If possible, the network is planned so the whole pipe system consists of loops, and no pipes come to a dead end. In this way, water can flow to any point in the system from two or more directions. This eliminates the need to cut off the water supply for maintenance work or to repair breaks. Older water systems frequently were expanded without planning and developed into a treelike system. This consists of a single main that decreases in size as it leaves the source and progresses through the area originally served.
Smaller pipelines branch off the main and divide again, much like the trunk and branches of a tree.
A treelike system is not desirable because the size of the old main limits the expansion of the system needed to meet increasing demands. Also, there are many dead ends in the system where water remains for long periods, causing undesirable tastes and odors in nearby service lines. In general, mains should be located so they are clear of other structures and should be adjacent and parallel to streets but not within roadways, if possible. Mains also should be separated from other utilities to ensure the safety of potable water and to lessen interference with other utilities during maintenance. The purpose of installing shutoff valves in water mains at various locations within the distribution system is to allow sections of the system to be taken out of service for repairs or maintenance without significantly curtailing service over large areas.