3. The water hammer phenomenon in this case stems from the inertia/momentum of the large volume of water inside the long length of horizontal pipe run at roof. When the pump stops running, gravity acts against the upward inertia of those vertical columns of water inside pipe risers A and B, causing the upward movement of the water columns to slow down quickly. For the volume of water inside the long horizontal pipe run at roof, its inertia towards the right is countered by the gravity of the short water column inside riser B but the effect is minimal. On the other hand, water column inside riser A quickly comes to a standstill when the non-return valve at the pump discharge closes. This would lead to a lower than atmospheric water pressure at the pipe elbow EB1 to act against the right-ward movement of the horizontal length of water, bearing in mind that the pressure at the roof tank pipe discharge is atmospheric. Cavitation would occur at EB1 (see Figure 2) when the local water pressure drops below its own vapour pressure.