Rooftop Tank vs Underground Reservoir: Which Do You Need?

In most multi-storey buildings in Bangladesh, an underground reservoir (receiving and storing water from WASA supply or a tube well) and a rooftop/overhead tank (fed by a pump from the underground reservoir, providing gravity-fed pressure to upper floors) work together as two stages of the same supply system, rather than being alternative choices between which a building owner picks one — nearly every multi-storey building needs both, each serving a distinct function.

The underground reservoir's primary role is to buffer against variability in the incoming supply — WASA supply that may not be continuously available at consistent pressure throughout the day, or a tube well that may need periodic rest between pumping cycles — by storing a reserve volume that can supply the building even during a temporary interruption in incoming supply. Sizing this reservoir against the building's actual daily demand and the typical duration of supply interruptions in the specific area is the relevant design consideration, not simply "as large as the available basement space allows."

The rooftop tank's role is different — it receives water pumped up from the underground reservoir and then supplies the building's upper floors by gravity, providing more consistent pressure than would otherwise be achievable relying only on direct pumping to every floor, and providing a further short-term buffer if the transfer pump is briefly unavailable. Rooftop tank sizing should reflect the building's expected peak-hour demand (morning and evening peak usage patterns, similar to the demand-peaking considerations covered in our hostel/dormitory water-treatment article) rather than simply average daily consumption.

Both tank types need periodic cleaning to remove accumulated sediment and prevent biofilm growth, but their maintenance considerations differ somewhat — underground reservoirs are more prone to groundwater infiltration if not properly sealed and are less visible/less frequently inspected simply due to being out of sight, while rooftop tanks are more exposed to direct sunlight (a concern for algae growth if not properly shielded, similar to the concern noted in our rainwater-harvesting storage-tank article) and weather exposure. Both are commonly neglected specifically because they are physically out of the way of daily building activity, making a scheduled (rather than purely reactive) cleaning and inspection routine for both a genuinely worthwhile, low-cost precaution.

Where a building adds water treatment (RO for drinking water, a whole-house filter, or similar), the correct placement within this two-tank system depends on the treatment's purpose — a whole-house filter is typically placed on the line feeding the rooftop tank (or on the supply to each floor) so that all distributed water benefits, while a drinking-water RO system is typically installed at the point of use (kitchen tap) since it does not need to treat water intended for non-drinking uses. Treatment should not be assumed to belong at any single point in this system by default — its correct placement depends on what the specific treatment step is meant to address and for which specific end uses.