To find out whether your water tank is leaking, fill it, stop all use, mark the water level and check it again after eight to twelve hours. A drop means water is escaping through the tank, a fitting or the pipes. In an underground tank that matters twice over, because any gap that lets clean water out will let groundwater and sewage in when the tank is low.
This guide covers the signs, the tests you can do yourself, where leaks usually are in Karachi’s tank systems, and how to prevent them.
Understanding Water Tank Systems in Karachi
A standard system has an underground concrete tank that receives line and tanker water, a pump, a riser pipe, an overhead tank, and distribution pipes down to the taps. Leaks happen at every stage, but four places account for most of them:
- Cracks and joints in the underground tank, especially where the wall meets the floor and where pipes pass through.
- The outlet connector at the base of the overhead tank.
- The float valve, which fails and sends water down the overflow.
- Toilet cisterns and hidden pipe joints in the house.
For the layout of each tank type, see underground water tanks and overhead water tanks.
The Importance of Leak Detection
Leaks work in both directions
When an underground tank is full, water pushes out through a crack. When the tank is low and the ground around it is wet, from rain, a leaking sewer or a soak pit, dirty water pushes in through the same crack. Researchers who tested Karachi’s water in 2017 found E. coli in a third of samples and attributed it to sewage mixing with drinking water (Dialogue Earth). A cracked sump next to a sewer line is exactly how that mixing happens at household level.
Water is scarce
Karachi receives about 650 million gallons a day against a need of about 1,200 (Geo News). A tap dripping once a second loses around 20 litres a day. A float valve stuck open can empty a tanker’s worth of water down the overflow in a few nights. If you buy tankers, a leak is money running into the ground.
Leaks damage buildings
Water escaping under a porch washes out the soil and can undermine paving and footings. A leaking roof tank saturates the slab, rusts the steel inside it and brings down ceiling plaster.
Signs of Water Tank Leaks
Underground tank
- The tank empties faster than your use explains.
- The pump runs more often, or you order tankers more often, with no change in the household.
- Damp, sunken or cracked paving around the cover.
- The water level rises when no supply has come. This means water is coming in from the ground.
- Water turns salty, smelly or cloudy after rain.
Overhead tank
- A damp patch, white salt marks or green growth on the roof around the tank.
- A stain or peeling paint on the ceiling below.
- Water trickling from the overflow pipe.
- A wet line down the outside of the tank from a fitting or a crack.
House plumbing
- The overhead tank loses water overnight.
- Damp patches on bathroom or kitchen walls.
- The sound of running water when everything is off.
- A cistern that hisses or refills on its own.
Detecting Leaks: DIY Methods
1. The overnight level test
- Fill the tank and switch off the pump and the incoming supply.
- Mark the water level with a waterproof marker or a strip of tape.
- Make sure no water is used. For the overhead tank, close the outlet valve if it has one.
- Check the mark after 8 to 12 hours.
In a covered tank the level should not move. A drop of more than a few millimetres means a leak.
2. Narrowing it down
Repeat the overhead test twice: once with the outlet valve closed and once with it open but no taps used.
- Drops with the valve closed: the tank or its fittings.
- Holds with the valve closed, drops with it open: the house plumbing or a cistern.
3. The cistern dye test
Put a few drops of food colouring in each toilet cistern and do not flush. If colour appears in the bowl within half an hour, the flush valve is leaking. This is the most common hidden leak in any house.
4. The dry-surface check
Wipe the outside of the overhead tank and every fitting dry, then come back in an hour. Fresh wet marks show you where the leak is. On a concrete roof tank, look for damp lines and white salt deposits on the outside face.
5. Inspection when empty
The best time to find a leak in an underground tank is when it is empty and clean. Cracks, damp patches that reappear after the wall dries, rust stains from reinforcing steel and roots coming through joints are all visible then. This is one reason to clean on a schedule: the tank gets inspected twice a year. Do not go into the tank yourself. It is a confined space. See our safety page.
6. The inflow check
After the tank has been emptied and cleaned, leave it empty for a few hours with the supply off. If water collects on the floor, it is seeping in from outside, and the tank is not safe for drinking water until it is sealed.
Professional Leak Detection Services
If your tests show a loss you cannot locate, bring in a specialist. Plumbers and leak detection firms use pressure testing of pipe sections, acoustic listening devices, moisture meters and thermal cameras to find leaks inside walls and under floors without breaking them open.
Our own work is cleaning and disinfection. While the tank is empty, our crew will show you any crack, damp patch or failed fitting they see, so that you can have it repaired by a mason or plumber before the tank refills. The cleaning steps are on our service procedure page.
Preventing Water Tank Leaks
Build it right
- Use dense, well-compacted concrete with proper cover over the steel, and waterproof the tank inside with a product approved for drinking water.
- Round the corners where walls meet the floor. Sharp corners crack first.
- Seal around every pipe that passes through the wall with a proper puddle flange or sealant, not just mortar.
- In saline ground near the coast, use sulphate-resisting cement. See choosing the right tank material.
- Keep the tank clear of large trees and of foundations. See choosing the right location.
Install plastic tanks properly
- Put the tank on a flat base that supports its whole floor.
- Use proper tank connectors with rubber washers on both sides, tightened by hand plus a quarter turn. Over-tightening cracks the wall.
- Support the pipes so their weight does not hang from the tank fitting.
- Keep the tank out of contact with sharp edges and hot metal.
Maintain it
- Clean and inspect both tanks every 6 months. See how often to clean your water tank.
- Test the float valve monthly and replace worn washers.
- Seal hairline cracks as soon as they are found. They only grow.
- Renew waterproofing in concrete tanks when it wears or peels.
- Remove scale from valves and fittings before it stops them from closing. See hard water stains and deposits.
- Service the pump. Vibration from a worn pump loosens pipe joints. See pump system maintenance.
The general routine is in how to maintain your water tank after cleaning.
Addressing Common Water Tank Issues in Karachi
- Old concrete tanks. Houses and apartment blocks built decades ago often have sumps with worn plaster and fine cracks. They need inspection at every cleaning and re-waterproofing every few years.
- Ground movement and heavy vehicles. Tanks under driveways crack when loads exceed what the slab was built for.
- Salty groundwater. In coastal areas, chlorides rust the steel inside concrete. Rust expands and splits the concrete from within.
- Monsoon. Saturated ground after heavy rain is when inward seepage is worst. If the water quality changes after rain, suspect the underground tank. See climate change and Karachi’s water tanks.
- Sun-damaged plastic. After years on a roof, cheap tanks turn chalky and brittle and split at the base or around fittings.
- Illegal or badly made connections. Poor joints on the incoming line leak and also let dirt in when the line is empty.
Other faults that look like leaks, such as low pressure and overflow, are covered in common water tank problems.
What to Do If Your Underground Tank Is Letting Water In
- Stop using the water for drinking and cooking.
- Have the tank emptied and cleaned so the damage can be seen.
- Get the crack or joint repaired and the tank re-waterproofed by a competent mason.
- Once the repair has cured, have the tank disinfected and rinsed before use. See how to disinfect a water tank.
- Find and fix the source outside the tank: a leaking gutter line, a soak pit too close, or surface water running to the cover.
Regulatory Compliance and Water Tank Safety
Pakistan’s National Standards for Drinking Water Quality require that no E. coli or coliform bacteria be detectable in drinking water (S.R.O. 1063(I)/2010). A tank that admits groundwater cannot meet that. For households this is a health matter. For landlords, building unions, schools, restaurants and factories it is also a matter of responsibility to the people who use the water. See who is responsible for tank cleaning in apartment buildings and government regulations on water quality.
Structural repairs to large tanks, and any work inside a deep tank, should be done by trained people with ventilation and a person stationed outside.
Frequently Asked Questions
How can I tell if my underground water tank is leaking?
Do the overnight level test with supply and pump off. A falling level means a leak. A rising level means water is seeping in.
Is a small leak a problem?
Yes. It wastes water every hour of every day, it grows, and in an underground tank it is a route for contamination.
Can a leaking concrete tank be repaired?
Usually. Cracks are cut out, filled and the surface re-waterproofed with a drinking-water-safe product. The tank must be empty, clean and dry.
Can a leaking plastic tank be repaired?
A leaking fitting can be replaced. A cracked tank body should be replaced.
Why does my tank water get dirty after it rains?
Either rainwater is running in around the cover, or groundwater is seeping in through cracks. Both need fixing, and the tank needs cleaning and disinfecting afterwards.
Have the Tank Emptied, Cleaned and Looked At
A cleaning is the easiest way to see the true state of your tank. Call or WhatsApp 0313-2796778. We work across Karachi, including Bahadurabad, PECHS and Tariq Road. Rates are on the charges page, and all our service areas are listed on the site.
About the author: Ahmar Shoaib is the founder of Karachi Tank Cleaning. He holds a BS in Chemical Engineering from Karachi University and has run the company’s cleaning and disinfection work since 2007. This article follows our editorial policy.
Sources: PCRWR water testing for the Sindh High Court judicial commission (2017), reported by Dialogue Earth; PCRWR figures on Karachi’s water demand and supply, reported by Geo News (May 2025); National Standards for Drinking Water Quality, S.R.O. 1063(I)/2010.
