Tiling and shower waterproofing

Tile is not waterproof and grout is not waterproof. Both are decorative surfaces over something that is, or should be. Every shower failure in a house comes down to what was or was not behind the tile, and by the time it shows on the ceiling below, the repair is the whole shower.

Step one: the substrate. Drywall does not belong in a wet area. The Code requires tile on walls around bathtubs or showers to be applied over moisture resistant backing (OBC 9.29.10.4, Moisture Resistant Backing), meaning cement backer board, a foam board system or a glass-mat gypsum product rated for wet areas. Green board is not a waterproofing layer and is not adequate on its own in a shower.

Step two: the waterproofing membrane. This is the layer that actually keeps water in the shower. Two approaches dominate. Sheet membranes are bonded over the backer with thinset and overlapped at seams. Liquid-applied membranes are rolled or trowelled on in two coats with reinforcing fabric at the corners and changes of plane. Either works well when installed as a complete system, with the manufacturer's own corners, seam tape, pipe seals and drain flange. Mixing components from different systems is the most common way a membrane fails, and it usually voids the manufacturer's warranty.

Step three: the pan and the slope. A shower floor has to slope to the drain, conventionally about 2 per cent, which is 20 millimetres of fall per metre. That is achieved either with a pre-formed sloped foam or acrylic pan, which is faster and dimensionally reliable, or with a site-built mortar bed, which allows any shape and any drain position but depends entirely on the skill of the person floating it. The drain flange has to be integrated into the membrane rather than sealed to the tile, because water that gets past the grout must reach the drain rather than the subfloor.

Step four: the flood test. Before a single tile goes down, the drain is plugged, the pan is filled to just below the curb and left for 24 hours, and the ceiling below is checked. This costs almost nothing and is the only way to know the pan is sound. It is also the step most often skipped. Ask for it in writing and ask to see it, because once the tile is in nobody can test anything.

Step five: tiling and the joints that move. Large format tile needs a medium-bed mortar and back-buttering to avoid hollow spots. Movement joints belong at changes of plane, at the perimeter and at the tile-to-tub joint, which the Code requires be caulked with a material conforming to CAN/CGSB-19.22-M (OBC 9.29.10.5, Joints between Tiles and Bathtub). Grout in a corner cracks. Sealant does not.

How long it takes. A standard shower enclosure of about 4.5 square metres of tile, from backer board to grouted and sealed, is four to seven working days including cure times and the flood test. Large format tile and mosaic floors take longer. A full bathroom floor and shower together is usually a week to ten days. Cure times are not padding: thinset under large tile needs time before grouting, and grout needs time before the shower is used.

What it costs in Ottawa in 2026. Indicative. Tiling labour commonly runs $80 to $160 per square metre for standard format on walls and floors, rising to $160 to $260 for large format, mosaic, herringbone or anything that needs a mitred edge. A complete waterproofing system for a standard shower, materials and labour, is usually $1,500 to $3,500. A site-built mortar pan is $800 to $1,800. Niches are typically $300 to $700 each and benches $500 to $1,200. Tile material is separate and ranges from $30 to well over $200 per square metre.

What makes an Ottawa bathroom different. Exterior walls and the vapour question. A shower on an exterior wall in a climate that spends four months below freezing drives warm humid air toward a cold cavity, and if that air can get past the tile assembly it condenses inside the wall. A waterproofing membrane on the room side is also a vapour retarder, which is generally helpful here, but two vapour retarders with insulation between them can trap moisture. That is a detail worth asking about specifically on an exterior wall in an Ottawa house. The second factor is movement: much of the city sits on sensitive marine clay that shrinks and swells seasonally (NRC, Geotechnical properties of Leda clay in the Ottawa area), and houses on it move enough that rigid tile assemblies without proper movement joints crack along predictable lines. An uncoupling membrane under floor tile is cheap insurance in this city.

Who should do the work. Tiling and waterproofing are not licensed trades in Ontario and there is no certificate you can ask to see. What exists instead is manufacturer training, and it matters: several membrane manufacturers will only extend their system warranty where the installer is certified on that system. Ask which system, ask whether they are certified on it, and ask for the system warranty document rather than a verbal assurance. Liability insurance and WSIB coverage apply as they do to any contractor on your property (WSIB, Expanded Compulsory Coverage in Construction). Any change to the drain or supply piping is separate and belongs to a certified plumber, because plumber is a compulsory trade in Ontario (Skilled Trades Ontario, Plumber, trade code 306A).

What to ask before you hire. Which waterproofing system, by product name. Are you certified on it, and does that give me a system warranty. Are all components from that one system. Pre-formed pan or mortar bed. Will you flood test, and for how long. Is there an uncoupling membrane under the floor tile. What backer board.