How do I find where my water pipes run in the wall or slab?

A drill bit or anchor screw that hits a charged water line turns a small project into a flood inside the wall. The repair means cutting open drywall, drying the cavity, and watching for mold, on top of fixing the pipe itself. Knowing the pipe route first is cheaper than any of that.

The risk is highest in a few spots. Lines tend to rise vertically out of the floor or drop from the ceiling near fixtures: under sinks, behind toilets, around the water heater, and at the tub or shower valve. Walls near a bathroom or kitchen are the most likely to carry a pipe. An exterior wall on the north side of a Phoenix home can hold a hose bib line too.

Two patterns hold in most homes and help you predict the route. Supply lines usually take the shortest sensible path, so they run straight up a wall and then turn at the top plate or the floor, rarely on a diagonal. And a pipe almost never travels alone in a wet wall, so where you find one line, the matching hot or cold line and a drain are often within a foot. Mapping one pipe tells you roughly where its neighbors sit.

Mapping the route also protects the pipes from temperature swings. The U.S. Department of Energy notes that "southern states generally start having issues with frozen pipes when the temperature reaches about 20 degrees Fahrenheit." Knowing which lines sit in an unheated garage, attic, or exterior wall tells you which ones to insulate before a cold snap, and which ones to avoid when you drill.

The cheapest method costs nothing but a little time. If you have the home's original construction plans or a remodel set, the plumbing sheets show the supply and drain routing. Check a moving folder, a kitchen drawer of house papers, or ask the previous owner or builder if the home is newer.

No plans at home? The City of Phoenix Planning & Development Department keeps permit and inspection records, and you can request building records and plans for a specific address. Permits for a re-pipe, a water heater swap, or a bathroom remodel often include drawings or notes that show where lines were run. Records are searchable by address, so even a partial history helps you guess the route before you open a wall.

Plans are a starting map, not a guarantee. Crews sometimes shift a line a few inches around a stud or a vent, and later remodels move things without a permit. Treat the drawing as your best first guess, then confirm with a scanner before any bit touches the wall.

If your home is older and the records are thin, the age and material of the plumbing still tells you something. A pre-1960 Phoenix house may have galvanized steel supply lines, which a metal scanner reads easily. A home re-piped in the last couple of decades likely runs PEX or CPVC, which a metal tool will miss entirely. Knowing the material in advance tells you which scanner to trust.

For an interior wall, a handheld scanner is the next step. The two tools overlap, but they are not the same.

1. 1Stud finder with metal and live-wire detection. A stud finder locates framing by sensing a change in density behind the wall, and many models add a metal-scan mode and a live-wire AC mode. As Zircon, a stud finder maker, explains, its scanners "detect both metal and the electrical field radiating from live AC wires." To make a metal pipe stand out, turn the water on so the line is full and reads as a solid object, then mark every hit.
2. 2Wall scanner rated for plastic pipe. Most Phoenix re-pipes use PEX or CPVC, and a plain metal detector or basic stud finder misses both. You need a wall scanner that advertises non-metallic or plastic pipe detection, which reads density rather than metal alone. Run it slowly in a grid, horizontal and vertical, and mark each return.

Scan a wider area than your drill spot and mark studs, wires, and pipes in different colors. If a tool flags anything in your target zone, move the hole. Drill shallow at first and stop the moment you feel a change in resistance.

When a scanner is unclear, three more methods help you read the route without opening the wall.

• Infrared or thermal camera. Run hot water through the line for a few minutes, then scan the wall with a thermal camera or a phone thermal attachment. The hot supply line warms the surface above it and shows as a bright stripe, which traces the hot run. This works best on the hot side, since the cold line stays near room temperature.
• Listen acoustically. With the house quiet, open a faucet and put your ear or a cup against the wall. Moving water makes a faint rush you can follow along the pipe path. It is rough, but it narrows the search before you scan.
• Trace logical runs from what you can see. Look at visible fixtures and shutoffs: the lines under a sink, the stub-outs behind a toilet, the valves at the water heater, and any exposed pipe in the garage or attic. Supply lines usually run straight up or straight over from these points, so a pipe you can see at the floor often continues directly up the wall above it.

These methods stack. A warm stripe on the thermal camera, a scanner hit in the same spot, and a fixture directly below all pointing to one line gives you a confident mark. Two of three agreeing is good enough to drill carefully a few inches to the side.

One habit makes all of these safer: drill short. Once you have marked the route and chosen a spot that clears it, set your bit or anchor for the shallowest depth the job allows, drill in stages, and stop to check the moment resistance changes. Drywall gives almost no resistance, so any sudden firmness can mean wood, conduit, or pipe. Pulling back early beats pushing through a line you missed.

Here is where many Phoenix homes differ from the wall problem. Most are built slab-on-grade, with the supply lines running in or under the concrete slab rather than through a basement or crawlspace. A wall scanner cannot see through several inches of concrete, and a stud finder is useless on a slab. Surface clues like a warm spot underfoot or the sound of running water with everything off can hint at a hot line below, and those same signs can mean a slab leak, which is covered on our page on the signs of a slab leak in Arizona.

To map a slab line, a plumber uses a line locator or leak-detection gear: acoustic listening equipment, a pipe-tracing transmitter, or a thermal scan of the floor. This is the same toolkit used for hidden leaks, which our guide on how plumbers find hidden leaks explains in more detail. The work is non-invasive and pinpoints the spot, so you avoid breaking open the whole slab to guess.

Call a pro before you cut concrete, before any major remodel that moves plumbing, or any time a wall scan is unclear and the hole has to land in a high-risk zone near a bathroom or kitchen. The cost of a locate is small next to a repair. The U.S. EPA's WaterSense program reports that household leaks waste nearly a trillion gallons of water a year nationwide, and a pipe nicked during a project is one fast way to join that total. A short visit to map the line first, the kind HQ Plumbing & Air handles across metro Phoenix, keeps a simple job simple.