Put an underground water tank on the high, street-facing side of the plot, as far as possible from sewer lines and soak pits, with its cover raised above the floor. Put an overhead tank over a beam or column, as high as is practical, shaded if possible, and where someone can reach the lid. Those two rules prevent most of the contamination, pressure and leak problems we are called to deal with.
Location is decided once, usually by the mason on the day of digging, and the household lives with it for decades. This guide sets out what to weigh before that day, for both tanks, with the conditions specific to Karachi.
Factors to Consider When Choosing a Location
Six things decide whether a tank position is good or bad:
- Contamination risk. How close is it to sewage, drains, rubbish and floodwater?
- Structure. Can the ground or the roof carry the weight safely?
- Hydraulics. Will water flow in easily and reach the taps with enough pressure?
- Access. Can a person open it, look inside, and clean it? Can a tanker hose reach it?
- Heat and light. Will the water sit in full sun?
- Cost. How much pipe, excavation and structure does this spot require?
A position that fails on contamination or structure should be rejected however convenient it is. The others are trade-offs.
Water Usage Needs Assessment
You cannot place a tank until you know how big it is. Size comes from the number of people, their daily use, and the gap between supplies. The WHO puts basic needs at 50 to 100 litres per person per day (UN), and a household with flush toilets and a washing machine typically uses around 135.
| Household | Daily use | Overhead tank | Weight on the roof when full |
|---|---|---|---|
| 2 to 3 people | 270 to 400 L | 500 L | about 0.5 tonne |
| 4 to 6 people | 540 to 810 L | 1,000 to 1,500 L | 1 to 1.5 tonnes |
| 7 to 10 people | 950 to 1,350 L | 2,000 L | about 2 tonnes |
The underground tank is sized for the days between supplies and for the tanker load you buy. The full method, with worked examples, is in how to choose the right size water tank. Bring those two numbers to the site before deciding where anything goes.
Overhead vs. Underground Water Tank Systems
Most Karachi properties need both, because they do different jobs.
| Underground tank | Overhead tank | |
|---|---|---|
| Job | Receives line and tanker water | Delivers water to taps by gravity |
| Fills by | Gravity from the street main or a tanker hose | Pump from the underground tank |
| Main location risk | Sewage seepage and floodwater | Roof overload, heat, open lids |
| Key location rule | High side of plot, away from sewers | Over a beam or column, raised and shaded |
| Access needed for | Tanker hose, monthly check, cleaning crew | Lid, float valve, cleaning crew |
Each is covered on its own in pros and cons of underground tanks and benefits of overhead tanks. Other designs are described in types of water storage tanks.
Locating the underground tank
- Away from sewage. Keep it as far as the plot allows from the septic tank, soak pit, gutter line and any manhole. Never build it against a wall shared with any of them. Sewage finds cracks.
- On the higher side. Rain and wash water should run away from the cover. If the whole plot is flat, raise the neck of the tank 15 to 30 cm above the finished floor.
- Towards the street. A short pipe from the main connection, and a tanker hose can reach the cover without crossing the house.
- Clear of foundations. Digging next to a column footing or boundary wall can undermine it. If the tank must be close, an engineer should design the excavation and the tank wall.
- Away from large trees. Roots crack tank walls and enter through joints.
- Not under a permanent load. Do not put the cover where a car always stands, under a staircase, or beneath a planter. It has to be opened.
- Close to the pump. A short, straight suction pipe keeps its prime and lasts longer.
Locating the overhead tank
- Over structure. Place the tank above a beam, column or load-bearing wall. The middle of a slab is the weakest point. For any tank bigger than the roof was designed for, ask an engineer.
- High. Every metre of height between the tank and a tap adds about 0.1 bar of pressure. The roof of the stair tower, or a stand of 1.5 to 2 metres, makes the top-floor bathroom usable.
- Shaded. The north side of the stair tower or a simple canopy keeps the water cooler and slows bacterial growth and algae. See preventing algae growth.
- Above the bathrooms it serves. Short pipe runs lose less pressure. A tank at one end of a long roof gives a weak flow at the other.
- Reachable. Leave space to stand beside it, open the lid and work. A tank wedged into a corner or under a low shed does not get checked or cleaned.
- On a flat, full base. A plastic tank needs a level platform under its whole floor.
- Clear of bird perches and trees. Wires, antenna poles and branches above the tank mean droppings on the lid. See bird contamination in rooftop tanks.
- Secured against wind. An empty plastic tank is light. Strap it or box it in where the roof is exposed.
Pressure by tank height
| Height of tank base above the tap | Approximate pressure | What it is enough for |
|---|---|---|
| 1 m | 0.1 bar | Slow tap flow only |
| 3 m | 0.3 bar | Taps, toilet cisterns, basic shower |
| 5 m | 0.5 bar | Comfortable shower, washing machine |
| 10 m | 1 bar | Most fittings and many instant geysers |
This is why ground-floor taps are strong and top-floor taps are weak in the same house. If raising the tank is not possible, a small pressure pump on the outlet line solves it.
Site-Specific Considerations for Karachi
Flooding
Karachi’s monsoon arrives in bursts. August 2020 brought 484 mm, the wettest August since 1931 (Wikipedia), and ground floors in many neighbourhoods were under water. An underground tank cover at floor level in such a street will be submerged. Raise the neck, fit a cover that seals, and slope the porch away from it. More on this in climate change and Karachi’s water tanks.
Old pipes and sewer lines
In older parts of the city, water and sewer lines run close together and both leak. When the Pakistan Council of Research in Water Resources tested Karachi’s water in 2017, a third of samples contained E. coli, which the researchers attributed to sewage mixing with supply lines (Dialogue Earth). Find out where the street sewer and your own drain run before choosing the tank’s side of the plot.
Coastal ground
Near the sea the water table is high and the groundwater saline. Salts attack concrete and steel, and an empty tank in waterlogged ground can be pushed upwards. In these areas the tank needs sulphate-resisting cement, external waterproofing and a structural design. See choosing the right tank material.
Sun and heat
Rooftop water in summer gets uncomfortably hot. Heat drives off chlorine and favours bacteria, including Naegleria fowleri, which causes deaths in Karachi most summers (Dawn). Shade and an insulated multi-layer tank are worth more here than in a cooler city.
Small plots
On 80 and 120 square yard plots there is often only one place the underground tank can go: under the porch, a few feet from the gutter line. If that is your situation, the answer is construction quality: a properly waterproofed tank, a raised sealed cover, and water pipes laid above the sewer pipes, never below them. Then keep to a strict cleaning schedule.
Tanker access
If you buy tankers, check that the cover is within hose reach of where the tanker can park, and that the opening is wide enough for the hose.
Apartment and commercial buildings
Large buildings put the underground tank in the basement or compound and the overhead tanks on the roof slab, sized and placed by the structural engineer. The practical issue is access: tanks boxed in by generators, parked cars or locked plant rooms are the ones that go years without cleaning. See who is responsible for tank cleaning in apartment buildings.
Installation and Maintenance
Installation points that depend on location
- Lay the incoming water line above any sewer line it crosses.
- Fit the outlet a few centimetres above the tank floor, and a washout at the lowest point.
- Mesh every vent and overflow.
- Run the overflow where you will see it. An overflow hidden behind a parapet wastes water unnoticed.
- Keep electrical connections for the pump out of any area that gets wet.
- Wash and disinfect a new tank before first use. See how to disinfect a water tank.
Maintenance that a good location makes easy
- A monthly look inside with a torch.
- A check of lids and covers after every storm.
- Cleaning and disinfection every 6 months. See how often to clean your water tank.
- Inspection for cracks and leaks whenever the tank is empty. See detecting and preventing leaks.
When our crew arrives, they need to get a pump, hose, pressure washer and vacuum to both tanks and reach a drain. A well-placed tank makes that a 2 to 4 hour job. The steps are on our service procedure page.
Cost Analysis
Location changes cost in four ways:
- Excavation and structure. A tank near foundations or in waterlogged ground needs engineering and extra concrete.
- Pipework. Every extra metre between main, tank, pump and bathrooms is pipe, fittings and lost pressure.
- Stand or platform. Raising an overhead tank costs steel or masonry, but far less than living with weak pressure or running a pressure pump for years.
- Running costs. A pump placed far from the tank works harder and fails sooner. See pump system maintenance.
Material and labour prices move too quickly for any figure here to stay accurate, so get itemised quotations. The one place not to save is waterproofing and the cover of the underground tank. A leak there costs far more later than it saves today.
Common Location Mistakes
- Underground tank sharing a wall with the gutter or soak pit.
- Cover flush with the floor in a street that floods.
- Overhead tank in the middle of the slab.
- Overhead tank sitting directly on the roof with the bathroom just below, giving almost no pressure.
- Tank boxed in so that the lid cannot be opened fully.
- Cover under the car.
- Plastic tank on bricks or an uneven base.
If your existing tank has one of these faults, you may not need to move it. Raising a cover, adding a stand, sealing a wall or simply cleaning more often will deal with most of them. Our guide to common water tank problems covers the fixes.
Frequently Asked Questions
Where is the best place for an underground water tank?
On the high side of the plot near the street, away from sewer lines and foundations, with a raised, sealed cover that is easy to open.
Where should an overhead tank be placed on the roof?
Over a beam or column, as high as practical, close to the bathrooms it serves, shaded if possible, with room to open the lid.
Can I put the underground tank next to the septic tank?
No. Keep them as far apart as the plot allows and never share a wall. Any crack will let sewage into your drinking water.
Does the location affect how often I need to clean the tank?
Yes. Tanks near sewers, in flood-prone streets or in full sun get dirty faster. The baseline is every 6 months.
My tank is already in a bad spot. What can I do?
Raise and seal the cover, waterproof the inside, shade or raise the overhead tank, and keep to the cleaning schedule. Moving the tank is a last resort.
Tank Cleaning Wherever Your Tank Is
We clean tanks in tight basements, under porches and on crowded roofs across Karachi, including Gulshan-e-Maymar, Malir and North Karachi. Call or WhatsApp 0313-2796778, or see our charges.
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; Wikipedia, 2020 Karachi floods; PCRWR water testing for the Sindh High Court judicial commission (2017), reported by Dialogue Earth; health officials’ statements on Naegleria fowleri, reported by Dawn (June 2025).
