amounts of the materials. Do not try to judge by eye when mixing if more liquid needs to be added weigh and measure the amounts before mixing so you know you are getting the correct mix ratio. Any left over mix which there should be very little of put it on the floor or throw it away. Whichever you use start by using a cheap brush 3-4 inch wide 1cm thick to apply compound into the corners and the bath/ tray top to board. It is crucial that you do not use too much mix that allows it to pool on top of the bath creating lumps when it dries, brush it to a thin coat. Once done then use a large paint roller to apply over the main sections of the wall. Once finished, throw the brush and roller in the bin do not wash them the chemicals are not good for the environment. You'll need a second brush and roller for the second coat. Tanking compound can be hard work to apply to backer boards and you'll find that it dries very very fast. The second coat is often ready to start where the first coat began immediately after finishing the first coat. Pull the masking tape and plastic sheet off immediately whilst the mix is still a little wet. Once done the tiling can start.

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 the 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 really designed for then the added 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 to 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 5 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 5 years. Done right it should last 25 years. So how much does bathroom waterproofing cost? Using the methods outlined above no more than 1 days labour during installation and the cost of the tanking kit itself. All the kits mentioned retail from £45-£70. It's a small price to pay in comparison to having to get the bathroom done again in only a few years.

Waterproofing Wetrooms

The waterproofing of a wetroom is slightly different from that of the standard walk in shower or shower over bath as described above. It is not critical that the tile backer boards sit on top of the shower tray or bath instead with a wet room the walls can be boarded first and then the required recess cut out of the floor for timber subfloors to accommodate the wetdeck. In this case the edge of the deck will be up against the bottom of the face of the board. The joint is then sealed using a butyl based joint sealant tape. A similar type waterproofing membrane is then applied to the floor over the deck and the remaining floor area to waterproof the wetroom. In comparison to the tanking compound mentioned previously this membrane is expensive hence one of the reasons wetrooms cost more than a walk in shower. It is of course recommended to install the membrane on the walls too however with the cost of the membrane it is possible to use it solely on the floor along with the joint sealing tape and then apply standard tanking compound to the walls.