Anyone Can Build a Shower Base ... Can’t They?
November 12, 2013
Most times a shower base is taken for granted and rarely is the topic of conversation….until it doesn’t function properly. There is really no excuse for shower base failure since there are specific guidelines provided in the TCNA Handbook and ANSI Specification that are time-proven. Guess what? They work!
We will look at the integral parts of a shower base or as it is listed in the Handbook, “Shower Receptors.” In many regions of the country, the plumber may be responsible for certain components of the shower pan. If this is the case, be certain to check that the sloped fill and appropriate shower pan membrane are included and installed correctly as illustrated by the Handbook detail B415.
However, in other parts of the country, the tile installer is responsible for providing a waterproof shower, and it is vitally important that this work be performed correctly the first time by a qualified tile installer who has been tested and certified according to established industry standards.
Sloped Fill: This term may be misunderstood by some people because it is known most widely as a preslope or pitched pan. Whatever you call it; this is one of the most critical and least used or understood components of the assembly. The sloped fill does exactly as it describes: It provides the slope for the shower pan membrane to direct the water travelling through the mortar bed to the weep holes in the drain body and out to the sewer. ANSI method A108.1A.2.3.4 calls for the slope to be 1/4” per foot (20 mm/m) to the drain weep holes.
If the sloped fill is not provided, the pan will fail. It may not leak, but it can develop two issues that will only go away if the shower is no longer used. The first is a very disturbing odor. This problem arises when the water doesn’t flow to the drain and is allowed to collect and remain on the flat floor which builds up to the thickness of the drain body, usually about 3/8” to 5/8”. This water combines with body oil, shampoo, cream rinse and soap residue to form a nasty black sludge that grows bacteria, which is the culprit that produces the less-than-pleasing stench that could fill the otherwise beautiful shower.
The second aesthetically non-pleasing issue occurs when the mortar bed doesn’t evacuate the water which keeps the mortar bed constantly wet. This moisture wicks up through the bed and causes the grout joints to darken only in the areas above the trapped water; yielding some dark joints while others are light. If natural stone is used on the shower floor with trapped moisture under the mortar bed, the stone tile will darken, again yielding dark areas while the dry areas are much lighter in tone as seen in the photo.
Shower Pan Membrane: The majority of pan membrane materials are either made of chlorinated polyethylene (CPE) or polyvinyl acetate (PVC), generally between 30 and 40 mils thick. This product lays directly on top of the sloped fill, providing an easy path for the water to travel to the weep holes and down the drain.
In preparation for the membrane installation over the sloped fill, calculations need to be made to determine the size which is needed. The pan liner (according to the Handbook) must turn up the walls a minimum of 3” above the shower curb, or 6” above the floor in showers without curbs. Determine the size of the pan and add the height of the curb plus the 3” required above for each of the two sides. For example, add the pan size (42”) plus the height of the curb (4 1/2”) plus 3” to be above the curb for each side. So, 42” + 7 1/2” (which is the height of the curb plus 3”) + 7 1/2” equals 57”, which would be taken from a 60” wide roll. For the length, do the same exercise except the membrane must also go up the inside, across the top and down the outside of the curb to the bath floor (7 1/2” + 42” + 4 1/2” + 3 1/2” + 4 1/2” = 62”), which is cut from the length of the roll.
Now mark the same measurements on the pan membrane and fold it on the lines marked. With the upturns now folded toward the center, fold the membrane in half front to back and then fold the liner in half right to left, similar to a folded bath towel. Take the folded pan liner and place it (without turning it), in the upper left corner of the shower. Unfold the membrane left to right and then back to front. The pan liner should fit like a glove. Next, lift the wall upturns to the wood blocking and fasten at the top with galvanized roofing nails.
Now we have a waterproof pan, right? Well, almost. There are two more critical aspects yet to be described. Very importantly, no fasteners may penetrate the pan liner below the 3” requirement, regardless if the wall substrate is a mortar bed or backer board. Let me repeat that. No fasteners.
Whether the shower has a curb or not, the second critical part of a waterproof pan is the application of the four, manufacturer produced, pre-formed outside corner dams at the intersection of the curb (or the floor, if curbless) and the wall. These corners, when installed with the manufacturer’s recommended sealant or adhesive, will provide a watertight connection. Unfortunately, many unqualified installers take shortcuts by either not using the corners at all and/or using a cheaper adhesive that most likely will not bond the corners to the membrane.
An installation tip that will provide flat shower walls comes from the Handbook which states, “Fur out [the] studs above shower pan membrane or notch out [the] studs behind the shower pan membrane so folds/corners of shower pan membrane do not cause backer board to bow inward.”
This can be accomplished by applying lattice strips to each stud above the membrane prior to the backer board or by cutting each stud about 1/8” deep. Then, use a hammer and chisel to remove the wood below the saw cut. This notch will allow the three folds in the membrane to be neatly recessed in the notch. Also apply a generous bead of the sealant along the folded membrane to keep water from wicking up the fold.
Shower Drain Weep Hole Protection: If all of the work mentioned above is completed correctly but the weep holes are not kept open, the mortar bed will fill with water and fail. In order to keep the weep holes open and able to evacuate the water, some sort of weep protection must be in place. A small amount of pea gravel, a handful of tile spacers or a manufactured plastic ring may be used as a filter to keep the water flowing.
Reinforced Mortar Bed: ANSI A108.1A.2.3 lists several important requirements for the mortar bed. The mortar bed mix is to be one part Portland cement and four parts damp sand by volume and is to be a minimum thickness of 1 1/2” (38 mm). This mixture should only contain enough water to bind the materials together. If water leaks out when a snowball-sized portion is pressed between two hands, the mix is too wet. Many people who are not acquainted with a proper shower pan mix will say it looks too dry to activate the cement, but it is actually probably about right.
Section 2.3.5 states: “Galvanized reinforcing wire shall be suspended near the middle of the mortar bed. Reinforcing wire shall not butt against vertical surfaces. When installing mortar bed mix in a defined space (contained by four walls) of 65 square feet or less, wire reinforcing is not required unless specified by the architect or design professional.” This relatively new statement eliminates the need when working with showers of that size, but is still required in showers larger than 65 square feet or not in a defined space.
The Curb: The curb is another area where unqualified tile placers (they cannot actually be called installers) carelessly place backer board over the membrane and attach it to the wood curb framing through the membrane with a couple of drywall screws. This compromises the integrity of the membrane and will eventually fail. Why do these people continue to flourish in the tile industry when they consistently do substandard work?
This brief overview of a properly installed shower receptor does not include every detail necessary for its completion, but it does point to the necessity of using a qualified tile professional to complete the work. The cooperative efforts of the Ceramic Tile Education Foundation (CTEF), the International Masonry Institute (IMI), the International Union of Bricklayers and Allied Craftworkers (IUBAC), the National Tile Contractors Association (NTCA) and the Tile Contractors Association of America (TCAA) have formulated a series of tests known as the Advanced Certifications for Tile Installers (ACT). The installers who take the certification tests for Shower Receptors have proven they possess the skills, knowledge and hand skills necessary to complete the job successfully.
An ACT-certified installer may cost a few dollars more, but this small difference far outweighs the aggravation of a failed installation and an unhappy customer.
Scott Carothers is the director of certification and training for the Ceramic Tile Education Foundation headquartered in Pendleton, SC. He is responsible for the creation of the Certified Tile Installer (CTI) program and is also involved in the creation of the new Advanced Certifications for Tile Installers (ACT) program, as well as providing training to others in the tile industry. He has been involved in the ceramic tile industry for over 33 years and was the owner of a successful retail store and installation firm prior to coming to CTEF. He has served as president of the NTCA, chairman of the NTCA Technical Committee and named the NTCA Tile Person of the Year in 2005. He is a voting member of the ANSI and the TCNA Handbook committees. He may be contacted at 864-222-2131 or email@example.com.