How should waterproofing be done in bathrooms to reduce water seepage?

The bathroom waterproofing issue you mentioned is a critical element in hotel projects that directly affects operational quality and guest experience. From the perspective of an MEP director, water seepage is often not the failure of a single procedure, but a systemic deficiency in design, material selection, construction, interface coordination, and subsequent operation and maintenance. To minimize water seepage, full-process control is required across the following five core dimensions:

I. Design Phase: Establish the Dual Defense Line of "Structural Self-Waterproofing" and "Additional Waterproofing Layer"

Many water seepage problems originate from the neglect of structural layer defects during the design phase. We cannot rely solely on waterproof coatings; we must address the root cause.

1. Structural Layer Requirements:

Standard Reference: According to Article 3.1.5 of GB 50037-2013 *Code for Design of Building Ground* and Article 4.1.2 of JGJ 298-2013 *Technical Code for Interior Waterproofing Engineering of Residential Buildings*, the bathroom structural floor slab should preferably adopt cast-in-place reinforced concrete slabs, with a thickness of not less than 100mm. In hotel projects, I require that structural slabs must be cast in one continuous pour; post-cast strips or construction joints are strictly prohibited within bathroom areas. This is the foundation of structural self-waterproofing.
Depressed Slab Design: For bathrooms with same-floor drainage, the depressed slab depth must accommodate the drainage pipe slope and backfill layer thickness. I typically require a minimum depressed slab depth of 300mm (including backfill layer and waterproofing layer). Insufficient depression leads to inadequate drainage slope, and long-term standing water will penetrate through capillary action.

2. Waterproofing Layer Design and Material Selection:

Rigid + Flexible Composite: In hotel projects, I typically require "polymer cement waterproofing coating (JS)" as the base flexible waterproofing layer (to accommodate minor structural deformation), followed by a layer of "cement-based capillary crystalline waterproofing material" as the surface rigid waterproofing layer (to enhance adhesion and facilitate tile installation). This is more reliable than a single coating.
Key Node Reinforcement: According to Article 5.3.4 of JGJ 298-2013, at all pipe roots, floor drains, internal and external corners, and wall corners, non-woven fabric or fiberglass mesh must be added as reinforcement layers, along with an additional waterproofing layer. I require a "dome" shape at pipe roots and rounded corners at wall bases (R≥50mm) to avoid stress concentration that could cause cracking.
Height Requirements: The waterproofing layer in shower areas should turn up no less than 2000mm (typically to the underside of the ceiling); the washbasin area no less than 1200mm; other areas no less than 300mm. This is a mandatory check item in hotel fit-out drawing reviews.

II. Construction Workmanship: Strictly Control the Details of "Three Parts Material, Seven Parts Workmanship"

The construction phase is the primary cause of water seepage, with emphasis on substrate preparation, coating application, and water-tightness testing.

1. Substrate Preparation:

Flatness and Dryness: Before waterproofing application, the substrate must be solid, flat, free of sanding, and free of standing water. Using a 2m straightedge, the flatness deviation shall not exceed 3mm. The moisture content must be below 9% (tested by the plastic film cover method). If the substrate is uneven, cement mortar leveling is required; otherwise, the waterproofing layer is prone to hollowing and detachment.
Cleaning and Wetting: Remove loose dust and oil stains. Spray water to moisten the substrate before application, but no standing water shall remain. This enhances the adhesion between the JS coating and the substrate.

2. Coating Application:

Cross-Hatch Method: Application must be done in 2-3 coats, with each coat applied in mutually perpendicular directions (e.g., first coat horizontal, second coat vertical) to ensure no missed areas or puddling. The interval between coats depends on product instructions (typically 2-4 hours), judged by touch-dry condition.
Thickness Control: The total thickness of the waterproofing layer shall not be less than 1.5mm (Article 5.3.3 of JGJ 298-2013). In hotel projects, I require spot checks using a wet film thickness gauge at every 10㎡ to ensure uniform thickness. Too thin fails; too thick cracks.
Pipe Roots and Floor Drains: Floor drains and pipe roots must be densely packed with waterproof plugging compound or dedicated sealant, with reinforcement treatment. For bathtubs and shower enclosures common in hotels, the joints with the floor must be treated with flexible waterproof tape or sealant, not relying on coating alone.

3. Water-Tightness Test and Protection:

Water-Tightness Test: After the waterproofing layer has cured (typically 48 hours), a 24-hour water-tightness test must be conducted (Article 5.3.7 of JGJ 298-2013). The minimum water depth at the shallowest point shall be no less than 20mm, and the corresponding location on the floor below shall be observed for no leakage or dampness. After the test, it must be signed off by the supervisor, contractor, and owner.
Protection Layer: After passing the water-tightness test, a cement mortar protection layer (20mm thick) must be applied immediately to prevent damage to the waterproofing layer during subsequent MEP pipe routing and floor tile installation. This is a common "secondary damage" risk point in hotel projects—workers stepping on or dropping tools causing damage to the waterproofing layer.

III. Fit-Out and MEP Interface Coordination: Prevent "Destructive" Water Seepage

As an MEP director, I pay particular attention to damage to the waterproofing layer caused by fit-out construction.

1. Pipe Chasing and Sealing: If MEP chases are made in bathroom walls or floors, they must be densely sealed with waterproof mortar or waterproof plugging compound, and the waterproofing layer at the chase opening must be extended outward by 100mm for reinforcement. Chasing directly on the waterproofing layer is strictly prohibited.
2. Floor Tile Installation: Floor tiles should be installed using the thin-set method (semi-dry mortar) to avoid excessive cement mortar thickness causing deformation of the waterproofing layer under pressure. Tile joints must be densely filled with grout or waterproof sealant to prevent water penetration through tile joints.
3. Floor Drain Installation: Floor drains must be installed at the lowest point of the waterproofing layer, and the connection to the drainage pipe must be sealed (using sealing rings or waterproof putty). Floor tiles around the floor drain should be laid in a picture-frame pattern or four-side slope to ensure smooth drainage without standing water.

IV. Acceptance and O&M: Establish a Long-Term Waterproofing Mechanism

1. Phased Acceptance: In addition to the water-tightness test, a shower test (simulating usage conditions, continuous showering for 2 hours to check for leakage) and post-water-retention observation (observing for 24 hours after water retention to confirm no dampness) are required.
2. O&M Records: During hotel operations, establish a waterproofing layer inspection archive. Annually conduct infrared thermography scans of weak points such as pipe roots, floor drains, and wall corners (temperature differentials can reveal internal water seepage), or conduct periodic water injection tests (simulating usage) for public area bathrooms.

Summary: Core Principles for Reducing Water Seepage

Design Level: Structural slab depression + rigid/flexible composite waterproofing + node reinforcement.
Construction Level: Substrate preparation → cross-hatch coating → thickness control → water-tightness test → protection layer.
Interface Level: Fit-out and MEP coordination to prevent chasing and tile installation from damaging waterproofing.
O&M Level: Establish inspection and infrared thermography detection mechanisms.

Finally, you must require the project department to strictly implement the sample-first system. In hotel projects, I usually require a standard guest room bathroom to be constructed first; after all waterproofing, water-tightness testing, and tiling processes are completed, the design, supervision, construction, and hotel management parties conduct a joint acceptance inspection together. Only after confirming the workmanship is correct should large-scale work begin. This is the most effective management measure to avoid systemic water seepage.

Engineering reference only — verify against project conditions and the latest codes.

How should waterproofing be done in bathrooms to reduce water seepage?