Planning Plumbing

A home’s plumbing system is a complex network of water supply pipes, drain pipes, vent pipes, and more. Because plumbing is complicated and one of the costliest systems to install in a home, it pays to understand

how the system works if you’re planning a new home building project or a major remodel.

Understanding the fundamentals of a plumbing system allows you to do two things. First, it helps you design a system that will work properly and pass plumbing codes. A properly designed system will deliver water to the various faucets, fixtures, and water-using appliances efficiently and carry away waste water without clogs. And second, it may save you money. By planning wisely, you can often reduce the overall plumbing expense significantly by locating bathrooms, kitchens, or laundry rooms near one another so that they can share parts of the system.

A house’s pipes are mostly hidden in walls and floors. However, you can usually see enough—most often in a basement or crawl space—to figure out the paths your pipes follow.

Outside the house
For most people, water is supplied by a local municipality or utility company. The water arrives purified to meet safety standards for drinking. It also should be of sufficient pressure so all showers, taps, and appliances can run efficiently. In most areas, wastewater exits the house and goes to a municipal sewage treatment facility where it is treated and made pure again.

If you live in a rural area, your water may come from a well on your property. If this is the case, you are responsible for maintaining the well’s pump and periodically testing the water to make sure it is safe to drink. Instead of a municipal sewage system, you may have a septic system in your yard that treats the wastewater.

In order to conserve water, some local systems now recycle “gray water”-- wastewater from sinks, laundry, and tubs but not from toilets. Gray water can often be used for irrigation and other non-drinking purposes, thereby cutting down on fresh water usage.

Supply system

Copper is the most common material for supply pipe; it resists corrosion, so water runs freely and pipes don’t leak for many decades. In an older home, you may find galvanized steel pipe, which tends to clog with minerals and rust over time and can develop leaks. Nowadays, many homes have plastic supply pipe, most commonly cross-linked polyethylene (PEX), which installs quickly and is expected to last virtually forever.

Water arrives via a main supply pipe, which is typically 1 inch in diameter or larger. (A pipe is measured by its inside dimension, so a 1-inch copper pipe is about 1 1/8 inches in outside diameter.) In most cases, the pipe runs through at least one main shutoff valve, located outside the house in a “Buffalo box” buried in the yard near the house or just inside the basement or crawl space. It then usually passes through a water meter, and there is likely another main shutoff after the water meter.

The main supply line usually runs to the water heater, where it divides into cold and hot water pipes. From there, supply pipes almost always travel in pairs, hot and cold. Pipes from the water heater are typically 3/4 inch but may be 1/2 inch. Horizontal pairs run to below walls and then vertical pairs, called risers, run up to the various rooms.

In newer homes, there are separate lines running from the water heater to each room, so water use in one area does not affect use in another area. In an older home, a single line may loop throughout the house, meaning, for instance, that if someone flushes a toilet downstairs the cold water supplying a shower upstairs will have lessened pressure, causing the shower water to suddenly become hot.

Drain, waste & vent (DWV) system
Drain pipes carry water out of the house. Waste pipes are those drain pipes that carry sewage from toilets. Vent pipes supply air to the pipes to keep things running smoothly.

A house has at least one main stack, a vertical pipe that runs from above the roof down to the main sewer line. The house’s various toilets, faucets, tubs, and appliances have horizontal pipes that run into the main stack. Horizontal pipes must be sloped so water cannot settle in them.

Venting is a sometimes complicated matter, but the principle is straightforward: Like that little air hole in a gas can, a vent pipe allows air to come behind the drain water so it flows smoothly. Without venting, drain water can gurgle, much like water coming out of an upturned thin-necked bottle. Local and national codes have very specific requirements for vent pipes.

Each fixture has a trap, which is usually shaped like a sideways P and so is called a P trap. The curved portion of the trap holds water in such a way that noxious gases cannot back up into the house. A toilet has a built-in trap.