An overhead water tank is a storage tank placed on the roof or on a raised stand so that water flows to the taps by gravity. Its main benefits are steady pressure without running a pump for every tap, water during load-shedding, low cost and easy inspection. In Karachi it works as the second half of a pair: an underground tank catches the supply, a pump lifts it, and the overhead tank delivers it.
This guide explains how the system works, the benefits, the types on the market, how to choose and install one, and the upkeep it needs in Karachi’s heat. We clean overhead tanks on houses and buildings across the city every day, and the advice here comes from what we see on those roofs.
What Is an Overhead Water Tank?
“Overhead tank” simply means a tank that sits higher than the taps it serves. Height is what makes it work. Every metre of height between the water level and a tap adds about 0.1 bar of pressure, so a tank whose base is 3 metres above a shower head gives about 0.3 bar. That is enough for taps, toilets and a basic shower. It is not enough for most instant geysers or rain showers, which is why some houses add a small pressure pump on the outlet.
How the supply system works
- Line or tanker water enters the underground tank.
- A pump lifts it through a riser pipe to the overhead tank.
- A float valve or automatic level switch stops the pump when the tank is full.
- Water flows down by gravity through the distribution pipes to each floor.
- An overflow pipe carries away excess if the float fails, and a vent lets air in and out.
The underground half is described in pros and cons of underground water tanks.
Parts of an overhead tank and what each does
| Part | Job | What goes wrong |
|---|---|---|
| Lid | Keeps out light, dust, insects and birds | Cracks, warps or blows off |
| Inlet and float valve | Lets water in and shuts it off when full | Sticks with grit or scale, so the tank overflows or stays empty |
| Outlet | Feeds the house. Should sit a few centimetres above the floor. | Blocks with sediment if set too low |
| Overflow pipe | Carries away excess if the float fails | Lets insects in if not meshed |
| Vent | Lets air in and out as the level changes | Lets insects and light in if not screened |
| Washout or drain | Empties the tank for cleaning | Often missing, which makes cleaning slower |
| Base or stand | Carries the weight and adds height | Uneven support cracks the tank |
Benefits of Overhead Water Tanks
Water without electricity
Once the tank is full, the house has water for hours or days with the power off. In a city with regular load-shedding, this is the benefit people value most. Pairing the tank with a solar-powered pump takes the grid out of the picture altogether.
Steady, quiet pressure
Gravity pressure does not surge or drop when a second tap opens, and the pump runs once or twice a day instead of every time someone washes their hands. The motor lasts longer and the electricity bill is lower. See pump system maintenance.
Low cost and quick installation
A plastic tank can be carried up, placed on a prepared base and connected in a day. There is no excavation and no concrete work.
Easy to inspect
You can lift the lid and see the whole inside. Dirt, algae and a failing float are obvious, and leaks show as damp patches on the roof. An underground tank hides all of these.
Away from sewage and floodwater
A rooftop tank cannot be reached by sewer seepage or street flooding, the two worst sources of contamination for underground tanks.
Simple to replace or enlarge
If the family grows, you can add a second tank beside the first and link them.
Drawbacks to Plan For
- Heat. A rooftop tank sits in full sun. The water gets hot, chlorine disappears faster and bacteria multiply faster. Warm, poorly chlorinated water is also where Naegleria fowleri survives. Karachi records deaths from it most summers, and health experts advise keeping overhead and underground tanks chlorinated (ARY News).
- Sunlight. Thin or light-coloured tanks let light through, and algae grows. See preventing algae growth.
- Open lids. Lids crack, warp or blow off, and then dust, insects and birds get in. See bird contamination in rooftop tanks.
- Weight. A 1,000-litre tank weighs a tonne when full. The roof must be able to carry it.
- Limited pressure. A tank sitting directly on the roof gives weak pressure on the top floor.
Every one of these has a straightforward fix, covered below.
Fixes at a glance
| Problem | Fix |
|---|---|
| Hot water from the cold tap | Shade or canopy, an insulated multi-layer tank, a light outer colour |
| Green walls | Opaque tank and a tight lid |
| Weak pressure upstairs | Raise the tank on a stand, or add a small pressure pump |
| Lid keeps coming off | Threaded or clamped lid, tied down in the monsoon |
| Overflowing | Clean or replace the float valve, or fit an automatic level switch |
| Sediment at the taps | Clean both tanks and raise the outlet |
More faults and their causes are in common water tank problems.
Types of Overhead Water Tanks
Plastic (polyethylene) tanks
The standard choice for houses. They are light, rust-free and available from a few hundred litres to several thousand. Choose a multi-layer tank: a UV-resistant outer layer, a black light-blocking middle layer and a white food-grade inner layer. Some add insulating foam, which keeps the water noticeably cooler. Avoid thin single-layer tanks that you can see daylight through.
Concrete (RCC) tanks
Built as part of the roof structure on many older houses and most apartment buildings. They hold more, keep water cooler and last for decades, but they are heavy, can crack and seep, and their rough surface holds biofilm. They need a smooth internal plaster and periodic waterproofing.
Fibreglass (GRP) tanks
Rigid and corrosion-free, sold as one-piece tanks and as bolted panel tanks for large commercial roofs. The inner gel coat must be intact.
Stainless steel tanks
Hygienic, smooth and completely light-proof. They cost more, heat quickly unless insulated, and cheaper grades corrode at the welds in sea air.
Galvanised steel tanks
Found on older roofs. Once they rust, replace them.
A side-by-side comparison of all materials is in types of water storage tanks and how to choose the right tank material.
Choosing the Right Overhead Water Tank
Size
The overhead tank should hold at least one full day of the household’s use, so that one pump run covers the day. The WHO puts basic needs at 50 to 100 litres per person per day (UN), and urban households with flush toilets and washing machines commonly use more. As a rough guide:
| People in the house | Suggested overhead capacity |
|---|---|
| 2 to 3 | 500 to 750 litres |
| 4 to 6 | 1,000 to 1,500 litres |
| 7 to 10 | 2,000 litres |
| Two families or more | 2,000 to 3,000 litres, or two linked tanks |
Do not oversize. Water that sits for days in a hot tank goes stale. The full calculation, including the underground tank, is in how to choose the right size water tank.
Material and build
For a house, a multi-layer plastic tank from an established maker. Check that the lid is a tight, threaded or clamped fit and that the tank has a moulded outlet above the base.
Shape
Vertical cylinders are the strongest and the easiest to clean. Horizontal and rectangular tanks fit low spaces but have more corners where sludge collects.
Colour
A light outer colour reflects heat, but the wall must still be opaque. White outside with a black inner layer is the best combination.
One tank or two?
Two smaller tanks linked at the base are often better than one large one. The load is spread across the roof, each tank is easier to handle and replace, and with a valve on each you can keep one in service while the other is cleaned. Link them with a pipe of the same size as the outlet, and give each its own lid and vent.
Stand height and pressure
| Tank base above the tap | Approximate pressure | Good for |
|---|---|---|
| 1 m | 0.1 bar | Slow tap flow |
| 3 m | 0.3 bar | Taps, cisterns, a basic shower |
| 5 m | 0.5 bar | A comfortable shower, washing machine |
A tank sitting directly on the roof slab gives the bathroom below barely a metre of head. A steel or masonry stand of 1.5 to 2 metres, properly designed for the weight, is the simplest cure.
Installation and Maintenance Tips
Installation
- Put it over a beam or column, not in the middle of a slab. Ask an engineer if the tank is larger than the one the roof was designed for.
- Give it a flat, full base. A plastic tank must rest on a level concrete or steel platform that supports its whole floor. Bricks under the edges will crack it.
- Raise it on a stand or the stair tower if you want better pressure on the top floor. Each extra metre adds about 0.1 bar.
- Shade it if you can. A simple canopy or the north side of the stair tower keeps the water cooler.
- Fit the outlet above the floor and add a separate washout at the lowest point for draining during cleaning.
- Mesh the overflow and vent. These are the doors that insects and lizards use.
- Use a float valve or automatic switch so the tank does not overflow and the pump does not run dry.
- Leave room to work. Someone has to open the lid and reach inside twice a year.
Placement is discussed further in how to choose the right location for your tank.
Maintenance
- Clean and disinfect every 6 months, together with the underground tank. See how often to clean your water tank.
- Check the lid after every storm and replace it if cracked.
- Look inside once a month for green walls, sediment or anything floating.
- Check the float and the overflow. A tank that overflows daily is wasting water and money.
- Look for damp patches or drips around fittings. See detecting and preventing leaks.
The full routine is in how to maintain your water tank after cleaning.
How an Overhead Tank Is Cleaned
The steps are the same as for any tank: drain, remove sludge, pressure-wash, vacuum out the waste water, disinfect with a chlorine-based solution and rinse. Overhead tanks are quicker than underground ones because access is easy and the walls are smooth. Our service procedure has the details.
For an overhead tank alone, our house rates run from Rs 2,500 to Rs 8,500 depending on plot size. Cleaning both tanks in one visit costs less than two separate jobs, and it is the only way to be sure the clean overhead tank is not refilled from a dirty underground one. See water tank cleaning charges.
Frequently Asked Questions
What does “overhead tank” mean?
A water tank placed above the level of the taps, usually on the roof, so that water reaches them by gravity.
What size overhead tank does a house need?
About one day’s use: roughly 1,000 to 1,500 litres for a family of four to six.
How high should an overhead tank be?
As high as is practical. Each metre above the highest tap adds about 0.1 bar. A stand of 1.5 to 2 metres makes a clear difference to a top-floor shower.
Why is the water from my overhead tank so hot?
The tank is in direct sun. Shade it, choose an insulated multi-layer tank, or paint a dark single-layer tank with a light reflective coating on the outside only.
Is overhead tank water safe to drink?
Only if the tank is covered, cleaned every 6 months and filled from a clean underground tank. Even then, boil or filter drinking water in Karachi.
How often should an overhead tank be cleaned?
Every 6 months, and immediately if the lid has been off or the walls have turned green.
Overhead Tank Cleaning in Karachi
Call or WhatsApp 0313-2796778 to book, or use the contact page. We clean rooftop tanks on houses, flats and commercial buildings across the city, including Gulistan-e-Johar, Gulshan-e-Iqbal and Tariq Road. The calculator on our home page gives your rate instantly.
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: UN Water for Life Decade media brief, citing WHO on water needs per person; health experts’ advice on Naegleria fowleri, reported by ARY News (July 2026).
