Kerdi Membrane Over Plywood

The Kerdi shower wall products, manufactured by Schluter Systems, are designed to provide a waterproof substrate for the installation of tile in showers. While one might argue that the tile and grout itself is waterproof, building code requires the use of water-resistant drywall or other waterproof products under tile around bathtubs and in shower stalls.

Various products are used to meet this need, including Greenboard, Wonderboard, Durock and HardieBoard. Each has their advantages, although they are not all waterproof. In fact, most are not, relying on their ability to withstand moist, rather than any waterproofing capability they might have. So, water seeping through cracks in the grout could still cause damage, even if it doesn’t damage these substrate materials.

The difference between these products and the Kerdi line of products is that Kerdi membrane and Kerdi board are designed to provide a waterproof barrier, thereby protecting the substrate they are mounted to. Kerdi membrane can be installed directly over normal drywall. There is no need to use a water-resistant product as the substrate.

For the Kerdi products to fully waterproof a tub or shower enclosure it is necessary to use the complete system, including the special Kerdi-Band tape and the seals to go around the shower’s plumbing fixtures. The company also provides shower pans, drains, curbs and insets for fully customizing the shower.

Kerdi Membrane System

The Kerdi Membrane system consists of the membrane itself, along with the Kerdi-Band tape and Kerdi corner. This provides the greatest flexibility in use, because the membrane can follow curved surfaces or be wrapped around corners.

Kerdi membrane is a flexible polyethylene sheet, with non-woven polypropylene on both sides. The polyethylene sheet provides an excellent moist barrier, which is flexible and does not break easily. But polyethylene itself is resistant to adhesive bonding, a problem which is solved by the addition of the non-woven polypropylene on both sides. The sheet has limited gap-bridging capability, allowing it to be used over seams in drywall or plywood, as well as surface fissures in plywood.

The Kerdi membrane system requires a 2” overlap at all seams; either with the membrane itself overlapping adjacent piece or with the use of Kerdi-Band tape. Special corner pieces are provided so that inside and outside corners can be covered in a seamless way. The tape then overlaps the corners, by a minimum of 2” to ensure that the entire surface is covered, without any gaps.

The Kerdi system is installed using thin-set mortar. Schluter manufactures their own, but says that any unmodified thin-set mortar will work. Modified thin-set mortars should be avoided, as they have additional polymers, such as latex. The mortar needs to be thinned out more than typical, but should still be able to hold the tooth from running a toothed trowel over it.

Since thin-set mortar works over almost any substrate, the Kerdi system can be used over drywall, plywood, or the various cement boards mentioned above. Some might feel that it is unnecessary to use both Kerdi and cement board, but the combination provides both waterproofing and water resistance, essentially a double protection in case of damage.

Once the Kerdi membrane system is installed and the mortar has been allowed to set, ceramic tile or stone can be applied directly over it using the thin-bed method.


At times, it might be better to use Kerdi-Board, rather than the Kerdi membrane system. Kerdi-Board is an extruded polystyrene foam panel, with reinforcing material on both sides, covered by fleece webbing for effective anchoring of thin-set mortar. This product is available in 3/16”, 3/8”, 1/2”, 5/8” 3/4”, 1” 1-1/2” and 2” thicknesses, allowing for a wide variety of installation options.

Being made of reinforced extruded polystyrene, Kerdi-Board can be easily cut with a utility knife, eliminating the need for saws and more complex cutting systems. thicker panels can be used to make countertops, benches and other structural elements in the bath. Panels are installed using special screws, with a pierced ash, roughly the same size as a fender washer. These come in the kit with the board or can be purchased separately.

Since Kerdi-Board provides structural strength, it is not necessary to install it over drywall or plywood. One-half inch thick Kerdi-Board can be mounted directly onto studs set at 16” on center. If the studs are set farther apart, it is recommended to add additional structure to support the boards.

As with the Kerdi membrane system, all seams on Kerdi-Board need to be sealed with Kerdi-Band tape and the Kerdi corner designed for use with the tape. Failure to install the tape eliminates the waterproof nature of the product, as water can always seep through the seams.

Kerdi-Board is also available in the Kerdi-Board-V configuration. This is a board which has been grooved vertically, allowing it to be installed around curved surfaces, such as a curved wall or around a column. Like the Kerdi-Board and Kerdi membrane, the Kerdi-Board-V provides a waterproof surface, when properly installed.

Kerdi-Board can also be used to make a shower curb, although an actual curb is included in the product line. It is available in 38”, 48” and 60” lengths, which can be joined together for larger shower enclosures.

Showers Pans

The Kerdi system also includes a variety of different size shower pans, designed to complete the waterproofing of the shower enclosure. These pans help to ensure a full seal with the walls and with the drain, eliminating one of the prime places that leaks happen in showers. There is no need for a mortar bed when using these shower pans, as they are waterproofed in the same way the rest of the Kerdi system is.

The system includes not only the shower, but the drain and seal to ensure that the shower pan will not leak around the drain.

Trays are made of expanded polystyrene with sloped sides to the center drains. They come in 38” x 60”, 48” x 60”, 48” x 72”, 60” x 60” and 72” x 72” sizes. An offset tray, the Kerdi-shower-TS is available in a 38”x 60” size for bathtub replacement. All of these, and their TT thin perimeter line (in the same sizes), can be cut with a utility knife for ease of adjustment.

For a larger shower enclosure or one that has an unusual shape, Kerdi shower pans can be used in conjunction with mortar to fill in areas around the pan. As the pan and mortar will be covered with tile, the transition between the pan and the mortar will not be visible, as long as they are blended together with a trowel.

kerdi, waterproof membrane
Kerdi Membrane, Andrew Hart


To complete a custom shower installation, the Kerdi waterproofing system also includes prefabricated shower niches, which are fully sealed, ready to be inset into the walls. These will work with either the Kerdi-board system, or when using Kerdi membrane over another substrate.

All of the shower niches are 12” wide, and come in 6”, 12”, 20” and 28” heights. They all have a 2” wide, ½” thick flange all the way around, allowing them to be inset into drywall or Kerdi-Board. The flange provides the necessary 2” of width for the installation of the Kerdi-Band tape, ensuring that there are no leaks around the edges of the niche.

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