Why flexible cement suits internal wet areas
Flexible cement is a two-component material: a water-based resin blended on site with inorganic aggregate, usually cement-based. It is essentially cement with elasticity, and its compatibility with concrete substrates is the best of any waterproofing family.
Three practical considerations matter indoors: no solvent odour, so no extended ventilation; tiles can be laid directly over it, with sufficient adhesion; and sensible cost for areas that are small but demand reliability.
Standard sequence
1. Substrate preparation — remove laitance, level uneven areas, and clear oil, dust, debris and organic growth. Moisture content below 10%.
2. Details first — treat gully surrounds, pipe penetrations and internal corners with sealant or reinforcing mesh before the main field, not afterwards.
3. Primer — apply a water-based primer evenly to consolidate and seal the substrate, around 0.1 kg/m².
4. Flexible cement — approximately 1.0 kg/m² horizontal and 0.8 kg/m² vertical per coat. Carry the membrane at least 300 mm up walls, and to 1800 mm or above within shower enclosures. Touch dry in about 1 hour; allow at least 1 hour between coats.
5. Flood test — carry out a 24-hour flood test on completion and confirm there is no ingress before following trades proceed.
When to use the heavy-build grade
Where greater film thickness is required in a single pass, the heavy-build flexible cement achieves up to 3 mm without delamination, at around 1.2 kg/m² per 1 mm horizontally and 0.6 kg/m² per 0.5 mm vertically. It suits poorer substrates, or where levelling and waterproofing are combined.
Dealing with an existing leak
A bathroom that is already leaking cannot be resolved by recoating the surface. Ingress through cracks or joints should first be treated by injection — a water-based acrylic injection material replaces solvent-based foaming grades, is ready to use, fast-curing, highly elastic and does not remain tacky once dry.
Diffuse seepage is treated with a crystalline penetrating material, which can be applied to damp surfaces and reacts with the concrete to seal pores. One unit applied in two coats covers approximately 25–33 m² at 1–1.5 mm thickness.
Detailing decides the outcome
- Gully surrounds — the membrane must be carried into the drain body itself; this is the single most common leak path
- Pipe penetrations — seal the annulus between pipe and slab before applying the membrane
- Internal corners — form a cove or add reinforcing mesh to avoid stress concentration at the right angle
- Thresholds — carry the membrane beyond the threshold and terminate it properly
- Around baths — confirm the bath position first; the membrane must extend beneath it