Renovation guide · Bathrooms
Waterproofing a shower: what goes behind the tile
Tile is the part you see, and the part people assume keeps the water in. It isn’t. What keeps a shower dry is the layer under it.
6 min readUpdated By SS Carpentry and Renovations
Key takeaways
- Tile isn’t waterproof: water gets through the grout into the mortar and backing. The membrane under the tile is what keeps a shower dry.
- Ontario’s Building Code asks for a waterproof finish at least 1.8 m up a shower wall, over moisture-resistant backing.
- A traditional pan liner lets water through the mortar and drains it out; a bonded membrane stops it right under the tile, so the shower dries between uses.
- A flood test before any tile goes on proves the membrane holds. Ask for one.
Isn’t tile waterproof?
Not on its own. The Ontario Building Code lists tile among the materials it calls a “waterproof finish”, and that’s where the idea comes from. The makers of shower waterproofing see it differently. In one handbook’s words, “contrary to popular belief, the tile covering itself is not waterproof”: water gets through the grout lines into the mortar and the backing behind.
Both are true. The Code sets the minimum finish a shower must have; what keeps the wall and floor behind it dry for years is a waterproof layer under the tile, and how well it’s built.
- A: Tile and grout: the finish the Code calls waterproof, though water gets through the grout lines
- B: Waterproof membrane, continuous from wall to floor
- C: Moisture-resistant backing on the studs
- D: Floor sloped to the drain
- E: Drain with a flange the membrane bonds to
- F: Waterproof finish to at least 1.8 m above the floor (Ontario Building Code)
What does the Building Code require?
| Article | What it requires |
|---|---|
| 9.29.2.1 | A waterproof finish at least 1.8 m above the floor in a shower stall, 1.2 m above the rim of a tub with a shower, and 400 mm above the rim of a tub without one. |
| 9.29.2.2 | The finish can be ceramic, plastic or metal tile, sheet vinyl, tempered hardboard, laminated decorative sheets or linoleum. |
| 9.29.10.4 | Tile on walls around tubs and showers goes over moisture-resistant backing. |
| 9.29.10.5 | The joint between wall tile and a tub or shower is caulked with a mildew-resistant sealant. |
Read together, the Code sets the height and the backing. It doesn’t tell you which waterproofing method to use behind the tile. That choice is yours and your contractor’s, and it’s the one that decides how the shower ages.
What are the two ways to waterproof a shower?
The traditional method builds the waterproofing deep in the floor. A mortar bed is sloped to the drain, a pan liner goes over it and is clamped into the drain, and a second mortar bed goes on top for the tile. On the walls, a moisture barrier behind the backing laps over the liner. Water passes the tile, soaks through the top bed and runs out through small weep holes in the drain. The membrane maker calls it a “water in, water out” system, and notes the bed can stay wet if the slope under the liner is skipped or the weep holes clog.
A bonded membrane moves the waterproofing to just under the tile. The membrane is bonded over the sloped bed or a foam tray, up the walls over the backing, and to a flange on the drain; tile is set directly on it. Water stops at the membrane, the bed and backing stay dry, and the shower can dry out completely between uses. The membrane also slows water vapour into the wall.
Traditional pan liner
Water passes the tile and drains through the mortar
- Tile and grout
- Thinset
- Mortar bed
- Sloped mortar bed (pre-slope)
- Subfloor
Bonded membrane
Water stops right under the tile
- Tile and grout
- Thinset
- Thinset
- Sloped mortar bed or foam tray
- Subfloor
Sheet or liquid membrane?
Bonded membranes come in two forms. Sheet membranes are a fabric set in thinset, with bands over the seams and pre-formed pieces at the corners. Liquid membranes are rolled or brushed on, usually in more than one coat, and cure into a continuous rubbery skin.
Either way, the maker’s instructions govern the installation, including the drying time before testing: one sheet system asks for at least 24 hours before a flood test, and one liquid membrane at least 12, longer in cool or damp conditions.
What is a flood test?
It’s the one moment anyone can see the waterproofing work. Once the membrane has cured, the drain is plugged, the shower base is filled with water and left, and the water level is checked for any drop. A membrane maker calls it strongly recommended before setting tile, because after the tile is on, a leak is found the expensive way.
What keeps it dry for years?
- An exhaust fan. Whatever the waterproofing, the membrane maker says a bathroom needs adequate ventilation to manage water vapour and condensation.
- Sealant where the Code asks for it. The joint between wall tile and a tub or shower takes a mildew-resistant sealant.
- A careful finish. At the final walkthrough on our bathrooms, every tile joint and silicone bead is checked with you.
Does it need a permit, and how long does it take?
Replacing a shower where the drain and supply stay put is replacing an existing fixture, which the City of Ottawa leaves off its permit list. Moving the drain or the shower is a plumbing alteration, which needs a building permit; our guide to which renovation work needs a building permit in Ottawa has the City’s lists. On the bathrooms we renovate, plumbing is done by a licensed plumber and the waterproofing and tile are ours.
A bathroom typically takes 3 to 4 weeks on site. For the decisions that come before that, from layout to fixtures, see how to plan a bathroom renovation step by step. And if the shower is one part of a bigger home renovation in Ottawa, the waterproofing is still worth its own question at the estimate.
Guidance only, not a design or code ruling. The Ontario Building Code and each product’s installation instructions set what a shower must meet.