Mortar Bed Shower Installation ImagePorcelain, ceramic, and stone tile can be used in tile lined showers to create beautiful, durable, and long lasting installations. Starlike® Waterproof Tile Grout is perfect when installing new shower tile. In this section, we discuss the various installation methods used today.

In Figure A, we see a mortar bed shower installation. The walls are constructed and tiled in the same manner as that in figure A or B in the section on “Walls”. The key difference is in the shower pan construction. The shower pan must be completely waterproof.

Industry standards for the construction of shower pans and wall details are found (for the United States) in The Tile Council of America Handbook (2009), the American National Standards Institute Manual (2005), and the Uniform Plumbing Code.

There are two types of tiled shower pan installations. The first is the pan constructed with a dam or curb. The second is one without a dam. The common tiled shower pan has a dam formed of three or four 2″ x 4″ studs laid flat and attached securely to the sub floor. These two by fours are attached to wood sub-floors with nails or screws capable of penetrating the lumber and sub-floor to provide secure fastening. Each two by four is placed and secured one at a time in the desired location. The two by fours should also be “toe nailed” into the adjoining wall studs when possible.

Tile Doctor Tip: All horizontal surfaces within a shower or tub need to slope toward the drain to allow water to flow off their surface. This includes the top of the dam, shampoo shelves, and seats.

If the sub floor is concrete, the first two by four is attached using a powder actuated nail gun with fasteners capable of penetrating the lumber and concrete with sufficient depth to insure stable and secure fastening.

Shower dams on concrete slabs are also constructed today with the use of masonry like cinder block or other appropriate masonry. There are also companies who manufacture dams made of high density foam. The recommendation for all horizontal surfaces to slope to drain should still be followed with these alternate methods.

Tiled shower pans without a dam or curb, as in the case of handicapped showers or other specific architectural detail, need to have sufficient clearance below sub floor level to accommodate all necessary elements in the illustration including tile thickness and required slope to drain.

For the placement of the two by fours, remember that there will be 3/4″ to 1-1/4″ of membrane, reinforcing, mortars, and tile on both sides of the dam when the installation is finished. These overall measurements will have to be accounted for prior to fastening the first board.

For the tile lined shower pan, a two piece shower drain assembly will need to be installed next. These plumbing fixtures are available in an “ABS” plastic and cast iron. There are six holes on the upper part of the two piece drain. Three of the holes are for the bolts that clamp the shower drain together. The other three holes are the weep holes. The weep holes and their importance are discussed later.

Tile Doctor Tip: The installed two piece drain should have the two stainless steel screws oriented either parallel, for “soldier course” tile, or at a 45° angle to the dam for tile set on a diagonal. This makes a much nicer finished look.

The two piece assembly is necessary for the waterproofing of the shower pan liner. The two pieces of the drain assembly “sandwiches” the waterproof membrane between the upper and lower part of the assembly. This forms a continuous watertight system between the drainpipe and membrane.

The plumber or installer disassembles the two piece drain and attaches the lower part to the drainpipe. The lower drain assembly is installed level with or just above sub floor level. At this point the waterproof membrane is installed and laps over the lower part of the drain with a hole cut out for the drainpipe. Once the membrane is installed, the upper part of the shower drain assembly is installed and clamped using the assembly bolts over the waterproof membrane. This creates a watertight seal for the whole installation.

Over the years, many types of shower pan liners were available. Shower pan liners have been made of copper, lead, and other metals. The newer and more common od was “hot-mopping” in which several layers of 15 pound roofing felt were cut, installed, and mopped into place using molten tar. This is still a good method. However, a skilled technician is needed to fabricate and complete this type of installation.

Today, flexible, waterproof pan liners are available specifically for use in showers. There are Manufacturers with pan liners that are self-adhesive. During the application and use of flexible shower pan liners, the Manufacturer’s instructions need to be followed closely. The success or failure of the installation depends on the liners ability to be and stay water-tight.

Whether the shower pan is hot-mopped or flexible membrane depends on the customer, architect, or local building code. How it is installed is critical. Blocking should be installed between the wall studs to adequately support the installation of the membrane. The blocking needs to be flush with the stud surface. Two by twelve joist boards cut to size work well for this application. The boards are toe nailed or screwed into place between the studs.

The membrane must be installed at least 3″-4″ above dam height or 6″ above non-dam or non-curb shower pan. The membrane must have the necessary adhesives or caulking applied between the lower flange and membrane only. Do not block the weep holes that are part of the upper flange. Ant corner reinforcing should be applied as directed by shower pan manufacturer.

The membrane must lap over the dam and must not be punctured anywhere below 1″ above dam height or 6″ above a non-dam/curb. Fasteners should be used to secure the membrane to the front of the dam and to the supporting blocking/wall studs above the 3″ minimum. The reason for this is simple. When tested, the shower pan must hold water without any leakage for 24 hours when filled to the top of the dam. This test should be completed before any other work begins.

The following question often arises concerning fastening reinforcing wire when puncturing the waterproofing is not allowed. How is the wire reinforcing attached inside shower pans or tile lined tubs? The answer is that it is not attached. The trick is to bend the reinforcing back toward the walls so that when the reinforcing is attached, it will naturally cling to the wall at the bottom.

The other critical element is that the floor below the membrane must be sloped 1/4″ to the foot toward the drain. The most common failure in tile lined showers, besides leaking, is water accumulating between the mortar bed and membrane. This accumulated water causes a host of problems including mold and mildew formation, reinforcing corrosion, efflorescence, and eventual failure of the tiled assembly.

Use this method to pre-slope the sub-floor. Measure from the drainpipe to the wall farthest away. For every foot, 1/4″ is added together revealing the required slope. For example, for a three foot distance, the measurement would be 3/4″. Make a mark, using a spirit level, on the base plate inside the enclosure the required distance above drain height. Form “working lines” all the way around the enclosure. Use these lines to pre-float a bed of Portland cement mortar using a wood or metal straight edge.

There is another cause of water accumulation between the mortar bed and membrane and that is the failure to leave the weep holes in the two piece shower drain open. The weep holes are the 3 small holes visible in the upper part of the two piece drain assembly. This is accomplished by placing crushed tile or small stones over the weep holes so that the mortar bed will not block them.

In summing up the installation steps in order:

  • The first step is placing and securing the dam or curb in its proper position.
  • The lower part of the two piece drain is then installed.
  • The sub floor is filled to allow a slope of 1/4″ per foot toward the drain.
  • The membrane is properly installed, fastened, and reinforced if necessary.
  • Then the upper part of the shower drain assembly is properly installed.

At this point, the wall mortar is installed using the method described in Figure B in the section on “Walls”. Look in this section to decide whether the installation will be tiled using “surface” or “conventional” trim, as the thickness of the mortar bed will have to be adjusted to accommodate each type differently.

How the walls will interface with the dam tile and trim will have to be decided also. If the installer has decided to have conventional trim, the wall mortar will need to extend 1/2″ in thickness beyond the abutting walls. There is a host of trim shapes that can be used in this type of installation. The most common is the quarter round.

If the quarter round is to extend down to the dam and run directly across the face, the wall membrane and reinforcement should be installed directly in line with the dam two by fours and shower membrane.

If “surface” trim is to be installed, the wall membrane and reinforcement will need to extend beyond the dam and pan membrane allowing room for mortar, bond coat, and tile to meet at the dam properly.

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These choices are just examples. The installer or designer will have to carefully decide these issues in advance.

Remember that prior to the floor mortar installation, crushed tile or small stones are used to keep the weep holes open. Commonly, the shower walls are floated and tiled prior to the floor mortar being placed. In this event, great care must be exercised to protect the shower pan against puncture or damage.

Why would the walls be floated and tiled prior to the floor mortar installation? This question can be answered in shower layout. In all layouts, the goal is to use full tile if possible. Once the shower walls and dam are floated with mortar, covered in the section on “Walls”, a measurement is taken from the drain to the shower wall furthest away. For every foot of distance, 1/4″ is added up. A mark using that measurement is made using a level on the wall mortar above drain height. This mark represents the required slope to the drain.

“Working lines” are then made all the way around the shower walls and dam. The walls and dam are tiled using full tiles above the working lines leaving only the floor mortar and tile left to be installed. Therefore, it is the lower row of wall tiles that gauge the level for the floor mortar and tile.

The floor mortar can be installed immediately after the wall mortar if desired. Working lines are established in a similar fashion as mentioned above. However, it might be a little more difficult to keep the floor mortar straight when installed in this way.

As seen in the illustration, reinforcing is necessary in the floor mortar and should be 2″ X 2″ 16/16-wire mesh placed in the center of the bed. The reinforcing should not touch the shower wall mortar and should not touch the drain. Also, a mortar admix should be used in an attempt to make the floor mortar more water-resistant.

The acceptable mortar mix ratios for shower floors are found in ANSI 108.1 A- and are 1 part Portland cement to 4 parts damp sand. You will note a richer mixture for shower pan mortar than regular floor mortar. Less sand (4 parts) means more cement (1 part) by volume.

A mortar admix should be used according to the Manufacturers instructions. The admix should be specifically made for use in shower pan mortar. The admix and richer mortar mixture is designed to make a very stable and water-resistant mortar on which to set the shower floor tile.

Shower pan mortar should be mixed like floor mortar, mentioned in the “Floors” section and should be packed and thoroughly tamped into place forming a dense mortar structure.

Tile can be set using a trowelable paste made of Portland cement and water on a non-cured mortar bed or dry set/latex modified Portland cement mortars on a cured bed. More on tiling using this method will be discussed later in this section.

In Figure B, we see a stone or ceramic tile lined shower using backer board walls. As discussed in the “Walls” section, some Manufacturers do not require a membrane between the backer board and wall studs. Local, state, or federal building codes may require the membrane. The reason some Manufacturers do not require the membranes is that the membrane itself is water-resistant. Best to check the codes and Manufacturers instructions for the product intended for use.

If a water-resistant membrane is required behind the backer board, use 15-pound roofing felt or 4 mil. Polyethylene-sheeting. Remember that not all backer boards are recommended for wet areas. If the backer board abuts existing drywall, a 1/8″ gap should be left and filled with a silicon sealant. This step is designed to deter water from wicking from the backer board into the drywall.

The backer boards are available in a variety of thickness and sizes. The most common are in the 3′ X 5′ size and 1/2″ thickness. Again with the backer board method, the framing must be true, square, and in plane to within 1/4″ in 10 feet.

The units should be fastened to wood or metal studs, at 16″ on center, using corrosion resistant screws or ring shank nails spaced 6″ on center. The fasteners should not be countersunk as the strength of the unit is most often in the top layer of the unit. Manufacturers will recommend a certain type of fastener that will work best with their product.

Nevertheless, the backer board is installed in the desired location in place of the mortar bed discussed in Figure A. Noted in the illustration is the need for furring strips behind the backer board. These furring strips are designed to bring the board out from the studs so that enough room exists for the shower pan membrane.

If the installer placed the backer boards directly on the studs over the membrane, a significant change in plane would result. If the shower pan membrane was 1/4″ thick when fastened correctly, the backer board would need to be furred out from the stud’s 1/4″. If a water-resistant membrane was required, it should be installed over the furring strips and allowed to lap over the shower membrane at least 4″.

When installing the units, a 1/8″-1/4″ gap should be left between the sheets, which is filled with bonding mortar. The joints and 90° corners should be taped using alkali resistant tape and bonding mortar. This step is designed to give the units a monolithic performance. Or a performance like a mortar bed.

Some backer boards can be installed so they nearly touch the sub floor and be locked into place by the shower pan mortar. Other backer boards will not perform well placed in that manner. Check what the Manufacturer recommends.

The shower dam is treated as in Figure A and is floated with wall mortar. There are Manufacturers of backer units that have prefabricated backer board units that are formed to fit over two by four dams and membranes. Carefully following Manufacturers instructions concerning their use is highly recommended.

Certain backer boards that are designed for use in showers require that the end of the unit be waterproofed if used next to a tiled shower pan. In this instance, a sealant is applied to the lower exposed end and the shower pan mortar is brought up tight to the backer board. It might be best to use the type of backer board that can be installed just above the membrane and be locked in by the floor mortar.

Do not rest the backer board on the membrane! Use removable shims or some device to keep the backer board off the membrane approximately 1/4″ when fastening. Be sure to be extra cautious not to puncture the membrane in this process.

The tiled shower pan is constructed in the same manner as Figure A.

Tile and trim can be installed using dry set or latex modified Portland cement mortar or certain organic adhesives (type 1 only) as directed by the Manufacturer.

Again, the use of a prefabricated shower pan is nothing more than one of the methods for prefabricated tubs found in “Tubs”.

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