Transition fittings

Entry in Fittings and valves · Updated October 29, 2026 · Written by the Know Your Pipes editors from the sources listed under References

A transition fitting is a fitting made to join two pipes that cannot be joined directly, because they are of different materials, use different joining methods, or follow different sizing systems. Plumbing codes generally call for such fittings; Massachusetts, for example, requires unlike piping materials to be joined with adapters, transition fittings or prefabricated sealing rings or sleeves unless the code says otherwise.^[1]

Transitions are needed because each pipe has its own joining method: copper is soldered or pressed, CPVC and PVC are solvent-welded, steel is threaded, and cast iron uses couplings. A transition fitting carries one joining method on each end.

1 Water supply transitions

For CPVC pipe, one maker warns that all-plastic threaded male adapters used on hot water may fail, and recommends brass-threaded transition fittings made from low-lead brass for hot and cold lines.^[2] The same maker advises against compression fittings with brass ferrules on CPVC where water will exceed 140°F.^[2] It also recommends avoiding a metal male thread screwed into a plastic female thread, because the metal does not compress and all the stress falls on the plastic.^[2] See threaded pipe joints for the thread rules.

The IPS-to-CTS coupling exists because CPVC hot-and-cold tubing follows copper tube sizes while Schedule 40 and 80 pipe follows iron pipe sizes, so the two do not fit the same sockets.^[3] Other common water transitions, such as PEX to copper, use adapters supplied within the PEX fitting maker's system.

2 Drain and sewer transitions

Drain pipes of unlike materials are usually joined with elastomeric couplings. Above ground, shielded couplings made to ASTM C1460 connect unlike DWV materials in sizes through 10 in.^[5] Below ground, ASTM C1173 sets requirements for flexible couplings on buried drains and sewers, where the two pipes may differ in material, in size, or both.^[6] These are close relatives of no-hub couplings.

ABS and PVC drain pipe can also be joined with a special transition cement under ASTM D3138; the standard restricts it to gravity and other non-pressure lines of 25 psi or below, giving an ABS house drain meeting a PVC sewer as the kind of joint intended.^[4] Acceptance of that cement varies by code; ordinary PVC or ABS cement is not made for the joint. See solvent-weld joints.

3 Selection

Pressure, temperature, drinking-water listing and the code in force all decide which transition is acceptable, and a transition that suits a drain line is not automatically suitable for a pressurized supply line. The fitting maker's listing and the local plumbing inspector are the authorities on a given joint.