Cleaning the Overhead Water Tank After Rain or a Flood
Do it in five stages: empty the tank, scrub out the sludge, disinfect it with a product made for water tanks at the strength on its label, rinse and flush the pipes, then refill. Until all five are done, boil or treat every drop you drink.
In brief
- When: after floodwater or muddy runoff may have entered the tank, after a dead bird or rat is found, or when the water smells or looks wrong.
- Time needed: most of a day, including the disinfectant’s contact time.
- Two people minimum: one on the roof, one below. Never alone.
- Never: climb inside a closed tank, or mix bleach with any other cleaner.
- Meanwhile: drink only boiled, chlorine-treated or sealed bottled water.
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After heavy rain, a lot of families notice it at the tap first. A brown tinge. A faint smell of drain. Sometimes a leaf, or worse. The overhead tanki sits on the roof for years with nobody looking inside, and floods are when its hidden sludge, cracked lid or broken overflow pipe finally shows up in your glass. Here’s how to clean the overhead water tank after rain or a flood, safely and properly. It’s part of our Barsaat and Flood Season series.
Why the tank needs cleaning after a flood
A roof tank is like a teapot nobody ever washes, where fine silt settles at the bottom year after year and slowly builds into a soft brown layer you’d never see unless you climbed up and lifted the lid. Germs love it. In a flood, dirty water can get in through a cracked lid, a loose inlet, an underground sump that took in drain water, or a municipal line that sucked in sewage when pressure dropped.
Once contaminated water is sitting in that sludge, simply letting the tank refill won’t flush it out, because the germs cling to the slime on the walls and the floor and carry on quietly feeding into every tap in the house. Scrub, don’t just drain.
Safety first: roofs, ladders and closed tanks
People skip this part. Don’t.
- Wet roofs are slippery. Wait for a dry spell, wear rubber-soled shoes, and keep children off the roof completely.
- Switch off the motor pump at the mains before you start, and keep hands and wet cloths away from any wiring near the tank. Water and electricity on a roof is how people die in the rains.
- Don’t climb into a closed concrete tank. Air inside a sealed space can lack oxygen, and cleaning fumes build up fast. For a tank big enough to enter, hire a trained crew with the right gear.
- Never mix bleach with acid cleaners (toilet cleaners, tile acids) or with ammonia products. The gas it makes can badly hurt your lungs.
- Wear rubber gloves, closed shoes and eye protection when handling any disinfectant.
- Cover the open tank the moment you step away. A child can drown in very little water. Our piece on buckets, water tanks and child drowning covers lids and locks.
How to clean the overhead water tank, step by step
- Store drinking water first. Boil or treat enough for the day before you empty anything.
- Close the inlet valve and switch off the pump at the mains.
- Drain the tank by opening the drain valve or running the taps. Use this water for flushing toilets or washing floors, not for drinking.
- Scoop out the sludge from the bottom with a plastic mug and bucket, and carry it well away from any drain that feeds a water source.
- Scrub every inside surface with a stiff brush or a clean scrubber and a little detergent: walls, floor, the underside of the lid and the pipe openings. A long-handled brush lets you reach from outside a plastic tank.
- Rinse out the detergent with clean water until no foam is left, and drain again.
- Disinfect. Use a product made for disinfecting water tanks, or a plain unscented bleach, at exactly the strength and contact time printed on its label or given by your local health or water authority. Wet all the inside surfaces and keep them wet for the full contact time.
- Flush the pipes. Refill part way, open each tap in the house one by one until you smell the disinfectant, then close it. This cleans the pipes too.
- Drain and refill with clean water. Run each tap until the strong smell fades.
- Fix the cause. Replace a cracked lid, put mesh on the overflow and vent pipes, and seal gaps where rainwater, birds or insects get in.
Mark it on the calendar
Stick a note inside the kitchen cupboard with the date you cleaned the tank. A good rhythm is a clean before the monsoon and one after it, plus any time floodwater or an animal gets in.
Plastic tank vs concrete tank: what’s different
| Tank type | Cleaning from outside? | Watch for |
|---|---|---|
| Plastic (roof) | Usually yes, with a long brush | Cracked lids, algae from sunlight |
| Concrete (roof) | Often needs someone inside | Confined-space danger; hire a trained crew |
| Underground sump | No | Drain seepage in floods; professional cleaning |
Algae growing inside a plastic tank is a sign that sunlight is getting in through thin walls or a gap at the lid, so a darker, opaque tank and a lid that fits snugly are the simplest way to keep it down.
Myth: a spoon of phitkari in the tank keeps it clean
The truth: alum pulls mud together so it sinks, making water look clearer. It doesn’t disinfect the tank or reliably kill germs, and it builds up more sludge at the bottom over time.
Why people believe it: the water looks sparkling afterwards. Clear and safe aren’t the same thing, which is why a proper scrub and disinfection still matter.
Until the tank is done: drinking water rules
Keep boiling drinking water for one full minute at a rolling boil, or use chlorine water tablets exactly as the pack says, or use sealed bottled water. Use the same safe water for brushing teeth, washing fruit that’s eaten raw, and making ice. Even after the tank is clean, municipal supply can stay contaminated for a while after a flood, so keep boiling until your area’s water is declared safe. Our monsoon hygiene routine covers hand-washing, feet and food in the same season.
If someone at home has diarrhoea with blood, vomiting that won’t let them keep fluids down, very little urine, sunken eyes or drowsiness, get medical help the same day and start packet ORS made up as the packet says. Fever of 39 °C or above, fever for more than 3 days, or yellow eyes after a flood need a doctor too, because typhoid and hepatitis spread through dirty water. If anyone falls from a roof or ladder, feels chest tightness or coughs badly after breathing cleaning fumes, or gets an electric shock, call your local emergency number (1122 in Pakistan, 112 in India).
Water tank cleaning: common questions
How often should an overhead water tank be cleaned?
At least twice a year, ideally before and after the monsoon. Clean it again any time floodwater, a dead animal or dirty runoff may have got in, or the water smells or looks off.
How much bleach do I use to clean a water tank?
Use the strength and contact time printed on the product label, or the method given by your local health or water authority. Bleach strengths differ from bottle to bottle, so a number copied from somewhere else may be far too weak or too strong.
Can I use Dettol or phenyl to clean a water tank?
No. Antiseptic liquids and floor cleaners are not made for drinking water, and they leave residue that is hard to rinse out. Use a product labelled for water tanks or plain unscented bleach per its label.
Can I clean the tank without emptying it?
Not properly. The germs live in the sludge and slime, so you have to drain, scrub and remove the sludge. Adding a chemical to a full tank leaves that layer in place.
How do I know the tank water is contaminated?
Brown or cloudy water, a drain or rotten smell, insects or worms in the water, or stomach upsets spreading through the family are all warning signs. Clear water can still carry germs, so clean after any flood regardless.
Is it safe to bathe with water from a dirty tank?
Try not to, especially with cuts, scratches or eye infections, and don’t let children swallow it. Use it for flushing and floors until the tank is cleaned and disinfected.
Should I hire professional tank cleaners?
For concrete tanks that need someone to go inside, and for underground sumps, yes. A trained crew knows how to handle confined spaces and fumes. Ask them which disinfectant they use and whether it’s meant for drinking-water tanks.
How long after cleaning can I drink the tank water?
Once the tank and pipes have been rinsed and the strong disinfectant smell has faded, the water is fine for washing. For drinking, keep boiling or treating it until your local supply is known to be safe again after the flood.
How do I stop mosquitoes breeding in the tank?
Keep the lid tight and put fine mesh over the overflow and vent pipes. Mosquitoes that spread dengue lay eggs in still, clean water, so an open tank or a leaking lid is an invitation.