how to find pipes behind walls

How to Find Pipes Behind Walls (Speed Up Your Remodeling Project!)

ByDino Pelle

The first step in remodeling a plumbing system is learning how to find pipes behind walls. Pipes seem to play hide and seek with homeowners, but I’m going to show you how to win this game and speed up your project.

Where is the Vertical Drain-Vent Stack?

The vertical drain-vent stack, which will determine the location of added fixtures, is not difficult to find.

Its top is the roof vent. And its bottom should have an exposed cleanout. It is the biggest pipe in the house, at least 4 inches in diameter. In older homes, it will probably be made of cast iron. And it generally runs almost straight up and down, with only minor jogs.

PRO TIP: When remodeling, you’ll want to map the fixtures that drain into the stack. You should not add a new sink drain to the stack downstream from a washing machine or toilet unless you also vent the new drain separately.

how to find pipes behind walls - draigram of vertical vent stack
Notice how the large vertical pipe with a roof vent at the top and a cleanout at the base. This is the vertical drain-vent stack.

Where are the Hot & Cold Supply Lines?

The hot and cold water supply lines may be alongside the drain stack, but in many houses, they twist around in the wall in unexpected ways.

The best way to track them is to turn on the water, one faucet at a time, and listen for the flow of water with your ear against the wall.

If necessary, drill small exploratory holes (be careful not to punch a hole through a pipe!) until you can hear the water. If you locate one (hot or cold), the other is likely to be within 6 inches of it.

PRO TIP: Before remodeling, make sure to turn off the water at the main shutoff valve and drain both lines by opening the lowest-placed faucets. Do this before you begin opening the walls.

how to find pipes behind walls - hot and cold water pipes diagram
As you can see in this diagram, hot and cold water supply pipes can take crazy turns throughout your home. These are more difficult to find than the vertical drain-vent stack.

Instructions: How to Find Pipes Behind Walls

When taking on a project like remodeling or extending your plumbing, there are some important tips to keep in mind.

  1. Do not be afrain to open a large hole in the wall
  2. A rectangle that spans the space between 2 studs (about 16 inches usually) will make the work easier, and will be no more difficult to close than a small opening.
  3. Make sure that a drain stack is solidly anchored before you cut into it. Slippage of even an inch will be enough to break the roof seal at the top of the vent and cause a leak.
  4. If the drain stack is cast iron, it will also be very heavy. To anchor the stack, install additional stack clamps. The clamp above the section to be removed will support the weight of the stack above. The clamp below will immobilize the lower stack while you cut.
  5. Leave both in place when the job is finished.

1. Gaining Access

  • When you have verified the stack location by drilling an exploratory hole, use a keyhole saw to cut a hole large enough so that you can (a) see where you wil tie into the stack and (b) locate the adjacent wood studs in the wall.
  • Insert a steel tape ruler to find how far the stud edges are from the hole.
  • Mark the locations of the stud edges on the wall.
how to find pipes behind walls - use a keyhole saw on drywall
A keyhole saw is a great tool for gaining access through drywall.

2. Opening the Wall

  • Drill starter holes 24 inches apart vertically just inside each stud line.
  • Cut the rectangle (24 inches high from stud to stud) with a keyhole saw.
  • Cut only to the stud edges (cleats will be nailed later to the studs to form a mailing lip for a wall patch).
  • If this opening does not uncover the supply lines, locate them and make a similar hole over them.
drill starter holes inside each stud line
When drilling the starter holes, make sure they are about 2 feet apart (vertically, one on top of the other) and inside each stud line.

3. Anchoring the Stack

  • To support heavy cast-iron drainpipe while you work, place a stack clamp (two shaped pieces of strap steel held together at the ends with bolts) just above and below the section you will tie into. This will ensure stability and alignment.
cast iron stack clamp
Make sure you use clamps or some other (strong) anchoring device to keep the vertical stack stable and correctly aligned.

Final Thoughts

I hope these tips have helped you understand what you need to know when undertaking a large project that requires you to find plumbing pipes behind your walls.

Call 1-Tom-Plumber

Don’t hesitate to contact us here or call us at 1-Tom-Plumber (1-866-758-6237) if you would like to know more about how to find pipes behind walls.

1-Tom-Plumber’s certified team of plumbers and drain technicians respond immediately to any emergency plumbing, drain cleaning, or water damage problem. We also handle the excavation of underground water lines and sewer main lines. Our immediate-response team is available every day and night of the year, even on holidays.

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