How to Find Underground Cast Iron Pipes with a Metal Detector

It’s estimated that 76 million homes in the United States have cast iron pipes. For much of the 20th century, cast iron was the material of choice for household plumbing. It was relatively inexpensive, strong enough for use in water, gas and sewage lines, and had a service-life expectancy of 50 to 100 years. 

However, factors like internal corrosion, external soil conditions, and extreme temperature changes can reduce the service life of cast iron plumbing by more than 60%.

In the early 1980s, PVC (polyvinyl chloride) plastic pipe was introduced and quickly became the national standard for plumbing pipes. It's much cheaper than cast iron, lasts longer, and the pipe doesn't corrode or contaminate water.

If your house was built before the mid-1980s, it may have cast iron plumbing that is reaching the end of its life and needs to be replaced. These pipes are often buried in the foundation, within walls, or underground, making it difficult to notice when they’re starting to fail, and even harder to locate.

In this article, we’ll first explore the reasons why cast iron pipes fail and need to be replaced. Next, we’ll learn about some of the signs of a failing pipe, so that you know what to watch out for. Lastly, we’ll show you how to locate a buried cast iron pipe with a special type of metal detector called a magnetic locator.

Why Cast Iron Pipes Fail

There are three major factors that contribute to cast iron pipe failures: internal corrosion, external soil conditions, and temperature changes.

Internal Corrosion

In the course of a drinking water pipe’s service-life, bare iron particles are continually exposed to high-pressure water. The iron combines with minerals in the water in a process called electrochemical corrosion. This process transfers iron from the walls of the pipe to irregularly-shaped protrusions (called tubercles) of metal inside the pipe.

Over time, this type of corrosion weakens the structural integrity of the pipe, and the protrusions restrict the flow of water, resulting in clogs and bacterial buildup.

Sewer and drainage pipes from bathrooms and kitchens corrode much more quickly than those that carry drinking water.

Decaying organic matter in sewage causes hydrogen sulfide gas to form. As this gas oxidizes, it produces sulfuric acid, which causes cast iron pipes to corrode from the inside, particularly on the top or “ceiling” of the pipe. Some drain cleaning products also contain sulfuric acid, which can accelerate the corrosive reaction, shortening the lifespan of the pipe.

External Soil Conditions

While your cast iron pipes are corroding on the inside, the soil that they’re buried in can be corroding them externally too. As you can see in the map below, highly corrosive soils are found in more than two-thirds of the United States.

A cast iron pipe may last 50+ years in the less corrosive soils of New England, 25+ years in the more corrosive clay of Texas, or 20-25 years in the salt-rich dirt of coastal Florida.

Sandy soils are considered the least corrosive. Clay soils, found in much of the Western and Southern U.S. are some of the most corrosive. Loams lie somewhere in the middle.

But what makes a particular type of soil more corrosive than others? Five factors—moisture content, temperature, aeration, pH, and mineralization are primary contributors.

Moisture Content is the most important factor in determining whether a soil is corrosive. If soil is completely dry (like sand), corrosion won't happen. As you add moisture, the soil becomes increasingly corrosive.

Likewise, as temperature increases (such as in high-humidity areas in the southeastern United States), the potential for corrosion increases too. 

Aeration, or the amount of air trapped within soil, plays a part in determining whether the water evaporates from the ground or corrodes the objects buried in it. As we learned earlier, water inside pipes can cause internal corrosion. Similarly, water retained in soil (the moisture content), can cause external corrosion.

If there's more air trapped in a particular soil, any water that's introduced is likely to evaporate quickly, meaning that highly-aerated soils are less likely to be highly corrosive. Red, brown, and yellow-colored soils often indicate high levels of aeration, whereas grey soil is typically poorly aerated.  

Mineralization, or the amount of naturally-occurring minerals present in soil, also affects corrosiveness. Soils with high levels of sulfates and salts (coastal areas) are some of the most corrosive. 

pH is a measure of the acidity or alkalinity of soil, and can range from 0 to 14. The lower the number, the more acidic a soil is—pH levels below 5 can lead to extreme rates of corrosion and premature pipe failure. Neutral levels of pH (around 7) minimize the potential for corrosion. High levels do not affect a soil's corrosive potential, and indicate that the corrosion is caused by other factors. 

Soil pH can be affected by rainfall, minerals present in the ground, gardening, and pet waste, among many things. 

Temperature Changes

According to the American Society of Civil Engineers, a sudden temperature change of as little as 10 degrees can increase stress on cast iron water mains and residential pipes, resulting in a greater risk of pipe failure. 

This is particularly an issue in the semi-arid and arid environments of the Western United States, where the weather can change drastically from one day to the next. In Colorado, for instance, we have recently seen temperature swings of 70+ degrees, going from record summer heat to snow in a matter of hours. 

As the effects of climate change cause extreme weather patterns to become more common, drastic temperature swings will likely increase the potential of pipe failures in the future. 

Six Signs of a Cast Iron Pipe Failure

There are six signs you may notice that point to a possible pipe failure: discolored water, leaks, slow draining and backups, odor, cracks, and an unusually healthy lawn.

Discolored Water

If you've ever turned on the faucet in your sink or shower and found brown or yellow-colored water flowing out, it can be unsettling. If you live in an older house and get your water supplied by your city or town, this discoloration is often a sign of damaged cast iron pipes. It results from corrosion that flakes off into the flowing water. 

Leaks

Frequent leaks can be another sign that your pipes are cracked or damaged by liquid drain cleaners. 

Backups or Slow Draining

A few years ago, I moved in to a condo with a toilet that was agonizingly slow at flushing. Within a few weeks, the toilet, sink, and shower were all backing up with sewage. It was a sure sign that the old cast iron sewer pipes were blocked by decades of corrosion and needed to be replaced. If you're experiencing slow draining or backups, have the pipes checked. 

Odor

If the smell of sewage wafts from your drains, however faint, it can be a sign of cracked cast-iron that's causing sewer gas to escape. 

Cracks

Speaking of cracks, if you notice any in your walls or foundation, it can signal that the main sewer line has an undetected leak that may have been destabilizing the ground for years, and can lead to sinkholes forming. 

A Strangely Healthy Lawn

Leaking sewage from a cracked or damaged cast iron pipe can act as a natural fertilizer for lawns and other plants. If you notice your yard looks unusually good, it may be a sign that your pipes are in poor condition.

Next, let’s look into how to find a buried cast iron pipe. 

Will a metal detector find cast iron pipes? 

Cast iron pipes run in straight or diagonal lines. They are buried 3-6’ beneath the surface, and often encased in concrete, hidden in walls, or otherwise physically inaccessible. Since they are made of metal, an obvious choice to many people is to locate them using a standard metal detector. Alas, it’s not that easy.  

A metal detector is designed to find small objects like rings, U.S. and international coins, bottle caps, buttons, etc. Many detectors have the ability to discriminate against certain metals; in other words, you can set them to ignore the types of metals you’re not looking for. In this case, we’re trying to find iron, so we would discriminate against everything else—aluminum, copper, silver, gold, and so on. 

But the insurmountable problem in using a standard metal detector to locate cast iron pipes is depth. Consumer-grade metal detectors can realistically detect objects between 2” and 12” (5cm to 30cm) below the surface. Cast iron pipes are 2-6x as deep, so we wouldn’t be able to detect them, even in ideal soil conditions. 

A Specialized Type of Metal Detector

However, there is a type of specialized metal detector that we can use to find buried cast iron or galvanized steel pipes. It’s called a Magnetic Locator, and it only detects iron and steel objects. 

Magnetic Locators measure the magnetic fields of buried iron and steel. Those magnetized objects then have their own magnetic field, which is received by the locator and displayed as signal strength. The stronger the signal, the closer you are to the buried object. 

With a magnetic locator, you can detect 4” (10cm) iron pipes that are buried up to 9’ (2.74m) underground. 

How to Find Cast Iron Pipes With a Magnetic Locator

For the purposes of this article, we're using a Schonstedt GA-92xtd Magnetic Locator, but the general information is applicable to any model you may have. To find a cast iron pipe with a magnetic locator, follow these steps: 

1. Turn the locator on. Set the GAIN (Sensitivity) to H.
2. Holding the locator vertically about 1' to 1.5' (0.3 - 0.46m) above the surface, determine the general direction of the pipe by sweeping across the ground. The signal strength will increase when you are nearing the pipe, and peak when you are directly above it.
   
   Once you know which direction is the pipe is going, you'll likely want to locate the pipe's joints as well. This is especially helpful if you need to repair the pipe by removing a damaged section.
3. To do so, set the GAIN to XH, and hold the locator vertically about 1' to 1.5' (0.3 - 0.46m) above the surface. Walk along the direction of the pipe, and mark the locations with maximum signal strength.
   
   
   
   As you can see in the above illustration, cast-iron pipes produce the strongest magnetic signals at their joints. Depending on the style of the joint, there may be two peaks; one at each end of the joint connection.
4. Once all the joints have been generally identified, go back to an area of maximum signal strength. Now, we'll more accurately pinpoint the pipe's joints.
5. Reduce the GAIN to H or M. Hold the locator several inches above the surface, and sweep across the pipe (both parallel and perpendicular to its direction) in an X-shape. The signal will peak when you are directly above the joints.