Water reaches homes under pressure, often from pumps where the water is cleaned. This means the pipes can be of small diameter. The water will flow up from under the ground to the top of the house because it is under pressure.
Water gets fed under mains pressure to the taps on the ground floor. There is also a feeder pipe to a tank in the loft (or in the top of a block of flats). This water is a kind of reservoir in case the mains should fail for a short while. The tank is kept full by means of a valve.
