Part 1 of 2: An estimated 76 million homes have cast-iron pipes, which can cost $10,000-$30,000 to replace in a residence.

By Donald Dunn and Ralph E. Moon, Ph.D.

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Cast-iron pipe corrosion is complex and involves a variety of biological, chemical and physical stress factors on the interior and exterior of cast iron pipe surfaces. However, other environmental factors may result in stoppages that are often not taken into account. (Credit: Donald Dunn)

The emergence of cast iron pipe (CIP) claims on the insurance landscape has placed adjusters, litigation defense and risk advisors in a difficult position because the damage to the pipe is not easily seen and the circumstances that contributed to discharges from pipe failures are not broadly understood.

When occasional backups, blockages, stoppages and overflows among pre-1970s homes with CIPs occur, the cause is often attributed to aging cast-iron drainpipes experiencing changes that diminish efficient waste transport and sometimes render the system unusable. CIP corrosion is complex and involves a variety of biological, chemical and physical stress factors on the interior and exterior of cast iron pipe surfaces. However, other environmental factors may result in stoppages that are often not taken into account.

How sanitary drains work

Most sanitary residential drains are gravity fed and water-assisted in their discharge of effluent. The sanitary sewer system is plumbed so that waste travels on a continuous downward slope from its origination point to its termination point in a municipal sanitary sewer, lift station or septic tank. The three to four-inch drain lines are pitched with a minimum of 1/8″ (1%) slope per linear foot so that the solid waste is carried away by the water used to clear the waste from the plumbing fixture in use. The sanitary drain line connections are directional and are intended to gradually direct the waste downstream while maintaining efficient flow and solids transport.

Sanitary lines are sized based on the number of fixtures that are served by the drain lines. Drain lines carrying solid waste are sized to run one-third full at minimal discharge and two-thirds full at maximum capacity. This design allows air to flow freely on the top of the pipe, preventing a vacuum lock.

Diagram 1: Comparison between turbulent and laminar flow. (Credit: Physics and Chemistry for IG and A Level/

A drain system’s design is intended to achieve turbulent flow to create a “scour zone” on the bottom of the pipe and helps keep the line clean. Pipe flow characteristics are critical to achieving optimal performance because corrosion is influenced by whether the pipe creates turbulent flow, which allows water and associated solids to flow down the pipe and not accumulate. Turbulent flow occurs when the water flow lines are intermixed and random, creating variations in pressures and velocities that move solids effectively through a pipe (Diagram 1).

Laminar flow occurs when the fluid flows in parallel lines with little to no interaction or pipe surface disruption. The absence of flow variation within the pipe encourages suspended matter to “fall out” of solution as a precipitate and allows organic matter (fat and grease) to adhere to the interior pipe walls.  Laminar flow also promotes the inefficient discharge of waste in specific areas of the pipe.

Turbulent flow accelerates the rate of erosion/corrosion and creates thinning along a narrow area on the bottom of the pipe. This thinning creates local areas of increased turbulence that increase the corrosion rate. This is a very long-term process that leads to what plumbers call “channel rot,” which leads to holes and breaches on the bottom of the pipe as the process progresses.

The sanitary drain configuration can exert a profound effect on turbulence and velocity. From an adjuster’s perspective, the layout of the drain lines could identify installations that include 90o corners (1/4 bends) and abrupt elevation changes due to connection offsets. Abrupt bends and elevation changes influence flow velocities that lead to residual water and debris accumulation and microbial corrosion.

CIP age and design

CIP diameters may differ depending on whether the residential community was built before 1920. At that time, the CIP manufacturing process lacked uniformity in precise pipe diameters. The earliest CIP manufacturing process originated from methods that were 300 years old known as “pit casting,” which involves workers pouring molten iron into vertical molds lined with sand. This casting method produced pipes of various sizes and irregular pipe diameters that altered wastewater flow characteristics at pipe connections and joints. The pit casting method predominated until 1921, when “centrifugal casting” was developed by a French engineer, Dimitri Sensaud deLavaud. This method established new standards in pipe uniformity by allowing the molten steel to be distributed evenly inside the cast pipe form.

An estimated 76 million American homes have cast-iron pipes. Replacing residential cast-iron pipes tends to range between $10,000-$30,000.

Developments in pipe connections and fittings

Pipe joining or fitting is the coupling method between two pipes. There are three methods to join CIP, and the type of joining method will help interpret possible failure locations. No matter the pipe age or joining method, corrosion occurs where water is unable to drain or where a local area of rapid flow erodes the pipe.

1. Bell and spigot with caulked joints

Figure 1: Addition of molten lead over the oakum to create a competent seal. (Credit:

The caulked joint was the only method of joining bell and spigot CIP before the late 1950s. A bell and spigot joint have one end of the pipe that was flared open and served as a socket to insert the opposite end of a pipe creating a joint (Figure 1). The gap between the spigot and bell was packed with oakum, a tar impregnated hemp or manila rope product. The oakum was packed into the joint to make a watertight seal. Molten lead was poured into the joint to fill the remaining part of the joint and hold the oakum in place.

2. Bell and spigot with compression gasket joint

A compression gasket was also used for a bell and spigot fitting. The compression gasket was a molded gasket that was made of vulcanized rubber following ASTM C-564 requirements. These requirements specified the hardness, elongation, tensile strength, tear strength and compression characteristics of the gasket. During installation, the gasket was inserted inside the hub and lubricated. The inserted pipe was aligned and inserted mechanically into the hub by attaching a lever and chain wrapped around the pipe.

3. Hubless joints

This joint was developed in the mid-1950s to hasten pipe jointing and require less-skilled craftsmen. Hubless cast iron soil pipe is joined by using the hubless coupling. Several different types of hubless couplings are available. The typical coupling consists of a rubber gasket and a stainless steel band (Photo 3). The rubber gasket is installed over one end of the pipe to the “stop” in the middle of the coupling. The other pipe is then inserted in a similar fashion so that the “stop” creates a small gap between the two pipes. A stainless steel band is then used to clamp the gasket in place and is tightened with a calibrated torque wrench.

Donald Dunn is a Florida-licensed master plumber and president of the Florida-based My Plumbing Company. In addition to providing residential and commercial plumbing services, he specializes in plumbing-related forensic and diagnostic analysis and teaches basic plumbing and plumbing-related cause and origin to insurance professionals and building scientists. 

Dr. Ralph E. Moon, Ph.D., is a forensic scientist and frequent speaker at insurance conferences and seminars. His current research interests are studying rates of wood deterioration and metal corrosion among building materials and plumbing components used in residential construction. He is employed by NV5, Inc. in Tampa, Florida. 

Part two of the series will look at the impact of government regulations and waste management have on CIP corrosion.