Expansion of Plastic Pipe Vs. Steel Pipe


Copper, steel and PVC (polyvinyl chloride) plastic pipe are all very common options for use in household plumbing systems. Copper and steel are typically used in larger constructions, but the low cost and lightweight qualities of PVC pipe make it an attractive option for homeowners or contractors building smaller houses. However, pipe qualities are a very important consideration when installing plumbing, and characteristics such as expansion can affect how the piping is installed.


  • Thermal expansion is literally the expansion of a material due to the presence of heat. Most materials, even metals, experience some type of thermal expansion. This can be caused simply by the energy given to the molecular structure of the substance, causing it to vibrate more quickly. It some substances, the molecules move relative to one another and inflate the structure, and in the presence of extreme heat the bonds of the molecules can stretch or fail, causing even greater expansion.


  • PVC and CPVC (chlorinated polyvinyl chloride, a variant), expand far more than metal in the presence of heat. Even a 50 degree temperature change is enough to cause PVC pipe to expand almost two inches per hundred feet. CPVC pipe expands even more, almost two and a half inches per hundred feet. Temperature changes this drastic tend to happen only in fires or near furnaces, but hot water can have slight effects of its own, too.

Fiberglass and Stainless Steel

  • Fiberglass and stainless steel pipe have almost the same thermal expansion characteristics. Steel pipe is sturdier and often quiet, while fiberglass pipe shares some of the lightweight and low cost properties of PVC pipe. But unlike PVC, fiberglass has a low expansion rate nearly identical to that of stainless steel; both only expand about half an inch per hundred feet in temperatures changes of 50 degrees.

Carbon Steel

  • Carbon steel is the most resistant to thermal expansion of the steel and plastic piping options. Like all steel, carbon steel is made by combining iron with trace amounts of carbon, but in carbon steel the carbon amounts are slightly increased to form a stronger and more brittle version of steel that is less likely to expand when heated. At 50 degree temperature increases, carbon steel does not even reach half an inch of thermal expansion.


  • Thermal expansion is important not only because of stress on the joints used in plumbing (which can crack and cause leaks), but also because of construction standards used in homes. Pipes that run through walls must be given space in which to expand in case of hot water, fire or even air heating. PVC pipes must be given much more room than steel pipes, which can make it difficult to replace metal pipes with plastic versions in some scenarios.

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