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Waterproofing Shower Walls

How to Build a Shower

One of the most advanced tasks you will likely encounter when laying tile or stone is building a shower. Whether doing a full bathroom remodel or starting from scratch, it is vitally important to use proper methods and techniques when building a shower. A shower not built correctly is susceptible to moisture getting behind the substrate leading to the frame swelling, the growth of mold, and other structural water damage. Tile and grout are not a moisture barrier or waterproof so all waterproofing must be addressed from behind the tile, before a single tile is laid.

Fortunately, there are several ways to build a shower. One traditional method involves building a layered mud bed with a pre-slope and pan liner for the shower base. Another method is to buy a pre-sloped shower pan or shower kit from a manufacturer such as Laticrete, Schluter or USG. These shower kits will include the basic components to build a shower including a preformed / pre-sloped shower pan, waterproofing membrane, and drain.

This guide will cover both the mud bed method and manufactured shower kits. Keep in mind that while manufacturers will provide detailed instructions for the installation of their shower kit, there are a variety of ways a mud bed shower base can be built. This guide is intended to only provide a general overview of how to build a shower.


Shower Base

Every shower must have a properly sloped shower floor. There are several methods for building a shower base that include using a dry pack to build up a sloped base, building a sloped shower pan with a hot mopping method, or buying a preformed shower pan from a manufacturer such as Laticrete, Schluter, or USG. Regardless of the method used for the shower, it is imperative for the floor to be completely waterproof because it is the area most prone to leaks. If the shower floor leaks, then everything is in the surrounding area is subject to mold and water damage.


Shower Curb

Before building the sloped shower floor, the curb must first be installed to create a "box" for the shower base. If the subfloor is wood, then kiln dried lumber is suitable for making the curb. Pressure treated lumber, however, should be avoided because of the excess moisture content. To create a curb using kiln dried or air dried 2 x 4's, begin by screwing the first 2 x 4 down to the floor. Next, stack another 2 x 4 on top and screw into the first 2 x 4, repeating this process until the desired height is achieved.

If the floor is concrete, then wood should not be used for the curb because it will absorb moisture from the concrete and begin to swell. To create a curb for a concrete floor, concrete bricks (without holes) can be stacked until the desire height is reached. Using thinset, the bricks can be adhered to the floor and to each other. Once the curb is in place, it is time to address the slope for the shower floor.


Pre-Slope

Building a shower floor requires a correct slope to direct water toward the drain. A pre-slope, however, is also necessary when using a pan liner between 2 layers of mud bed because if the liner lays flat, the water will not be drain properly. This can lead to stagnate water and other problems such as mold. With a correct pre-slope, however, the liner will direct water toward the weep holes in the drain.


Pre-Slope Height

To figure out the height of the slope from the floor, measure from center of the drain to the furthest corner. For every foot, the slope height needs to increase 1/4 inch. For example, if the distance measures 4 feet, the slope needs to be 1 inch high from the floor at the furthest corner. Once the slope height is determined, mark the height on a 2 x 4 wall stud and use a level to finish drawing the slope perimeter around rest of the shower.

For showers that are not perfectly square (most are not), the slope incline will vary with walls closer to the drain having a steeper incline... this is normal. The slope height at the walls, however, must be the same for each wall. While height of the pre-slope at the drain can vary from to drain to drain, the height should be level with or reach higher than the bottom flange of the drain (drains have a top and bottom flange).

The liner will then be supported by the pre-slope and will go in between the top and bottom flange of the drain to redirect water toward the weep holes in the drain. It is important to remember that at no point can the pre-slope be lower than the bottom flange of the drain. If any point is lower, the water will be unable flow towards the drain (water can only flow down an incline - not up).


Preparing the Shower Base

Before building the pre-slope, plastic or tar paper should be stapled to the wood subfloor with metal lathe nailed or stapled down over top. Stucco wire or galvanized wire mesh can also be used in place of metal lathe. The plastic acts as a "cleavage" membrane to prevent the wood from absorbing moisture from the pre-slope because if the wood absorbs moisture, it causes the pre-slope to cure too fast or not fully, significantly weakening the pre-slope. Note that the plastic does not make anything waterproof.

Once the plastic sheet is down, cut the metal lathe to the shape of the shower floor and staple or nail the lathe down. The lathe gives the dry pack something to grab onto. The lathe or stucco wire should also be fitted over the curb and secured as well. This will reinforce the mortar over the curb.

If the subfloor is concrete, lay down a layer of regular thinset slurry or thinset that has been mixed "loose" before creating the pre-slope. Thinset that has been mixed loose is simply thinset that has a bit more water added than the instructions call for in order to make it thinner. Just like the metal lathe gives something for the dry pack to adhere to with a wooden subfloor, the thinset slurry will give something for the dry pack to adhere to over a concrete subfloor. For larger showers, lay the thinset down in one workable section at a time.


Deck Mud

When building a shower base, a dry pack mortar, or deck mud, is commonly use to create the pre-slope. Deck mud consists of three components including portland cement, sand, and water. Mixing up deck mud requires getting the correct consistency for stability. Typically, a 5 parts sand to 1 part cement ration is used, but the ratio can vary from 4 or 6 parts sand to 1 part cement. Add just enough water to get the mixture damp - adding too much water and the deck mud will shrink as it cures causing it lose structural integrity. While a latex concrete additive can be added to the mixture increased strength and flexibility, it is absolutely is not necessary for a shower mortar bed.

There are also commercially available bagged products for shower floors and mud beds that require just the addition of water. Another option is to buy a bagged mixture of sand and portland cement and add more sand as needed to get correct ratio. For example, a 60 lb bagged product of 3 parts sand to 1 part cement would simply require an additional 30 lb bag of sand to bring the ratio up the correct 5 parts sand to 1 part cement. Whether buying a ready-to-use bagged product, adding sand to a bagged product to get the correct ratio, or mixing the mud from scratch, slowly add the water to get the correct consistency. The mixture should hold its shape when squeezing a handful without excess dripping water out.


Building the Pre-Slope

Once the pre-slope height has been marked, the subfloor prepared, and the deck mud properly mixed, it's time to start packing the mud in. Start at the walls and pack it in very densely by pounding it with a wooden or magnesium float... it should be packed in very tightly to fill any holes or crevices. The more densely it's packed, the more stable it's going to be. Pack it down to just over the marked pre-slope line around the perimeter. Place a 2 x 4 on the dry pack against the wall, and then with a hammer, tap the 2 x 4 so that the mud bed becomes even with the perimeter line.

Once the mud is level with the marked perimeter, continue packing the mud into the shower base from the wall down to the drain to create the pre-slope. Use a straight edge or similar, screed the mud from the wall to drain for an even slope. While the actual incline of the pre-slope will vary if the walls are a different distance from the drain, the pre-slope should be completely smooth from wall to drain with no dips or humps to prevent the water from becoming stagnate or pooling. Once finished, the shape will be similar to a very shallow bowl.

The curb should also have a layer of mud added with all sides covered - the metal lathe will help to hold the mud in place. Slope the mud layer on top of the curb toward the inside of the shower to drain water towards the shower pan. At this point, the bed will require about 24 hours to cure and will then be ready for the liner. The deck mud will still have a sandy finish once it has cured, but this is completely normal.


Waterproof Shower Pan Liner

When the pre-slope has cured, the waterproof liner can be installed next. Shower pan liners are made of a waterproof material such as polyvinyl chloride (PVC) or chlorinated polyethylene (CPE) and are installed on top of the pre-slope to prevent water from reaching the subfloor or frame. Before installing the pan liner, it is absolutely necessary to ensure the pre-slope has been properly built first so water will be directed toward the weep holes in the drain.

When purchasing the liner, select one that is at least a 1 ft longer than the shower base on each side. For example, if the shower base measures 3 ft x 4 ft, the liner needs to measure at least 4 ft x 5 ft. This will allow the liner to run up the wall 6" in each direction. However, specifications also state the liner must extend at least 3" up the wall over the height of the curb. For a curb higher than 3", a larger liner is needed than the additional foot in each direction. To be on the safe side, selecting a liner that measures 2 feet longer each way is a good option.


Preparing the Shower Base

Before installing the shower pan liner, ensure the pre-slope surface is smooth without any bumps or dips. For any uneven raised areas, use sandpaper or a razor blade to even the surface out. If there is an area with a significant dip, use more deck mud to fill in the area (make sure to put thinset down first so the mud has something to adhere to). Remember, the surface should be smooth and level - not flat. If it becomes flattened out, the water will not be able to drain.

Where the pan liner goes up the wall, it adds additional thickness to the wall studs. Once the cement backerboard (or other substrate) is installed over the wall studs and pan liner, the portion covering the liner is going to extrude slightly and result in the backerboard not being 100% vertical. In order to overcome this, the bottom portion of the wall studs, up to the height of the liner, can be notched out or shaved off about an 1/8" to accommodate the thickness of the liner.

The same principle will also apply to corners. The corners, however, will eventually be folded over creating additional thickness with 3 layers of liner. To accommodate the liner in the corners, one wall stud per corner should be notched out an additional 1/4" or so. Once the wall studs are notched out, the backboard will be flat when installed over the studs and liner.


Installing the Shower Pan Liner

Once the wall studs and pre-slope are ready, the shower pan liner can be laid out. Remove the top half of the drain flange (the part that bolts into the bottom half) but replace the bolts to serve as a guide when cutting the liner for the drain. Next, lay the liner out, covering the shower floor and wrapping it over the top of the curb and up the walls. The liner should be centered with equal amounts of the liner running up each wall and enough liner reaching over the curb to attach to the outside. Remember, the liner needs to extend up the wall 3 inches over top of the curb. Be sure to not cut anything until everything is correctly aligned and do not puncture the liner with staples or nails.

With the liner properly laid out, the hole for the drain can be cut. With a utility knife, start by poking a hole in the center of the drain placement and use a spiral cutting motion to cut the liner to just outside the bolts. Once the hole has been cut out, remove the bolts and place a bead of silicon under the liner and around the bottom flange perimeter without getting silicon in the bolt holes. Press the liner around the drain to confirm proper adhesion - this will prevent any minute moisture from getting under the liner.

After the liner is pressed around the drain with silicon, bolt the upper flange of the drain back into the lower half. While the liner should be firmly squeezed between the top and bottom flange, be careful not to over tighten the bolts because over tightening can cause the bottom flange to crack. Installing the drain clamp properly is essential for a leak proof shower, so take your time. With the top flange bolted back on, the drain is set and ready to go.

Now that the drain is in place, situate the liner up the walls and fit into the notched wall studs. To hold the liner in place, use tape or a roofing nail at the very top edge only. Do not puncture the liner anywhere below that very top edge along the walls. Even a small puncture in the lower portion of the liner can result in water getting underneath the liner. For the corners, fold them over and tuck into the notches in the wall stud- the liner should fit into the notches enough so that the backerboard will sit flush against the wall studs. Do not cut the liner in the corners in order to make them fit.


Waterproofing the Curb

Once the liner is fitted to the corners and wall studs, waterproofing the curb can be addressed. There are different methods for waterproofing the curb with this just being one approach. To start, make sure the liner is firmly pressed into the corner with the curb. Find the starting point for the cut by following the liner up to the top inside corner of the curb. If cement backerboard is being used with a plastic moisture barrier directly behind it, cut straight up, following the wall stud. If using a topical waterproofing membrane such as Laticrete Hydro Ban or Schluter Kerdi, cut straight outwards toward the outside of the curb.

Preformed corners can be bought and installed (highly recommended) to waterproof the top end of the curb where the cuts were made. The outside preformed corners are glued on the top end to cover the cut in the liner and make it waterproof. Remember to use to appropriate glue or adhesive per the manufacturer's instructions.

To finish waterproofing the curb, begin by placing a 2x4 on the floor against the inside of the curb so the liner will lay flat against the curb with no air pockets. With the liner pulled over the top, nail it to outside of the curb - only nail to outside of the curb and never the inside. Excess liner along the drywall can then be trimmed away and the remaining liner tucked into the wall.


Flood Test

Now that the liner and drain have been installed, the shower floor is a waterproof basin and ready to flood test. In applications requiring complete waterproofing (such as a shower), a flood test is always recommended, and in most cities and counties, required by plumbing code. The flood test is just to make sure the pan is waterproof up to this point - always flood test before laying any tile.

In order to do a flood test, the drain will need to be plugged up first. The drain can be stopped with a specialty plug or something as simple as a water balloon. Regardless of what is used, make sure it is pushed down the drain far enough to block the weep holes. Otherwise, water will seep through the weep holes and won't give an indication of if the liner is working properly.

With the plug in place, fill the shower base up with water to just below top of the curb and mark the water level with a marker. Wait 24 hours and check the water level again. If the water is at the same level and hasn't gone down, the shower is ready to go. Evaporation may also cause the water level to go down slightly, so a drop of less than 1/8" is also considered acceptable.


Top Mud Bed Installation

After a successful flood test, the top mud bed layer is ready to be installed. The top mud bed will be a consistent thickness and because the pre-slope has already been installed, the top mud bed will have the same slope. At this point, the backerboard and moisture barrier can be installed on the walls. Another option is to install the top mud bed layer first with backerboard and moisture barrier installed after. Installing the backerboard first, however, will help in achieving an even perimeter around the top edge of the mud bed and will add extra stability to the shower wall.

If installing the moisture barrier and backerboard prior to the top mud bed, leave a 1/2" - 1" gap over the shower base to prevent the backboard from mechanically damaging the liner. It also prevents water from wicking back up the backerboard. Scrap pieces of plywood or smooth backboard (avoid rough, cement backerboard that could damage the liner) work well as a spacer prior to securing the backerboard to the wall studs . Make sure to fasten the backerboard above the pan liner because a puncture in the liner can potentially lead to leaks. A gap should also be left when securing the backerboard of the curb.

Regardless of the option used, make sure the moisture barrier goes over top of the liner to ensure moisture is directed into the shower and not into the wall framing. The actual thickness of the mud bed is determined by how much vertical movement the drain has. The top of the drain (the center part that unscrews), or drain riser, will need to sit level with or just slightly below the mud bed with the tile installed. Many mud beds will be 1-1/4″ to 2″ thick.

Before building the top mud bed, pea gravel, tile spacers, or something similar over the weep holes of the drain collar to prevent the mud bed from clogging the holes. Using a piece of the floor tile placed at the bottom of the drain, unscrew the drain 1-1/4" making sure that the drain still reaches into the flange. As long as the drain still reaches the flange, the top mud bed can be 1-1/4". If the drain falls out at this height, unscrew it to only 1" for a top mud bed that will be 1" high.

With the drain height set, mark the proper mortar height on the shower walls using a straight edge. Set a 2 x 4 on top of the drain and measure from top of the liner to top of the 2 x 4. For many installations, this will measure around 4-3/4". With that measurement marked, continue to mark rest of the perimeter around the shower. If backerboard has not been installed yet, a laser level can be used to easily mark the perimeter.

Once the perimeter is marked, it is time to use deck mud to build the top slope. Just as before, mix up the deck mud. As the top mud bed is built out, it will correspond to the same slope of the pre-slope. Starting at the back, pack the mud in above the height of the marked perimeter. With a 2 x 4 placed flat against the wall on top of the mud, use a hammer to pound the 2x4 and pack the mud down tightly to the marked perimeter line.

Continue this process around rest of the perimeter. Once the perimeter is done, start packing in rest of the top mud bed with deck mud. From the perimeter down to the drain should be completely flat without any bumps or dips. Not that being flat is different from being level - it won't be level as it will follow the slope from the first mud bed installation. Be sure to pack the deck mud down as tightly as possible because the more dense it is, the better for the floor.

Once the top bed is complete, wait at least 12 hours, preferably 24 hours, for the deck mud to cure. After it has fully set, any remaining bumps or humps can be sanded down with sand paper to create a flat surface. It is imperative for the surface to be flat as possible because it will be directly tiled upon. If moisture barrier and backerboards have not been installed yet, do so now. With a completely waterproof base and walls, the shower is ready to be tiled.