Water tanks for 5 marla house in pak arab society lahore

Please advise how many water tanks should be made for a 5 marla house. Just one on the roof? What material should it be made of?

What is the purpose of having an underground water tank (on ground floor) and overhead water tank (on the roof)?

One Over head tank will be enough it can be of concrete or plastic tank it depends your own choice.

Undergournd water tank for extra water storage in case you dont have water in overhead tank you can utilize underground.

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