Waterproofing a bathroom floor and shower area before tiling
By Michał Białas · Last updated
Tile and grout are not waterproof; the waterproofing goes underneath. In a bathroom, a liquid membrane is rolled onto the floor in two coats and carried up the walls — about 4 in (100 mm) around the room and much higher in the shower — with sealing tape in every corner and collars around pipes and drains. A typical liquid membrane uses about 1.2 kg/m² per coat.
Why grout is not enough
Cement grout is porous, and even epoxy grout only protects the joints, not the cracks that can open later at corners and where floor meets wall. Water that gets through soaks into the screed, wood or drywall underneath, and the damage shows up months later as loose tiles, mold or stains below. A continuous membrane under the tile stops it.
What gets waterproofed
| Area | Typical treatment |
|---|---|
| Bathroom floor | Full membrane, turned up the walls about 4 in (100 mm) |
| Shower floor and walk-in shower | Full membrane; up the walls about 8 in (200 mm) on the floor area, and much higher on the shower walls |
| Floor-to-wall and wall-to-wall corners | Sealing tape, inside and outside corner pieces |
| Pipes through walls or floor | Pipe collars |
| Floor drain | Drain collar or the drain’s own sealing flange |
Shower walls are usually waterproofed well above the shower head height. The calculator covers the floor and its upstand; plan the wall area separately with the same coverage rate.
How much to buy
The membrane area is the floor plus the upstand strip around the walls: area + baseboard length × upstand height. Multiply by the coverage per coat and the number of coats. Sealing tape runs around the whole perimeter, plus 10% for overlaps. Inside corners come from the room shape; pipe and drain collars are counted by hand. The floor renovation calculator does this under Extras → Waterproofing.
Example 1 (meters): 2.4 × 1.8 m bathroom, 0.7 m door
- Floor 4.32 m²; perimeter 8.4 m; minus the door: 7.7 m of wall.
- Membrane area with a 100 mm upstand: 4.32 + 7.7 × 0.1 = 5.09 m².
- Two coats at 1.2 kg/m²: 5.09 × 1.2 × 2 = 12.2 kg → one 15 kg pail (three 5 kg pails would leave the same amount over).
- Sealing tape: 8.4 × 1.1 = 9.24 m → one 10 m roll. Inside corners: 4. Add a drain collar and pipe collars as needed.
- With a 200 mm upstand in a walk-in shower room: 14.1 kg → still one 15 kg pail, with less to spare.
Example 2 (feet): 8 × 6 ft bathroom, 28 in door
- Floor 48 sq ft = 4.46 m²; perimeter 28 ft = 8.53 m; wall length minus the door ≈ 25.7 ft.
- Membrane with a 4 in upstand: about 5.24 m² → two coats at 1.2 kg/m² = 12.6 kg (27.7 lb) → one 15 kg pail.
- Sealing tape: 8.53 m × 1.1 = 9.4 m (about 31 ft) → one 10 m roll; 4 inside corners.
Slopes and drains
A shower floor needs a slope toward the drain so water doesn’t stand on the tile. The slope is usually built into the screed or a pre-sloped tray before the membrane goes on, so the membrane follows it. Linear drains need the floor sloped in one direction only, which suits large tile; point drains need slopes from all sides, which usually means smaller tile or cuts toward the drain. Plan the drain height together with the screed thickness, because the drain body sets the lowest point of the floor.
Step by step
- Make sure the floor slopes to the drain where needed, is dry, clean and primed as the membrane requires.
- Embed sealing tape in the first coat at every floor-to-wall and wall-to-wall corner, with corner pieces at inside and outside corners.
- Fit collars around pipes and the drain and bond them into the membrane.
- Roll or brush the first coat over the whole area, including the upstands.
- Let it dry as the data sheet says, then apply the second coat, often at right angles to the first.
- Check the dry film thickness the product requires; skipped spots are the usual failure.
- Many installers do a flood test: plug the drain, fill with a little water and check for leaks after about 24 hours.
- Tile with a thinset that is compatible with the membrane.
Common mistakes
| Mistake | Result |
|---|---|
| One thin coat | Pinholes and thin spots let water through |
| No tape in the corners | Movement cracks the membrane where walls meet the floor |
| Stopping the membrane at floor level | Water gets in behind the baseboard tile |
| Forgetting pipe and drain collars | Leaks at the most common weak points |
| Grout in the corners instead of sealant | Cracks; use flexible sealant at changes of plane |
For choosing grout in wet rooms, see sanded, unsanded or epoxy grout.
Frequently asked questions
Do I need to waterproof the whole bathroom floor?
In most bathrooms, yes: the whole floor with an upstand along the walls. At the very least, the shower area and the area around the tub and toilet. Local building rules may set the minimum.
How many coats of liquid membrane?
Usually two, so that the required dry film thickness is reached without gaps. Follow the product’s data sheet.
Can I tile directly onto the membrane?
Yes, once it has cured, with a thinset the membrane maker approves.
Is a sheet membrane better than a liquid one?
Both work when installed correctly. Sheet membranes give a consistent thickness; liquid membranes are easier around pipes and odd shapes. The quantity method here is for liquid membranes.