GRP, epoxy and cementitious linings all waterproof a concrete water tank, but they suit different tanks. GRP is a tough fibreglass layer that bridges cracks; epoxy is a hard, smooth coating for sound concrete; cementitious coatings bond to damp concrete and suit underground tanks. For a drinking-water tank, the deciding rule is the same for all three: the product must be approved for potable water.
Why tanks leak in the first place
Most concrete tanks in villas and buildings leak at the same points: cracks in the walls and floor, the joints where walls meet the floor, and around pipe inlets, outlets and the overflow. Old linings also fail by blistering or peeling, which leaves bare concrete in contact with water. Signs include a damp wall or wet ground next to the tank, the level dropping when no water is used, rust-coloured water, and flakes of old coating in the water.
GRP (glass-reinforced plastic)
GRP is glass-fibre matting laid in liquid resin over the surface and finished with a top coat. It cures into a hard, seamless lining.
- Best for: tanks with cracks or joints that keep moving, old tanks with failed coatings, and tanks that need a very tough surface.
- Strengths: bridges hairline cracks, resists impact, no joints for water to pass through.
- Watch for: it needs a dry, sound surface or it will blister, and the resin and top coat must be a drinking-water grade.
Epoxy coatings
Epoxy is a two-part resin coating applied in layers to the concrete.
- Best for: structurally sound tanks where you want a smooth, hard surface that is easy to clean.
- Strengths: a hygienic, glossy finish; quick to apply on a good surface.
- Watch for: epoxy is rigid, so it can crack again if the concrete underneath is still cracking. Cracks should be repaired, often with injection, before coating.
Cementitious coatings
Cement-based waterproof coatings, including crystalline types that react with moisture in the concrete.
- Best for: underground tanks and tanks where the concrete cannot be fully dried, including where groundwater pushes from outside.
- Strengths: bond to damp concrete and can work against water pressure from the outside face.
- Watch for: they are less flexible than GRP and need a sound substrate.
How to choose
- Cracked tank, or an old lining has failed: GRP is usually the safest choice.
- Sound concrete, hygiene is the priority: epoxy.
- Underground tank with damp concrete or groundwater: cementitious or crystalline, sometimes with GRP or epoxy on top.
Many jobs combine systems: crack injection first, then the lining.
The drinking-water rule
Whatever the system, a tank that holds drinking water needs a lining approved for that use. Ask for the product's approval for potable water from the authority or a recognised certificate such as NSF/ANSI/CAN 61 or WRAS, and make sure it is cured fully and the tank is flushed and cleaned before refilling. A lining that is not made for drinking water can taint the water even if it stops the leak.
What a proper tank job looks like
- Drain, desludge and clean the tank.
- Inspect and repair cracks, joints and pipe entries.
- Prepare and dry the surface.
- Apply the lining, with extra reinforcement at corners and fittings.
- Cure, flush, clean and refill.
We line tanks with GRP or a coating chosen for the tank's condition, and every potable tank gets a drinking-water-grade system. Send photos of the tank and where it leaks, and we will recommend the right lining with a fixed quote. More on the full service: water tank waterproofing.