Plumbing Basics

Public Water Supply Systems

Typically, the water mains in residential areas are four inches to 12 inches in diameter, and run several feet below the street level. Smaller pipes, usually 1/2 inch or 3/4 inch diameter, run from city mains into buildings. The water is normally supplied at a pressure of 40 to 70 psi (pounds per square inch).


Water Service Piping: The 1/2 inch or 3/4 inch diameter service piping carries the water from the street mains to the house. Some early service pipes were 3/8 inch diameter. Most or all of this cannot be seen.


Lead piping was used between the street main and the house up until the 1950s. A good deal of lead supply line is still in use, and the health authorities indicate that as long as it is used regularly, there is no difficulty with it. If the water has not been run for some time, many recommend that the water be run for several minutes before using it. The life expectancy of lead piping is indefinite.

The purpose of a house plumbing system is twofold. On the supply side, the idea is to get water for drinking, washing and cooking to the appropriate areas of the house. The waste side of the plumbing system gets rid of liquid and solid waste.

The supply water is under pressure and the waste water flows by gravity. Serviced communities provide the fresh supply water and carry away the waste. In rural properties, wells, rivers or lakes supply fresh water and septic systems typically handle the waste.

The majority of the piping in a home, both supply and waste, is concealed in walls, ceilings and underground. Leakage, obstructions, or other problems may not be identified during an inspection.


Copper piping has been used extensively since the early 1950s for supply lines from the city main to the house. From 1950 to 1970, 1/2-inch diameter piping was used commonly. After 1970, 3/4-inch diameter copper service piping has been common. The life expectancy of copper piping is dependent on water conditions. In many areas, its life expectancy is indefi­nite. In harsh water or soil conditions, it may fail within 20 years.


Galvanized steel is not commonly used as a service pipe, although galvanized steel fittings may be found at the point of entry into the house. Where galvanized service piping is used, it is typically at least 1 1/4-inch diameter. The word galvanized means zinc-coated. The coating helps prevent the steel from rusting.


Plastic water service piping may be polybutylene (PB), polyethylene (PE), cross-linked AND TUBING polyethylene (PEX), polyvinyl chloride (PVC) or chlorinated polyvinyl chloride (CPVC). Most plastic piping is buried at least 18 inches deep. Exposed piping may be subject to mechanical damage and deterioration from sunlight.

Common Problems with Water Service Piping


Since the supply line from the street cannot be seen, no comment is offered during a home inspection. If there is a leak, it may go undetected for some time. In some cases, water can be heard running outside the basement wall. Water accumulating in the basement or a wet spot on the lawn is often the first indication. Leaks may be caused by building settlement, excava­tion, poor connections, faulty valves or a flaw in the pipe itself.

The underground water service line from the property line to the house is owned by the homeowner. Beyond the property line, the pipe is the responsibility of the city. A leak in the pipe requires excavation, and it is often difficult to know whether the leak is on the city’s or the homeowner’s side. The city is usually contacted and they excavate their section of the pipe, correcting the problem if they discover it. If no problem is found, the homeowner is left to correct the problem on his or her own. In some cases, the homeowner must pay for the city’s work if the city pipe is not at fault. Some municipalities use sophisticated leak detection equipment.


Poor water pressure in the house may be the result of a partially closed or obstructed valve in the street. It may also be because of blockage, such as a stone or other foreign body in the pipe.

New piping may be crimped during installation or become pinched under a rock during back-filling operations. This can also cause low water pressure.

City water mains may be undersized or deteriorated in older neighborhoods. Some cities have poor pumping and/or distribution systems. In these cases, low water pressure problems are usually experienced at every home in the neighborhood. The solution is to petition the city to improve its system.


In most new housing, the supply pipe from the street to the house is 3/4-inch diameter. In older houses, the piping was as small as 3/8-inch. Modern life styles and additional plumbing fixtures usually require a larger line, capable of providing more pressure and volume. Replacing this pipe is an expensive and disruptive job. It is often de­ferred as long as possible.


In some older semi-detached (attached) and row houses, a single supply line would run under a front lawn, and then split to feed two houses. This often yields unsatisfactory water pressure for both houses and is often replaced with two larger, separate lines.


Where municipal water pressure is above 80 psi, a regulator NEEDED should be provided to reduce the in-house pressure to pre­vent leaks at fixtures, stress on appliance hoses and possible broken pipe joints.


It is unusual, although not impossible, for the service pipe to be too close to the surface, and to freeze during very cold weather. Many service pipes extend above grade just before they enter the house. Freezing is a risk here in cold climates.


Up until World War II, most of the service pipes in built-up areas were lead. While these gener­ally provide good service, they are small in diameter and may have to be replaced. Also, lead is relatively soft, and if building settlement occurs, there is a chance of leakage or crimping the pipe. Leaks can also occur at connections as a result of long-term deterioration.


Many of the old lead service lines were connected to a galvanized nipple – a short piece of steel pipe that was often in contact with the soil. This pipe rusts on the outside and inside, and may be close to the end of its life. It is often wise to replace this as a precautionary measure. Galvanized steel service pipes typically last roughly 40 years.


Source: Carson Dunlop