You've probably seen the old Windows screensaver where a maze of pipes builds one upon another until the monitor screen is full. That's kind of what your house would look like if you could peel back the drywall, floors and ceilings. To the novice, a plumbing system may appear to be a complicated tangle of pipes, but it really is a very straightforward system.

Before you start any home improvement project, plumbing or otherwise, you need to familiarize yourself with the locations of your pipes. But first, let's get to know the plumbing system itself.

Water Supply

The purpose of the system is to get fresh water to a home and to take away wastewater. The system starts with a main supply line. It could come from a city source or from a private well. It almost always goes into a house's basement or crawlspace. If it is coming from a municipal supply, it will run through a water meter. If it is from a private well, it is pumped into a pressurized storage tank. Once the main line is in the home, it branches off into smaller pipelines. Cold lines run to all of the house's fixtures and the water heater, where hot water lines pipe water to appropriate fixtures. The diameter of the pipe allows for different water pressure.

Shut-off valves control the water supply to the house as well as individual fixtures. It's wise to know where the main shut-off valve is in case of an emergency.

DWV System

The rest of a house's pipes make up the drain, waste and vent (DWV) system, which gets rid of wastewater and prevents toxic sewer gases from coming into the home. The main line in this system is called the waste and vent stack. It's a thick vertical stack of pipes that consists of the main drainage line to a sewer or septic tank and the main vent line that runs to a roof vent.

In order to get rid of wastewater, each fixture in the home has a drainage pipe, which connects to the waste stack. The water first runs through a trap (the U-shaped pipes that are under sinks). The trap is designed so that a little of the water stays in the bend of the pipe. This prevents sewer gasses from coming into the home. The trip out of the house is all downhill from there. Even though branch drain lines, which are thicker than supply lines, appear horizontal, they have a slight incline so that the water flows to the waste stack and out the sewer line.

The purpose of the vent stack and vent pipes, which run up from a fixture's drainage line, is twofold. The roof vent allows air into the pipes. Without air, the water cannot flow into the drainage pipes. The vent system also vents sewer gases to the outside of the house.

Where are pipes located?

In short, plumbing pipes can be almost anywhere. They run through ceiling joists in the basement and floor joists in the attic. They are found between the studs and running through the studs in interior and exterior walls.

Just like you'd never dig a deep hole in your yard without contacting a one-call number to determine where the lines are, homeowners should never start a remodeling project without knowing what's behind the walls where they're working. Obviously, adding or moving a water fixture requires knowing exactly where supply, drainage and vent lines are located.

But unrelated projects can be affected by the location of plumbing as well. For example, maybe you want to cut a hole in the master closet floor and put a chute to the laundry room below. With proximity to the master bath, washing machine, utility sink and water heater, you'd better check for pipes before you cut.

Identifying Pipes

Different states, counties and cities have different codes as to the materials and sizes of plumbing pipes. A general rule is that the thinner lines which run both vertically and horizontally are hot and cold water supply lines. Thicker lines are for drainage as they carry not only water, but also debris and gases. They appear horizontal, but actually run downward. Lines that run vertically above a fixture's drain lines are vent pipes.

Knowing a little about residential plumbing systems and the location of pipes will benefit you in just about any situation that requires you to open up the walls. So before you go swinging a sledgehammer, be sure you won't be demolishing more than drywall. For best results, work with a reliable, licensed contractor and get an idea of the cost to repipe a house.

Updated July 23, 2018.