Why Most Bathroom Waterproofing Fails
Homeowners often assume that glazed ceramic tiles and grout make a bathroom waterproof. In truth, tile grout is porous: water continually seeps through microscopic micro-cracks between tiles and accumulates inside the sunken floor slab. If the structural concrete slab lacks an engineered elastomeric membrane, that trapped water migrates horizontally through walls and drips into the ceiling below.
The 5-Step GS Construction Bathroom Waterproofing System
- Surface Mechanical Cleaning: The bare concrete slab must be chipped clean of all mortar droppings, laitance, oil, and dust. Honeycombed concrete patches must be routed out and grouted with non-shrink structural polymer mortar.
- Angle Fillets (Coving): 90-degree right angles between the floor slab and vertical walls create high stress concentrations where coatings tear. Form a smooth 45-degree triangular fillet (coving) of 50mm x 50mm using rich polymer-modified cement mortar along all corners.
- Pipe Collar Sealing: Drainage pipes penetrating the concrete slab must be pre-sealed with expanding hydrophilic waterstop bars and high-grade polyurethane sealant around the sleeve joint.
- 2-Coat Polymer-Modified Cementitious Membrane: Apply two cross-directional coats of flexible polymer cementitious waterproofing (e.g. Dr. Fixit Fastflex / Fosroc Brushbond) across the entire sunken slab floor and extend the coating up the vertical walls to at least 300mm above finished floor level (and up to 2,100mm in the shower enclosure!).
- 48-Hour Ponding Test: Fill the sunken slab with 100mm of standing water. Inspect the slab underside from the room below for 48 continuous hours. Only after certifying zero dampness or moisture rings should lightweight cinder filler and screed mortar be placed!