For centuries, metal piping has been the backbone of plumbing systems, bringing durability and reliability to homes and buildings. Today, it faces competition from a less costly alternative—plastic piping. While each material has its own advantages, both metal and plastic come with unique limitations shaped by cost, longevity, installation ease, pressure and temperature ratings, building codes and fire resistance.

In this guide, we’ll weigh the pros and cons of the most commonly used metal and plastic piping in domestic water and drain-waste-vent (DWV) systems. By understanding the strengths and weaknesses of each, you’ll be better equipped to choose the right piping solution for your plumbing needs.

Although numerous types of plastic and metal piping are allowed by building codes, we will mention only those commonly used for residential and commercial buildings.

Plastic Piping

The three most common types of plastic pipe used in domestic water systems are Cross-linked Polyethylene (PEX), Polyvinyl Chloride (PVC) and Chlorinated Polyvinyl Chloride (CPVC).

PEX is a flexible tubing that comes in rolls of various lengths up to 300 feet. PEX connections are typically mechanical. In contrast, PVC and CPVC are more rigid and come in 20-foot lengths. Connections for  these two materials are a chemical weld made with solvent cement.

The two most common types of plastic materials used for DWV systems are Acrylonitrile Butadiene Styrene (ABS) and PVC DWV piping, which come in 20-foot lengths and are connected using solvent cement. For underground applications, PVC DWV pipe utilizes gasketed connections that are pushed together.

Pros of Plastic Piping

1. Longevity: Plastic pipes are more resistant to chemical corrosion than metal piping, so they do not generally degrade over time. Plastic is also more resistant to galvanic corrosion, which results when dissimilar metals are used in the same system.
2. Ease of Use: Plastic pipes are easy to install, making it a good option for DIY home repair.
3. Lower Cost: Plastic pipes are cost effective.

Cons of Plastic Piping

1. Less Fire Resistant: All plastic piping, except for CPVC and PEX, are typically not allowed by buildings codes for use in return-air plenums. The main reason for this is to avoid the distribution of smoke through the occupied space in case a fire engulfs the above ceiling plenum. This limitation does not apply to fully ducted return-air systems, since the ceiling area is not used for return air.
2. Expands More When Heated or Cooled: Plastic pipe has a higher coefficient of thermal expansion than metallic pipes and may require additional design features for long, straight runs.
3. Not Suitable for Higher Temperatures: PVC can only be used for domestic cold-water services. CPVC and PEX can be used for domestic hot water (up to 140 degrees Fahrenheit) and cold-water services.

Metal Piping

The most common type of metal piping for DWV systems is cast iron. Underground metal pipe systems typically utilize gasketed connections that are pushed together. For inside buildings, cast iron is connected with “no-hub” couplings, which utilize a rubber sleeve, stainless-steel shield and stainless-steel worm gear clamps.

Copper and stainless-steel piping are primarily used for hot and cold domestic water systems. Copper is usually connected by soldering, brazing or by utilizing mechanical couplings. Stainless steel connections are usually made with mechanical couplings.

Pros of Metal Piping

1. More Fire Resistant: Metal piping is more fire resistant than plastic and can be installed in above-ceiling return-air plenums with no limitations.
2. Easier Pipe Penetration: Pipe penetration of fire rated walls and floors is easier than with plastic pipes.
3. Needs Fewer Supports: Metal pipe is more ridged than plastic and requires fewer hangers and supports.
4. Higher Temperature Threshold: Metal piping can be used for higher temperature services, such as HVAC heating systems or high temperature domestic hot-water storage, which typically conveys liquids over 140 degrees Fahrenheit.
5. Higher Pressure Capacity: Metal piping also has a higher-pressure capacity than plastic piping, which may be necessary for primary supply piping in high-rise buildings with booster pumps.
6. Acoustical Benefits: Cast iron has acoustical benefits over plastic ABS or PVC drain piping.

Cons of Metal Piping

1. More Expensive: Metal piping is generally more expensive than plastic piping.
2. Less Resistant to Corrosion: Metal piping overall is less resistant to chemical corrosion than its plastic counterparts. Stainless steel is notably more resistant to corrosion than cast iron or copper. However, stainless steel can promote galvanic corrosion, if not properly isolated from copper.

As you can see, there are various types of plumbing pipe used today in domestic water and DWV systems. The type of material you choose for a repair, retrofit or new-build project depends on many factors. Hopefully, the above list of pros and cons of plastic and metal piping will help you decide.

If you’re ever in need of a plumbing expert witness, don’t forget about Ivey Engineering.

This article does not provide all considerations for selection of plumbing products and does not replace proper system engineering and design.