Water Tank Leakage Treatment for Overhead and Underground Tanks

The problem on site

A water tank that will not hold its level is one of the most persistent complaints we hear from builders, facility teams and housing societies across Delhi NCR. The tank is filled in the morning and by evening the level has dropped further than the day’s usage explains. The pump runs longer, the electricity bill climbs, and in summer the building runs short of water while the tank is nominally full.

The visible evidence usually appears somewhere other than the tank itself. On an overhead tank it shows as a damp patch spreading across the terrace slab soffit or the ceiling of the flat below, often with white salt staining and peeling paint. On an underground tank or sump it shows as a permanently wet patch on the adjoining basement wall, or water standing in the pump room that nobody can account for.

The common first response is to empty the tank, apply a coat of waterproof cement paint or a plaster patch on the inside face, and refill. It holds for a few weeks, sometimes a single season, and then the damp patch returns in exactly the same place. The reason is straightforward: a tank wall carries a standing head of water against it continuously, and a coating that merely sits on the surface is pushed off from behind every time the tank is filled.

Why it happens

A water tank is a harder duty than almost any other waterproofing job on a building. A terrace gets wet and then dries out. A tank wall never dries — it holds hydrostatic pressure twenty-four hours a day, every day, and any weakness in the concrete is under constant load. Water finds the path of least resistance and exploits it relentlessly.

In reinforced concrete tanks, that path is almost always one of the following:

• The base-to-wall construction joint: The single most common leak path in any RCC tank. The base slab and the walls are poured separately, and the kicker joint between them is a planned discontinuity in the concrete. If it was not properly prepared, or no water bar was cast in, it leaks.
• Drying shrinkage and thermal cracks: As concrete cures it shrinks, producing fine hairline cracks across the wall face. Individually they look harmless. Under a standing head of water they are continuous capillary routes straight through the section.
• Honeycombing behind congested reinforcement: Tank walls are heavily reinforced. Where concrete could not flow fully around the steel, a network of voids is left behind, and water moves through it freely under pressure.
• Tie-rod holes and form-tie sleeves: Formwork leaves openings right through the wall. If they were not plugged solid with a non-shrink material, each one is a direct route from the water to the outside face.
• Pipe penetrations: Inlet, outlet, overflow and scour pipes all pass through the wall or base. The annulus around an unsealed sleeve leaks reliably, and these are frequently missed because they sit behind pipework.
• Failed or absent internal treatment: Many older tanks were finished with ordinary cement plaster and nothing else, or with a surface paint that has since debonded and now traps water behind it.

There is a further complication on underground tanks and sumps. When the tank is full, water pushes outward from inside. When it is emptied for cleaning and the surrounding ground is saturated, water pushes inward from the soil. The wall is loaded from both directions at different times, which is precisely the condition that lifts a surface film off the concrete.

How Space Arc Engineering approaches it

Space Arc Engineering treats a leaking tank as a diagnostic problem before it is a materials problem. Applying a waterproofing system without first establishing where the water is actually going is how most tank repairs end up being done twice. As an authorised project distributor and technical solution provider, our involvement starts with finding the leak path, not with quoting a product.

The first question is whether the tank is leaking at all. A dripping outlet valve, a passing float valve or a buried supply line produces exactly the same symptoms as a leaking tank wall and costs nothing like as much to fix. We establish this by isolating the inlet and outlet, marking the water level, and observing the drop over a period with no draw-off. If the level holds with the plumbing isolated, the structure is sound and the problem is in the pipework.

If the level does drop, the tank is emptied and cleaned, and we inspect it dry alongside your site team. We map the cracks, trace the base-to-wall joint, check every penetration and sound the walls for honeycombing and hollow plaster. What this produces is the decision that governs the whole specification: is the water leaving through a small number of discrete defects, or is the wall generally porous across its full area? The first calls for local treatment and making good. The second calls for treating the whole internal surface.

We also establish at this stage whether the tank stores potable water, because that materially restricts what may be specified, and it is a question that needs answering before material is ordered rather than after it arrives on site.

Once the water path is mapped, Space Arc Engineering specifies the system, supplies the material, and supports your site team or waterproofing contractor through preparation, application and curing.

The system we supply

For the general internal waterproofing of a water-retaining structure, Space Arc Engineering supplies Fosroc Brushbond Crystal, manufactured by Fosroc. As an authorised project distributor for Fosroc, we supply genuine material with manufacturer technical data behind it.

Supplied by Space Arc Engineering · Manufactured by Fosroc

Fosroc Brushbond Crystal is a single-component crystalline capillary waterproofing slurry. It is supplied as a pre-blended cementitious powder, gauged on site with clean water alone, and applied by stiff brush or spray in two coats.

It does not work like a paint or a membrane. Its active chemicals migrate into the capillary pores of the concrete and react with moisture and the free lime already present in the hardened matrix to deposit insoluble crystals inside the concrete. The capillary routes that the water was using are progressively blocked from within, and the concrete itself becomes the barrier rather than carrying one on its surface.

Why this suits a water tank specifically

• It works under both positive and negative pressure: Because the waterproofing is inside the concrete rather than on it, it holds whether water is pushing outward from the stored water or inward from saturated ground. This is the property that matters most on a sump or underground tank, and the one a surface film cannot reliably offer, since negative pressure tends to debond a coating.
• It is integral, so there is no film to fail: There is nothing to peel, blister or puncture. A tank gets drained, scrubbed and brushed out periodically, and light abrasion of the surface does not compromise a crystalline structure that lives within the slab.
• It is chloride-free and non-toxic: This is why it is commonly specified for potable-water tanks and reservoirs. For any potable application, confirm the current Fosroc India technical data sheet and any potable-water clearance your specification requires.
• It seals fine hairline and shrinkage cracks: The crystalline growth closes the non-moving hairline cracking that causes most diffuse tank seepage, and residual unreacted chemistry can reactivate with later water ingress to re-seal fine cracks that form afterwards.

Where it is the wrong answer — and what we supply instead

Being straight about the limits is what makes the recommendation worth having. A crystalline slurry is not the answer to every tank leak.

• Active running leaks must be stopped first. No brush-applied slurry can be laid over flowing water. Where water is actively running, we supply Fosroc Renderoc Plug, a rapid-setting cementitious mortar for emergency leak stopping, to arrest the flow before broad-surface work begins.
• It will not bridge moving cracks. Brushbond Crystal seals fine, non-moving hairline cracks. Live, structural or moving cracks, and expansion and construction joints, need dedicated injection systems or joint sealants — not a crystalline coating.
• Where the structure genuinely moves, flexibility matters more. On a tank subject to real thermal or structural movement, a flexible crack-bridging system such as Fosroc Brushbond RFX is the more appropriate specification. Technical suitability decides this, not brand preference.
• It needs concrete to work on. The crystalline reaction depends on cement chemistry, so it is applied to sound concrete and cementitious substrates — not over brick, loose plaster or paint.

What to check before you order

The material is only as good as the preparation underneath it. Before ordering, your site supervision team should settle the following:

• Confirm it is the tank, not the plumbing. Isolate the inlet and outlet and watch the level before spending anything on waterproofing.
• Declare potable or non-potable up front. It changes what can be specified and it is far cheaper to answer now than after delivery.
• Strip the substrate back to sound concrete. Laitance, curing compound, old paint, loose plaster and oil all have to come off by mechanical means. A crystalline system needs an open capillary structure to penetrate; applied over old paint, it cannot work at all.
• Deal with the defects before the broad coat. Active leaks plugged, tie-rod holes and honeycombed areas routed out and made good with a compatible non-shrink repair mortar, the base-to-wall joint and every pipe penetration treated as a detail in its own right.
• Get the moisture state right. The substrate should be damp and saturated surface dry at the moment of application, with no standing water or free film on the surface.
• Arrange curing water before you start. This is the step most often skipped and the most common cause of disappointing results. The crystalline reaction needs the coating kept moist for the period stated on the data sheet. On an exposed terrace tank in a Ghaziabad summer, that requires planning, not good intentions.
• Plan the downtime and an alternative supply. The tank is out of service for draining, cleaning, preparation, two coats and curing. Buildings need to know this in advance.
• Measure properly. Take the wall area, the base, the underside of the cover slab if it is being treated, and all returns and fillets, so the quantity ordered covers a full two-coat application.

Confirm mixing ratio, coverage, pot life and curing duration against the current Fosroc India technical data sheet for your structure and exposure. We provide those figures with the quotation rather than asking you to work from a general web page.

Talk to our technical team

A tank that loses water wastes money every single day it stays unrepaired, and the damp it drives into the slab below causes damage that costs considerably more to put right than the leak itself. Space Arc Engineering supports contractors, consultants, facility managers and housing societies with site diagnosis, system selection, quantity estimation and application methodology for water-retaining structures.

Related reading on this site: our guide to waterproofing basements and underground structures covers the wider family of below-ground work, and our note on how to waterproof a water tank sets out the potable-water and testing requirements in more detail. You can also review the full Fosroc range we distribute and our waterproofing solutions.

To discuss a leaking tank or request a material quotation, contact us directly:

Space Arc Engineering
Authorised Project Distributor & Solution Provider — Construction Chemicals
Website: www.space-arc.in
Phone: +91 9999155255 (sales) · +91 7290089007 (technical)
Office: 11/155, Sector 3, Rajendra Nagar, Sahibabad, Ghaziabad, Uttar Pradesh 201005
Hours: Mon–Sat, 9:30 AM – 7:00 PM