Waterproofing a Bathroom: The Sequence That Keeps Water in the Room

Wet rooms leak at thresholds, drains and pipe penetrations, not through the tiles. This is the order of work, with the test that proves it before anything is covered.

Tiles and grout are not waterproof. Water passes through grout joints, around the drain, down the wall behind the tiles and across the threshold into the adjoining room, which is why a hidden waterproof layer is installed beneath the finish. Done in the right order, a bathroom stays dry for decades. Done out of order, the leak appears on the ceiling below six months after handover, and the repair means removing a finished room.

Before stage one

Before the waterproofing contractor arrives, the room has to be finished as a shell in the right sequence: all penetrations in place, falls formed, substrate sound and dry, and the drain fixed at the correct level. Waterproofing applied over an unfinished substrate will be cut into by whoever comes next, and every cut is a leak.

• All pipework and drains installed, pressure tested and fixed in final position.
• The substrate repaired, clean, free of laitance and dry to the level the system requires.
• Falls formed in the screed to the drain, with no flat areas or reverse falls.
• The drain body set so the finished tile surface will sit at the correct relative level.
• The system selected with its primer, detailing tapes and compatible adhesive for the tiles above.

Stage 1: Fix the services and prove them before anything covers them

Hot and cold supplies, the waste and any concealed cistern are installed, supported and pressure tested. A pressure test done after tiling is a test you cannot act on. Fix the drain body rigidly so it cannot move when a later trade stands on it, because a drain that shifts breaks the seal between the body and the membrane.

• Deliverable: A signed pressure test certificate for the pipework serving the room.
• Owner: Plumbing subcontractor, witnessed by the supervising engineer.
• Approval gate: No screed or waterproofing until the pressure test is accepted.

Stage 2: Form the falls and finish the substrate

The screed is laid to fall towards the drain across the whole wet area, with the fall continuous rather than created by a dish near the outlet. Check with a level and with water. The surface must be sound, free of cracks and laitance, and finished to the texture the waterproofing system needs, which is usually a wood float rather than a polished finish.

• Deliverable: A level survey or water test showing falls to the drain with no ponding.
• Owner: Screeding contractor.
• Approval gate: Falls verified by the engineer before the waterproofing applicator mobilises.

Stage 3: Treat the junctions and penetrations first

Wall to floor junctions, internal and external corners, pipe penetrations and the drain flange are reinforced with the system tapes, collars and bands before the main area is coated. These are where movement concentrates and where water finds a path, and they need the additional layers the manufacturer specifies.

• Deliverable: Photographs of every treated junction and penetration before the main coats go on.
• Owner: Approved waterproofing applicator.
• Approval gate: Junction treatment inspected and recorded as a hold point.

Stage 4: Apply the membrane to the full specified extent

Coverage extends over the whole floor, up the walls to the height in the specification, which is higher in a shower area, and across any threshold. Liquid systems are applied in the specified number of coats at the specified wet film thickness, with drying time between coats, and the quantity used is checked against the area covered as a crude but effective verification.

• Deliverable: An application record with product, batch, number of coats, area covered and material used.
• Owner: Approved applicator with the manufacturer technical representative where the warranty requires it.
• Approval gate: Coverage rate checked against the manufacturer figure before testing.

Stage 5: Flood test the room and hold it

Plug the drain, fill the floor with water to a shallow depth and leave it for the period the specification states, with the water level marked. Inspect the ceiling and walls of the space below, and check the level has not dropped beyond what evaporation explains. This is the only direct proof you will ever have that the membrane is continuous.

• Deliverable: A flood test record with start and end times, water levels and photographs of the space below.
• Owner: Contractor, witnessed by the supervising engineer.
• Approval gate: Tiling cannot start until the flood test is signed off.

Stage 6: Protect the membrane and tile over it correctly

Once the test passes, protect the membrane from the trades that follow. Tiling uses an adhesive compatible with the membrane, because some adhesives do not bond to some membranes, and that incompatibility is only discovered when tiles come loose. Movement joints in the tiling at the perimeter and at the thresholds are formed as drawn.

• Deliverable: A tiling setting out drawing with joint positions and a confirmed adhesive compatibility statement.
• Owner: Tiling subcontractor.
• Approval gate: Adhesive compatibility confirmed in writing before the first tile is fixed.

Stage 7: Finish the drain, thresholds and sanitaryware without piercing the layer

Fit the drain grating, the threshold and the sanitaryware with fixings that do not penetrate the membrane, or where a penetration is unavoidable, seal it with the system products. Wall hung sanitaryware fixed through a tiled wet wall is a frequent and avoidable breach, which is why the supporting frame should be installed before waterproofing.

• Deliverable: A fixing record showing how each item was secured and any penetration sealed.
• Owner: Installation subcontractor with the applicator consulted on penetrations.
• Approval gate: No fixing through a waterproofed surface without an approved detail.

Handovers between stages

The two handovers that cause leaks are from the plumber to the waterproofer, and from the waterproofer to the tiler. In the first, the critical transfer is that all penetrations are final and tested, because any pipe added afterwards means cutting the membrane. In the second, it is compatibility and protection: the tiler has to know which adhesive bonds to this membrane and has to work without damaging it. Make both handovers a short recorded inspection rather than a verbal confirmation between trades who may not share a language.

How you know it is finished

The room is finished, from a waterproofing point of view, when there is a flood test record signed and dated before tiling, an application record showing coverage, photographs of every junction and penetration before they were covered, and a compatibility confirmation for the adhesive. Keep those four documents per wet room. If a leak ever appears, they tell you immediately whether the membrane was ever proved, which decides how the conversation with the contractor goes.

Domira builds and supervises projects across Saudi Arabia, and this is the level of detail its site teams work at. Send the drawings or the site, and Domira will tell you what the specification has to say before anyone prices it.