Top five waterproofing mistakes with hard-surface installations

By Dale Kempster

Most tiled shower failures are completely avoidable and are due to mistakes made while waterproofing. Unfortunately, when a mistake is made the consequences can be severe and the repair cost can be excessive. Following installation guidelines in the TTMAC Specification Guide 09 30 00 for Tile Installation can help avoid these five all-too-common waterproofing mistakes:

1: Believing tile, grout and sealer are inherently waterproof

Tile used with epoxy grout, or a sealer applied over cementitious grout, is not waterproof, and is particularly susceptible to vapour and steam. One university dormitory had approximately 50 failed showers in four buildings. Only months after installation, water was leaking and running down the hallways. The installer assumed the tile was waterproof so the waterproofing was only applied 300 mm (12 in.) up the wall. To avoid this failure, waterproofing must be installed to at least the shower head, and preferably to the ceiling, as indicated in detail 319 SR of the specification guide.

2: Creating little or no slope to drain

When using a traditional mortar bed, there is a shower-pan liner that needs to have a pre-slope of approximately two per cent. This pre-slope is critical to ensure subsurface moisture can actively work its way to the two-stage drain. Additionally, weep holes must be kept open so moisture can evacuate from the area. Another option is to use a topical waterproofing membrane meeting American National Standards Institute A118.10, in combination with a drain that has an integrated bonding flange. This method eliminates the need for a pre-slope since the waterproofing is atop the substrate.

3: Creating penetrations through the waterproofing membrane

Causes of penetrations and punctures in waterproofing membranes can be as simple as nailing a piece of backerboard through the membrane on a curb, or using nails or screws as spacers to stack tile on a shower wall. Penetrations through the waterproofing layer should be avoided at all costs, especially on horizontal surfaces where water can easily collect. The 09 30 00 manual indicates that for shower receptors, “All openings and cuts must be treated to ensure waterproof integrity.”

4: Lack of coverage

The TTMAC 09 30 00 Tile Installation Manual specifies tiles installed in wet areas should have a minimum coverage of 95 per cent. The tile typically needs to be back-buttered, meaning a thin coat of mortar is applied to the tile’s back with the flat side of the trowel. When there is insufficient coverage, the tile or stone often displays some shading, and, since the voids collect water, the chances for efflorescence increase. There is also a much greater risk of tiles delaminating.

5: Inappropriate tile and stone selections

Stone such as travertine, limestone, or sandstone is often specified because the home or building owner likes the material’s esthetic. These are sedimentary stone products, which typically means they are quite porous and require ongoing maintenance such as frequent sealing. The 09 30 00 manual even goes to the extent to recommend: “Sealing of the stone prior to grouting may be required to effectively reduce the stone’s porosity.” An alternative is a porcelain tile, which can be made to look just like stone.

Glass tile has also become popular, especially as cheaper imports become more readily available. Often, these glass tiles are in mesh-mounted sheets. Before installing them, the coverage of the mesh should be verified. The adhesive holding the mesh should not cover the majority of the tile back. Too much adhesive is a bond-breaker as the thin-set will only bond to the adhesive rather than to the tile itself. A simple test is to place a sheet of the mosaic into a bucket of water to sit overnight. If the mesh comes off easily, the tile is unsuitable for use in a wet area.


A properly designed and installed tiled shower should last as long as the building it is in. By avoiding the most common waterproofing mistakes installers can ensure their projects are truly sustainable and have a lower long term cost. 

Dale Kempster is technical director with Schluter-Systems. The Terrazzo, Tile and Marble Association of Canada was founded in 1944 to promote standardized installation techniques and to provide technical resources.

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