Differences Between Black Pipe & Cast Iron Pipe

By Steve Sloane

Cast iron pipe has been used in sewer lines for centuries due to its durability, and in many areas is still accepted by building code regulations. Two other types of black pipe are used in household construction: black coated rigid steel gas pipe and ABS (Acrylonitrile Butadiene Styrene) sewer pipe. Gas pipe and sewer pipe are quite different from both each other and cast iron pipe.

Cast iron pipe is durable, yet brittle.

Diameter Sizes

Cast iron pipe is usually installed in 4-inch diameter 8-foot sections, where ABS sewer pipe comes in a diameter range of 1 1/2 to 4 inches (depending on the water fixture it connects to), and also in 8-foot lengths. Gas pipe, however, has smaller diameters of usually 1/2 or 3/4 inch and is available in many different lengths.

Installation Processes

The connecting process of each type of pipe is quite different. Where cast iron sections were once sealed together with oakum and melted lead, today a rubber gasket and stainless steel band are used. Gas pipes connect with female threaded steel couplings and threading compound, and ABS pipes fit together with ABS couplings and ABS cement.

Fixing/Removing Broken Pipe Sections

Though cast iron is very tough, it is also brittle and will crack/break if it receives a heavy direct knock. As cast iron cracks tend to increase in size over time, the cracked section must be cut out and replaced with either a new length of cast iron pipe or PVC pipe (where code permits). ABS pipe is also durable but will also crack/break if heavily knocked. Smaller cracks can be fixed with epoxy putty, but larger cracked sections must be replaced with new ABS pipe. If a gas pipe length is broken, it must always be replaced with a new section, to make sure there is no fear of leaks.

Other General Differences

ABS sewer pipe is cheaper and easier to install than cast iron, and as ABS is a lot lighter than cast iron, there is less danger of bodily injury during the installation process. However, cast iron pipe's thick walls act as an insulator against the noise of water/waste traveling along the pipeline. Cast iron pipe is cut with a device called a snap cutter, which generally has to be rented from DIY rental houses, where ABS is simply cut with a handsaw, circular saw or miter saw. Gas pipes are different from both cast iron and ABS pipes in that their ends are threaded to fit into the coupling. Consequently, if a gas pipe is cut through (using a tubing cutter or hacksaw), each side of the pipe cut must be threaded with a threading die before it is fitted back together.