Kumar IronUpdated
In this guide
Where bathroom leaks usually start
Tiles are waterproof, but grout lines, the joint between floor and wall, and the drain are not. Water goes through these weak points, runs behind the tiles and shows up later as damp in the room below or on the wall of the next room.
That is why waterproofing is done under the tiles, on the bare floor and the lower part of the walls, before any tiling starts.
Step 1: prepare the surface
- Chip out loose plaster, hollow patches and honeycomb. Fill them with cement mortar and let them cure.
- Fill cracks with a polymer modified mortar. Hairline cracks in a slab are common and should be opened into a small V groove first.
- Check the floor slope. Water should run toward the drain, not pond in the middle of the room.
- Clean away dust, oil, cement splatter and old paint. Remove any old coating that has peeled.
- Wet the surface lightly if the product needs a damp base, but do not leave standing water.
Step 2: cove the corners
The corner where the floor meets the wall is where most coats crack and leak. Build a cove, a curved fillet of polymer modified mortar, along that joint. Carry it up the wall for at least a few hundred millimetres, and take it to shower head height in the shower area.
Sources give different sizes for the cove, roughly 30 to 50 mm on each side. Use the size on your product's data sheet, and make it the same at every corner.
Step 3: treat the drain and pipe pockets
Pipes that pass through the floor or wall need the same care as the corners. Seal each pipe collar with a flexible sealant or a mesh reinforced coat, and do not leave a gap between the pipe and the slab.
The nahani trap, or floor trap, needs special attention. Fix its flange so the membrane can be tucked under it, and make sure the drain body does not wobble. A loose drain body lets water run behind the trap, and the coating cannot stop that. Check the trap before you coat, not after.
Step 4: apply the coating
There are three product families used in Indian bathrooms. The brands you will see at Dr. Fixit, Asian Paints, Birla and UltraTech all make products in these families. Pick the type your mason knows and the data sheet supports.
| Type | What it is | Where it suits |
|---|---|---|
| Cementitious slurry | Rigid cement based coat, applied to a damp base | Bond and basic coats on masonry or concrete |
| Polymer modified (SBR or acrylic) | Cement with a polymer added, flexible, brushed or rolled | Most bathroom floors and walls, with mesh at cracks |
| Polyurethane (PU) | Elastic liquid membrane, very flexible | Joints, cracks and areas with movement, usually under tiles |
Apply two coats. Let the first coat dry as the data sheet says, then apply the second coat at right angles to the first so that any pinhole in one coat is covered by the other. Use the full coverage rate on the pack. A thinned coat is the most common reason a good product fails.
Step 5: cure before testing
Curing means keeping the coat damp so it hardens properly. Many cement based products need light water spraying or a covering for several days, and the data sheet gives the exact time. Keep foot traffic and bucket work off the floor during this time.
Do not start the flood test until the coating is fully cured. Testing too early gives a false pass or a false fail.
Step 6: the flood test
Block the drain with a plug or tape. Flood the floor with clean water to about 25 to 50 mm, or 2 to 4 inches, and spread it so there are no dry patches. Mark the water level on the wall with a pencil or tape.
Leave the water for 24 to 72 hours. Many Indian masons test for 24 to 48 hours, while some specifications ask for 72. Check the level and look at the ceiling of the room below and the walls of nearby rooms. A level that holds and no damp anywhere means a pass. A falling level or a damp patch below means the coat has failed at that point, and you should find and repair that spot before tiling.
Step 7: tile after the test
Once the flood test passes, lay the protection screed or tile bed, then tile. Use a flexible tile adhesive and a flexible grout. At the floor and wall joint, finish with a silicone or polysulphide sealant rather than grout, because that joint moves.
Common mistakes
- Coating over dusty, oily or dry, absorbent concrete.
- Skipping the cove or the pipe collars because they look small.
- Thinning the product or applying one thick coat instead of two.
- Tiling before the coat has cured or before the flood test.
- Using a non-flexible cement coat where the floor will crack.
- Choosing a product with no data sheet or no coverage details.
Frequently asked questions
- How long should a bathroom flood test run?
- Typically 24 to 72 hours. Many masons use 24 to 48 hours. Use the test period in your product's data sheet or your contract, and mark the water level so you can see any drop.
- Can I tile over a waterproofing coat the same day?
- No. The coat must cure and pass the flood test first. Tiling early can trap moisture and break the bond.
- Which product is best for a bathroom floor?
- A polymer modified coat is a common choice for floors and walls. Where the floor has cracks or movement, a flexible PU or a mesh reinforced system is used at those points. Tell us your bathroom and we will suggest the type.
- Do I need to waterproof the walls too?
- Yes, at least up to a safe height on the walls and fully in the shower area. The exact height depends on your design and the data sheet, so ask your mason to follow the spec.