A landing valve should be placed such that its lowest point is approximately 750 mm above the floor level. Fire mains should preferably be protected by and put within a BS5041-4-compliant box. Landing valves for dry mains should be in accordance with BS 5041-2. The normal position for a landing valve is closed but it can be opened to discharge water if needed. Landings are usually controlled from a panel located near the street or some other public area. They can also be controlled from a remote location by telephone line or electrical cable. In commercial buildings, especially where there are many floors above ground level, it is important to protect the main supply pipe from damage caused by heavy loading objects being dropped onto them. This is done by installing a drop pipe at this point in the distribution system. The drop pipe should be large enough to allow water to drain out when the valve is open. It should be located below the highest floor in the building and should enter the wall at a height of between 1.8 m and 2.5 m from the floor level. Dry risers are used where there is no available space near the property boundary for a buried riser system. These pipes run across the roof of the building and their top ends are connected to the distribution system.