Water Tank Waterproofing

A water tank is the only structure in a building designed to hold water in rather than keep it out, and that inverts most of the usual rules. The treatment is permanently immersed, it is often in contact with drinking water, and a failure is measured in tanker deliveries and a monthly bill rather than a stain on a ceiling. Leakproofs by Betterwork treats overhead tanks, underground sumps, fire tanks and process tanks across Pune and Pimpri-Chinchwad as liquid-retaining structures, to a different standard from a roof.

First establish that it is actually leaking

A society convinced its underground sump is leaking is, surprisingly often, losing water somewhere else. Before any treatment is quoted, the loss has to be attributed:

• A drop test. Fill the tank, isolate the outlet valve, mark the level and leave it. A tank losing level with the outlet shut is leaking. A tank holding level has a distribution or float problem, not a structural one.
• Check the overflow and float valve first. A stuck ball valve overflowing continuously is the single most common cause of a “leaking” overhead tank, and it is a plumber’s twenty-minute job, not a waterproofing contract.
• Look for the wet face. Damp on the outside of an underground sump wall may be the tank losing water outward or groundwater pushing inward — opposite problems with opposite treatments.

We run this before quoting. If the answer is the float valve, we will say so.

Potable water changes the material list

Anything in contact with drinking water must be certified safe for potable contact. This rules out most general-purpose coatings, several solvent-borne systems and effectively all bituminous products, which can taint water for months. Suitable options are food-grade epoxies and specific cementitious and crystalline systems carrying potable certification.

Ask any contractor for the product’s potable-contact documentation before work starts, not after. We state which product is being used and supply the certification with the job file. For a society this matters twice over — the committee is accountable to residents for water quality, and “the contractor said it was fine” is not a defensible record.

Treat the water-retaining face

Tanks are treated from the inside wherever access allows, because that is the positive side — the face the water pushes against. Pressure then holds the system onto the structure rather than trying to lift it off. For an underground sump this also means the tank has to be drained and taken out of service, which is a scheduling problem worth planning rather than discovering.

Where an underground tank is also taking groundwater in from outside, you have two opposing pressures and the specification needs to address both — usually a crystalline system that works within the concrete rather than a film on one face.

Surfaces, coves and the details that fail

Tank failures concentrate at the same places:

• The wall–floor junction. Treated as a coved fillet, never a sharp internal angle. A right-angled corner concentrates stress and is where a rigid system cracks.
• Inlet, outlet and overflow penetrations. Each sealed individually around the puddle flange, not simply coated over.
• Construction joints in tanks cast in more than one pour.
• Honeycombing in the wall, common in narrow sump shutters, which must be cut out and made good before treatment.
• The soffit of an overhead tank, where leakage shows first on the ceiling below and is often mistaken for a terrace leak.

Cleaning, curing and getting the tank back in service

Treatment needs the tank drained, mechanically cleaned back to sound concrete, and dry enough for the chosen system. Cementitious and crystalline systems then need a curing period, and epoxies need a full cure before filling — typically several days end to end. Every system needs the tank filled, flushed and drained at least once before it returns to potable service.

For a society that means a planned water-supply interruption, announced in advance, usually bridged with tanker supply. Sequencing matters: where a building has two tanks, they are done one at a time so supply never stops entirely.

Standards and hold testing

Liquid-retaining structures in India are covered by IS 3370, which sets out design and testing for water-retaining concrete — a different code from the general waterproofing execution codes. The practical inheritance from it is the hold test: the tank is filled and the level monitored over a defined period, with allowance for evaporation, before it is accepted. See our guide to the Indian Standards and water tank waterproofing and leakage repair.

A tank that has not been hold-tested has not been finished. Leakproofs by Betterwork fills and monitors the completed tank before issuing the warranty document, so the cover starts from a demonstrated result. Eligible completed work carries a written ten-year warranty on the treated area, with exclusions stated on the estimate before you approve it.

Swimming pools and process tanks

The same principles harden further for pools — continuous immersion, chlorinated water, movement joints in the surround, and epoxy grout rather than cementitious in the tiled finish. Industrial process tanks and ETP structures add chemical attack, which narrows the system choice to chemically resistant epoxies and PU. In both cases the leak-versus-evaporation question has to be settled by a bucket test before anyone quotes a treatment.